RECENT ADVANCES IN YAK REPRODUCTION
CHAPTERS:
•
Introduction: The Yak as a Subject of Scientific Writing
•
Ecology and Biology of Yak Living in Qinghai-Tibetan Plateau
•
An Overview of the Reproductive Performance
•
Effects of Environment and Management on Yak Reproduction
•
Interspecies Hybridization between Yak, Bos taurus and Bos indicus and
Reproduction of the Hybrids
•
Reproductive Biotechnologies: Current Status in Yak Reproduction
•
Semen Characteristics and Artificial Insemination in Yak
•
Male Reproductive Physiology
•
Reproduction and Conservation of Wild Yaks
•
Artificial Control of Estrus and Ovulation in Female Yaks
•
Reproductive Endocrinology of the Female Yak
•
Ovarian Follicle Activity in Yak versus Cattle and Buffalo
Introduction: The Yak as a Subject of Scientific Writing
Introduction
The yak (Poephagus grunniens) belongs to the tribe/family Bovin/Bovidae with the bison (Bison),
buffalo (Bubalus) and cattle (Bos), and is the only species of the genus Poephagus. Linnaeus
(1766) named the domestic yak as Bos grunniens for its distinctive vocalization and to indicate its
relationship to cattle. More recently, the yak has been placed into its own genus Peophagus to
emphasize its difference from other bovines. The yak is a multipurpose species and an
indispensable livestock for the highlands of central Asian countries situated between 70o and 115o
of east longitude and between 27o and 55o of north latitude. Thanks to its thick hide,
subcutaneous fat layer, and dense hair, yaks survive the extremely cold weather (Fig. 1).
Figure 1. Yak herds stay outdoors all-year round in the pasture,
including in the snow at night as shown.
Because of its large heart, lungs and a high erythrocyte count, the yak can tolerate the low
oxygen content in the air of high altitude. Since it is the sole animal species of economic
importance which can work at cold, high mountains as a pack animal (Fig. 2 and Fig. 3) and
produce valuable products such as milk (Fig. 4), meat, hide, hair and dung, yaks play an
important role in the local economy of the central Asian mountainous regions. The recent trend of
the yak population decreasing at an alarming rate has become a serious concern to local yak
users, concerned government officials and those who promote the conservation of animal genetic
diversity. Since their habitats, particularly those of the wild type animal, are cold, desolate and
isolated areas, the yak remains one of the most neglected ungulate species.
Figure 2. Yaks are used for transportation of goods.
Figure 3. Yaks are used for farm transportation
Figure 4. Milking yaks
The yak population worldwide is estimated to be about 14 million, distributed in China, Mongolia,
Southern Russia, Tajikistan, Kirgies, Nepal, Bhutan, Myanmar, Pakistan and Afghanistan, of
which 85% is located in China.
The yak is indeed the only large domestic animal which populates the central Asian Highlands up
to altitudes of 6000m. It is therefore of extreme economic importance, providing the indigenous
nomads and farmers with the essentials for their livelihood. Milk, dairy products (Fig. 5) and meat
are the main foodstuffs. Skin, wool and dung are utilized to meet requirements for shelter,
housing and fuel. The animals are also used in farming and transportation of goods. Yaks thus
have a crucial role in the economy of peoples in the region.
Figure 5. Cheese making from yak milk
The relative isolation of yaks in the mountainous regions of central Asian is illustrated by the
dearth of writing about the yak in the West until relatively modern times. Currently, apart from
much documentation of the yak by Chinese authors and a substantial body of publications in
Russian, there are only a few books [1-3] and papers written in English, dealing with the Yak,
especially about its reproduction. Much of the wealth of articles in scientific journals, reports and
proceedings of technical meetings are mainly in Chinese or Russian languages and are not
accessible to international communities. It should be mentioned that publications of the three
international congresses on yak (1997 in Lanzhou, Gansu; 1999 in Xining, Qinghai; 2000 in
Lhasa, Tibet) [4-6] would be of great help to readers who are interested in yak production in Asian
highlands.
The present book has been developed to fill gaps in knowledge and understanding, worldwide,
and is intended not only for those concerned with the science of animal production, but also those
with an interest in the biology of the yak. The editors recognize the IVIS’s Recent Advances
Series as the best vehicle to distribute such information to veterinary clinicians, biologists and
students. The book is entitled "Recent Advances in Yak Reproduction" and aims to give an
account of the current knowledge, new findings and techniques on all aspects of yak
reproduction. Attempts have been made to keep a balance between the fundamental details and
useful practical information. We, as editors, kept our interference within the limits of usefulness,
without overstepping our role, and we express our thanks to the contributors for their
collaboration when asked for clarification, improvement and amendment.
Regarding the reference list for each chapter, the general trend has been to provide selected
articles and reviews to enable the reader to consult the literature. However, the difficulties of
translating Chinese into English may mean that some of the names of authors and titles and
sources in the reference list may not be totally correct in every detail, and that some of the
references listed will be difficult to access.
Ecology and Biology of Yak Living in Qinghai-Tibetan
R. C. Zhang
Department of Animal Science and Technology, Gansu Agricultural Univiersity, Lanzhou, Gansu,
China.
Summary
The ability of yaks to live in ecological conditions in which other bovines will not survive, or at
least, not thrive, suggests that yaks have developed special adaptations. Yaks can cope with cold
temperatures by conserving heat, rather than by generating it, which would require food that may
not be abundant. Heat conservation is achieved by a compact body conformation and a thick
fleece of coarse outer hair and an undercoat of fine down. Adaptation of yaks to low oxygen
content of the air involves their large chest, large lungs and large heart relative to their overall
body size. The skin is highly pigmented and the predominant hair color is black. Both of these
help to resist the damaging effects of solar radiation.
Introduction
Yak (Bos grunniens or poephagus gruniens) is the prominent livestock breed in the QinghaiTibetan plateau where other livestock species can hardly live but yaks can survive, reproduce
and produce milk, meat, wool and other by-products. Yaks live extensively on Chinese plateaus
in alpine and sub-alpine regions at altitudes ranging from 2000 to 4500 meters with a cold, semihumide climate (Fig. 1).
Figure 1. A yak herd in Qinghai-Tibetan highlands
The highest altitude at which yaks live normally is at about 5500 m (Tibetan Rongbushi region,
located on the northern slope of Himalayas); yaks used as pack animals are quite capable of
traveling terrains at 7200 m, playing an important role in the transportation of goods to Chinese
Mount climbers. The distribution areas extend from the southern slope of Himalayas in the south
to the Altai in the north and from the Pamir in the west to the Minshan Mountains in the east.
There are about 13 million yaks (of which about 15% are hybrids mostly with Bos Taurus cattle)
distributed in 210 counties in China. These form about 11% of the Chinese cattle population and
90% of World’s yak population. The available grassland for yaks in China is about 1.03 billions
ha, which is about 25.6% of the Chinese natural grassland [1]. Most of the yaks in Mongolia are
found in the Hangay Mountains of the western parts of the country and in the high altitude areas
of Altai, with the reminder in the mountains of mid-north Mongolia. Yaks in countries of the
Russian Federation are distributed on the narrow mountain areas on the borders with China and
Mongolia from Pamir in the west to Lake Bakail in the east. Yaks were also introduced to the high
alpine areas of the northern Caucasus in 1970, and to the Yakutsk Valley of Siberia as recently
as 1971 [2], to exploit the potential for meat production for otherwise inhospitable alpine
grasslands. The yaks of Nepal and Bhutan are on the southern slopes of the Himalayas [1,3]
while those of India are distributed in the high altitude northern provinces and in the small territory
of Sikkim [4]. Other yaks are found in alpine areas of Afghanistan and Pakistan, adjacent to the
Qinghai-Tibetan Plateau (Fig. 2).
Figure 2. Distribution of the Yak population in China and neighboring
countries
The basic features common to the environment where yaks live are extreme cold, mountainous
terrain, high altitudes with reduced oxygen content in the air, high solar radiation and short
growing season for herbage and a variable assortment of herbage, sparse in some areas. In
Qinghai-Tibetan plateau, the weather is rather cold and humid, the average relative humidity is
55 - 65%, the annual precipitation averages 350 - 650 mm, there are no absolute frost-free days
in this area and the growth period of plants is only about 120 days a year. The annual average
temperature is 0ºC, in Maduo County of Qinghai Province, the extreme lowest temperature is 41.8ºC.
In the yaks native regions, at present, the stocking density of yaks declines with the increase in
average annual temperature. The greatest concentrations of yaks are found at average annual
ambient temperatures between -3º and +3ºC. In the Qinghai province yaks are concentrated in
areas with annual temperatures between -3º and -4ºC, those in Tibet are densest at the range of 3º to -5ºC and in Sichuan Province between -1.6º and +3.4ºC.
Yaks live in the high mountainous areas where, with increased altitude, the oxygen content and
temperature decrease. At the elevation of 3500 m above sea level where most yaks live, the air
oxygen content is around 35% lower than that at sea level. On even higher grazing areas at an
altitude of 5000 m, the oxygen content is halved. Also, in most of the areas there are more than
2000 hours of sunshine and levels of solar radiation are between 130 and 165 Kcal/cm2(540 - 690
KJ/cm2). The ability of yaks to survive in such harsh conditions and the ability of people to derive
sustenance from them are classic examples of adaptation by both animals and human beings.
Anatomical Adaptations
The yak’s body is compact with a short neck, short limbs, no dewlap, small ears and a short tail.
The scrotum of the male is small, compact and cover with hair, and the udder of the female is
small and also covered with hair. The skin has few wrinkles and the surface area of the yak is
relatively small per unit bodyweight (0.016 m2/kg) [5]. Compared with common cattle, yaks have a
shorter and wider trachea to allow a high rate of air intake. The length of trachea in Tianzhu White
Yaks (Fig. 3) is 44 - 51 cm (65 cm for other cattle at the same age), but the diameter is
appreciably greater [6]. The section of trachea in yaks is crescent compared to the round shape
in common cattle [6].
Figure 3. Tianzhu White Yaks: a unique yak breed in Tianzhu area of
the Gansu Province of China
A large space exists between the two ends of cartilage, being 5 cm in the adult male and 2.2 - 3
cm in the female yak. The trachea in yaks is comparable to that of dogs and may be adaptive for
respiration in low oxygen content environment. This also allows yaks to breathe rapidly and to
quickly increase air intake into the lungs when conditions demand it.
Yaks have narrow and long ribs with a relatively larger space between ribs and a good
development of muscle in between. Compared to local cattle, yaks have a large thorax and chest
allowing the development of large lungs and a large heart, and these in turn, may be have come
about as adaptive strategies to assist in the intake and circulation of adequate amounts of oxygen
under conditions in which it is in low supply.
Yaks have 47 - 48 vertebrae, of which 14 - 15 are thoracic vertebrae and 14 pairs of ribs, one or
two more than in other cattle. This gives the yak a larger chest capacity. The heart and lungs
indexes (the ratio of heart and lung weight to body weight) are higher in yaks than in common
cattle. The heart index is 0.52% in female adult yaks and 0.42% in male adult yaks, the lung
index is 0.98% and 0.75%, respectively. Alveolar areas represent about 59% of cross-section
areas of the lung section, compared to 40% in Jargas cattle [7]. Thus, the heart and lungs are
exceptionally well developed in the yak, and the lungs of yaks have a relatively large surface area
from which to absorb air in order to compensate for the lower oxygen content of the air.
There are five lumbar vertebrae, one less than in cattle. The number of coccygeal vertebrae is
variable ranging from 12 to 16 (other cattle have 16). There are five sacral vertebrae and there
are seven cervical vertebrae, the same as in cattle. Total numbers of vertebrae are thus fewer
than for other species of cattle.
Yaks can walk freely in precipitous places at high altitudes which cannot be reached by horse or
sheep and they can cope well with marshy grounds. Yaks have strong limbs and small hooves of
compact texture, with a narrow and sharp hoof tip, hard hoof edges and a close hoof fork. These
hoof characteristics make deep imprints on the ground, and may be an adaptation to allow the
yak to control its movement when going downhill.
Physiological Adaptations
The capacity to intake sufficient air, by virtue of anatomic features, respiration rate and
physiological response, is clearly an important aspect of the yak’s adaptation to high altitude. It is
also important that absorption of air oxygen be adequate for their needs. The yak respiratory rate
is 9 - 77 times per minute and the heart rate is 45 - 79 times per minute, both of which are faster
than in common cattle. The concentration of red blood cells in yaks is higher than in other cattle,
being 6.60x1012/mm3 in Tianzhu White yaks and 10.38x1012/mm3 in Noregai yaks of Sichuan
[6,10]. The diameter of erythrocytes in yaks is larger than in common cattle. The diameter of red
blood cells in adult female yaks was reported to be 4.83 m, while that of local cattle was 4.38
m. This suggests that the oxygen content per unit of blood is higher in yaks than in local cattle
breeds.
Cai et al., [8] observed changes in respiration rates in 48 adult yaks on pasture at an altitude of
3450 m in July and August. The respiration rate was between 20 and 30 per minute when the
ambient temperature was below 13ºC, but the respiration rate rose rapidly when the temperature
increased. Respiration rate was significantly higher in the evening than in the morning, however,
respiration rate was not significantly correlated with humidity, wind speed or the prevailing
weather [4].
Zhao [9] examined the seasonal changes in respiration rates of 5 adult female yaks at an altitude
of 3400 m on the cold grassland. Over a one year period, the animals were observed each day
between 06:00 and 08:00 hr and again between 18:00 and 20:00 hr. Respiration rate was highest
in August and pulse rate highest in June. Both rates declined gradually after the warm season
ended and were at their lowest in March. Body temperature was virtually unaffected by season
and averaged 37.6ºC in the morning and 38.5ºC in the evening. These results suggested that
yaks alter their respiration rate not only in response to a changing need for oxygen, but also in
regulating body temperature. The yak, with its thick skin, absence of sweating and a heavy coat
has few means, other than respiration rate, at its disposal for heat dissipation. The lowest pulse
rate in March corresponds to the time of year when yaks are in the poorest body condition, often
close to exhaustion, and with low metabolic rate following a shortage of feed over winter, leading
to near-starvation.
The hemoglobin content in yaks is higher than in other cattle breeds, being 8.56 g/dl in Tianzhu
White yak and 12.71 g/dl in Nouregai yak of Sichuan, values which are significantly higher than
the 7.86 g/dl for local cattle [6]. It has also been reported that the hemoglobin content in adult
female yaks changes with the season, being 12.3 g/dl in winter and 10.2 g/dl in spring. Generally,
hemoglobin concentrations in blood increase with an increase in altitude [7].
These anatomical and physiological characteristics appear to be adaptations of yaks to ensure
survival in the harsh ecological conditions of yaks native environment. The well developed chest,
heart and lungs, short trachea, cardiac and respiration rates, and high hemoglobin content in
blood, contribute to provide oxygen to tissues and allow yaks to survive at altitudes greater than
3000 meters above sea level.
Reproduction Adaptation
Yaks in the Qinghai-Tibetan Plateau show many reproductive adaptations. The seasonal onset of
estrus in yaks appears to be delayed at increasing altitude. For example, at an altitude of 2100 to
2400 m, estrus starts on 10 - 15 June; at 2700 m, on 19 - 22 June; and at 3000 - 3800 m, females
who did not calve in the current year will show estrus on June 25th. In Naqu district only,
individuals will start to show heat at the beginning of July. This allows for calves to be born in
warmer weather or closer to the onset of such weather, and during, rather than before, the start of
significant growth of grass in the following warm season. This suggest that a delay in the ability to
breed is an adaptive response. However, yaks that are mated late in the season have a lesser
chance of being re-mated that same year (should conception fail), than females mated earlier in
the season. Thus, the adaptation is not ideal by any means. Also, calves that are born late in the
year may have insufficient time to achieve a good body condition to improve their chances of
surviving the rigors of their first winter [10]. Thus, conception at first estrus is important in this
species.
Compared with common cattle breeds, yaks have a shorter gestation period, averaging 255 days,
which is about 30 days shorter than that of common cattle. The shorter gestation period may
benefit the female by reducing the load on the heart and respiratory system, and might also be
the reason why neonates maintain the fetal type of hemoglobin (HbF2). Shorter gestation, with
consequently smaller calves, also leads to a less stressful and quicker parturition, which may be
a matter of some importance in the yak environment, especially in the face of danger from
wolves. Whether the resulting relatively-low birth weight of the calf is or is not a disadvantage to
the calf itself is a matter for debate [10].
Another aspect of reproduction in the yak, which might be regarded as adaptation to its
environment, is the fact that some female yaks show only one estrus during the breeding season,
and, if not pregnant, the next occurrence of estrus will be delayed until the following year.
Perhaps, in such instances, late in the breeding season, priority is given not to conception but to
the deposition of internal fat reserves.
Newborn yak calves contain four types of hemoglobin in their blood (Table 1) . Adult types of
hemoglobin (HbI and HbII) represent about 51% of total hemoglobin, and fetal hemoglobin (HbF1
and HbF2) represents 49%. The early fetal hemoglobin HbF2 is not totally replaced by HbF1.
Even though HbF2 represents only about 8.9% of the total hemoglobin, because of its higher
affinity for oxygen, it plays an important role in the survival of yak neonates in low oxygen
environments. In newborn calves in common cattle, there is reportedly only one fetal type of
hemoglobin. However, in a fetus collected from the abattoir (body weight 12 kg), there were two
types of fetal hemoglobin [7]. Changes of HbF1 and HbF2 in young yaks are shown in Table 2.
As indicated in Table 2, HbF2 disappeared at 45 days after birth and HbF1 at 90 days after birth.
Table 1. Mean (+/- SD) concentrations of adult and fetal types of hemoglobin
found in newborn yak calves
Hemoglobin
Type
Concentration (g/dL)
Range
Adult
HbI
11.3+1.27
3.76 - 19.56
HbII
39.7+1.36
34.70 - 45.60
HbF1
40.1+2.05
28.26 - 51.41
HbF2
8.9+0.73
5.70 - 12.30
Fetal
Table 2. Mean (+/- SD) concentrations (g/dL) of blood hemoglobin types in
young yak calves during the first 90 days of life
Age in days
Adult types
Fetal types
HbI
HbII
HbF1
HbF2
Newborn
11.3+1.27
39.7+1.36
40.1+1.36
8.9+0.73
15
21.1+0.87
51.8+0.97
21.8+0.83
5.3+0.60
30
28.5+1.23
57.1+0.78
13.6+2.43
0.8+0.35
45
32.0+1.24
58.7+0.72
9.3+1.24
-
60
34.5+0.74
60.8+0.49
4.7+0.44
-
90
36.7+0.70
63.3+0.70
-
-
Skin and Coat Adaptive Characteristics
There are few skin wrinkles on the body surface of the yak giving the animal a relatively small
radiation surface. The skin in yaks is thick, there are well-developed dermal and fat layers, and fat
is readily deposited during the warm season. Li et al., [5] reported that the average thickness of
skin is 6.2 mm in yak, 6.12 mm in Bos taurus, 5.62 mm in Bos indicus. Skin thicker in the yak'
s
back than in other parts of the body, perhaps because the back is the part of the animal that is
most exposed to wind, rain and snow. Li et al., [5] also measured the skin thickness on the
shoulder blade, the back and the knee on 7 live female yaks and found the average thickness at
the three anatomical regions to be 5.61+0.36, 7.5+0.83 and 5.6+0.40 mm, respectively. Sweat
glands are not well developed in yaks. They are apocrine glands with long and tortuous ductules
distributed in the skin over the entire body. Sweat secretion does not occur readily, reducing the
heat radiation surface. This appears to force the animal to retain heat in the body and to increase
its tolerance to cold [10]. Density of sweat glands per square cm was found to be greatest on the
forehead (89/cm2) and lowest on the rump (138/cm2), with an overall average of 399+251/cm2
[5,11].
The coat of yaks seems well suited to insulate animals from cold, protecting them from heat and
repelling moisture. All these factors are important to survive in the prevailing climatic conditions. A
thick winter coat is a general adaptation of animals living in extreme cold, e.g. arctic mammals.
Thus conservation of heat takes precedence over generation of additional heat. To generate
extra heat would ultimately require additional feed that is in short supply over winter. Interestingly,
one of the most successful of all yak breeds, the Jiulong yak of Sichuan Province, has a fiber
strain which produces between 3 and 5 times as much fleece as that of other types of yaks. This
breed also inhabits one of the coldest, dampest and most fogbound areas of all yak territory. It is
possible that the dense, heavy coat has allowed the Jiulong yak to survive under these prevaling
conditions.
The coat of yaks consists of three types of fiber: coarse, long fibers with a diameter over 52 ;
down fiber with a diameter smaller than 25 ; and, mid-type hairs with diameters between these
two values. The down fiber is a particular attribute of the winter coat of yaks and provides the
additional insulation required. Coats with a mixture of fiber types have been shown to maintain a
stable air temperature within the coat. Ouyang et al., [12] reported that the gradient in
temperature between the skin surface and the top of the hair surface is far greater in winter than
in summer for parts of the body trunk like shoulder, rump and belly, but that the seasonal
difference in the temperature gradient from skin surface to top of the hair surface is much less in
the extremities of the body, such as the ears, where vasoconstriction occurs during cold [9].
The function of the coat in helping yaks survive in very cold and wet conditions is enhanced by
the coat'
s low water absorption [12]. The erectores pilorum muscles are well developed in the
dermis of the yak [13] and their contraction makes the fibers stand up and effectively increase the
depth of the coat and reduces heat loss under stress from cold.
Hair growth and the composition of the coat changes with season in the yak. As ambient
temperature falls with the approach of winter, down fibers grow densely among the coarse hairs,
especially on the shoulder, back and rump. Ouyang et al., [12] found that, in winter, the proportion
of down fiber increased between 17.5% and 30.0% in winter, through the activation of down
follicles which had lain dormant [14]. The proportion of coarse hairs, therefore, correspondingly
decreased. As air temperature rises with the onset of the warm season, down fibers began to
shed from the fleece.
As a consequence of the abundant grazing in summer and early autumn, yaks are able, normally,
to deposit a layer of subcutaneous fat which then helps to insulate them from cold and also
provides an energy reserve to be used to withstand nutritional deprivation over winter and early
spring.
Energy Metabolism Adaptation
Hu [15] studied the energy metabolism of growing yaks at three different ages (1, 2, and 3 years
old) and compared it with that of the local yellow cattle, at three different altitudes (2261, 3250
and 4271 m). The author reported that heat production of the fasting yak remained fairly constant
irrespective of altitude, whereas that of yellow cattle rose markedly. This could well point to an
adaptive response of yaks to life at high altitude and to the nutritional deprivation which yaks
experience in winter [15]. Heat production of fasting yaks was higher than that of the yellow cattle
at the lowest of the three elevations, but not at the higher altitudes. In another experiment by the
same author, yaks generated a little more heat in the course of walking than did the somewhat
larger yellow cattle. The author attributed the differences in heat production to the difference in
body size, with smaller animals being expected to generate more heat.
An Overview of the Reproductive Performance
X. X. Zhao
Department of Veterinary Medicine, Gansu Agricultural University, Lanzhou, Gansu, China.
Summary
Yaks are seasonal breeders with mating and conception constrained to the temperate season of
the year. Usually, yaks are mated for the first time when they are 3 to 4 years old and are most
likely to calve once every 2 years or twice in 3 years, producing 4 - 5 calves in a lifetime.
Seasonal and general environmental conditions affect the reproductive rates rather markedly.
The behavioral changes of estrus are not usually as clear as in other domestic cattle. The
average length of the estrous cycle is approximately 20 days with the duration of estrus less than
a day. Gestation length is around 258 days, shorter than in other cattle breeds. Abortion and
other causes of premature termination of pregnancy are between 5 and 10%, however, the
percentage rises with inter-species hybridization. Bulls usually start to mate at 3 or 4 years of
age, they first have to establish a dominant position in the mating hierarchy of the herd and reach
the peak of the mating ability around the age of 6 - 7 years. There is sufficient evidence to
suggest that the reproductive rate of yaks can increase with improvements in a variety of
management techniques geared to increase estrus frequency and conception rates.
Introduction
Yaks are one of the most remarkable domestic herbivores living on the Qinghai-Tibetan plateau,
the "roof of the world". The plateau itself extends over 2.5 million sq km [1] as the most
widespread high elevation region on earth and the best grasslands in Asia. From the core
distribution area, yaks have also spread to the adjacent territories above the tree line where there
are nearly no crops and there is not an absolutely frost-free period. Some 14 million yaks live and
provide food, transport, shelter and fuel where few other animals will survive. It is quite possible
that without the existence of the yak living in such harsh conditions, human civilization might not
have reached and flourished in these remote areas.
Knowledge of the basic reproductive biology of yaks is necessary in order to improve the
production potential of the animal. However, the number of publications relating specifically to yak
reproduction is insignificant compared to other domestic species. The great majority of reports
have been published as abstracts, short communications or theses and in many instance, the
number of observations has been low. This review aims to document the reproductive
performance of the yak under various sets of circumstances. Consideration will also be given to
components of the reproductive process, in order to indicate which of these is the most limiting, or
most amenable to change in the yak.
Reproductive Anatomy
The structure of the reproductive organs of the female yak differs in some aspects from those of
dairy cows. The cervix in yaks has three transverse circles consisting of many tight folds, the
average length of the cervix is 5.0+0.9 cm long and the average external diameter is 3.2+0.7 cm.
The corpus uteri is rather short, being 2.1+0.8 cm only. A long and distinct septum (approx 6 cm
long) extends downward from the bifurcation of the uterine horn towards the uterine body.
Because of the short uterine body and the long septum it is easy to deposit the semen in an
optimal position such as the uterine body, uterine horn or the tips of the horn, especially since the
cervix is relatively free within the pelvic cavity and can be readily held [2]. The ovaries in the yak
are smaller than in dairy cows, the left ovary is 2.1 - 2.5 cm in length, 1.1 - 1 cm in surface to
surface distance and 0.8 - 1.2 cm from attached to free border and the reported weight is 1.4 - 4.0
g.
For the right ovary 1.8 - 2.4 cm in length, 1.2 - 4.8 cm in surface to surface distance, and 0.7 1.2 cm from attached to free border and the reported weight is 1 - 4.1 g. The length of the uterine
tube is 18 - 24 cm [3-4].
Breeding Seasonality
Yaks are considered seasonal breeders. However, information about the breeding season is
rather conflicting. The onset and the end of the breeding season are affected by ecological
factors such as climate, grass growth, latitude and altitude. When temperature and humidity start
to rise, grass starts to grow, body condition of yaks is improved following their long period of
deprivation and weight loss over the winter, and females come into the breeding season. On the
northwestern grasslands of Sichuan the season begins around June [5], at the higher elevation of
Laqu in Tibet the breeding season will not start until July. Similar observations are reported from
Kirgizia where the annual onset of the breeding season started on May 25th at an elevation of
1400 m and occur progressively later until at the altitude of 2700 m estrus started after June 22nd
[6]. The breeding season reaches its peak in July and August when temperature is at its highest
and grass growth is at its best. Thereafter, estrus frequency decreases and stops around
November. Two examples from Qinghai, China and Mongolia are presented in Figure 1 [7-8].
Semen quality and quantity also change with season. The highest values for volume, sperm
concentration and motility were obtained from June to September, with the peak occurring in
September in Dangxi, Tibet [9].
Figure 1. Distribution of estrus in different months of the year.
Puberty
First estrus in yaks generally occurs in the second or third summer and autumn following birth,
i.e., between 13 and 30 months of age. In Mongolia, around 10% of yaks come into the first
estrus in the second summer of their life and most females do not show estrus until their third
summer when they are more than 2 years old. Onset of first estrus is determined more by the
body condition at the beginning of the breeding season than by age [10]. Very similar results
based on observations on yak in Tuva Autonomous Republic were also reported [11].
In China, the majority of yaks are mated for the first time at the age of 3 years, i.e.,, in the 4th
warm season following birth, but under favorable conditions some yaks may be mated a year
earlier.
Such conditions prevailed among 197 primaparous Jiulong yaks in the Sichuan province studied:
32.5% calved at 3 years, 59.9% at 4, 6.1% at 5, and the remaining 1.5% at 6 years or age. At
these ages, yaks had reached between 75% and 100% of their mature weight [12]. In this
context, yaks in Tuva reached fertile estrus at approximately 90% of mature body weight
compared with 60% for Bos taurus cattle in that region [11]. First mating at the age of 2 years,
though it also occurs in China, is more common among yaks in some other countries.
Males start to show mounting behavior around the age of 6 months. This behavior continues and
intensifies to include searching and mounting females in the following year. No sperm, however,
are found in epididymal fluid of yak bulls before the age of 2 years. Puberty thus occurs in the
third warm season following birth, when the males are over 2 years old. In practice, bulls start to
mate from the age of 3, reaching their peak ability at around 6 - 7 years old after establishing their
dominant position in the mating hierarchy by fighting in the same herd (Fig. 2). After the age of 8,
yak bulls start to lose to younger bulls in the competition for females [9].
Figure 2. Two male yaks fighting to establish their dominant position in
the mating hierarchy of the herd
A sexually productive life expectancy of not more than 10 years for a yak bull was indicated from
results of an artificial insemination (AI) stud farm of 38 yak bulls in Tibet (elevation 4300 m). The
ejaculate volume, concentration and motility of sperm rose steadily from the age of 3 to 9 years
old and then declined [9].
In Mongolian yaks, the older the bull the more females it was able to serve, which was consistent
with the courtship behavior and dominance hierarchy of bulls. However, it was also found that the
younger bulls, with fewer females at their disposal, mounted their mates more frequently [10].
Fertilization seemed to be more dependent on the number of services than on the age of the bull,
which showed that overall pregnancy rate of females increased with the number of services.
Estrus
The average length of estrous cycle in yaks has been reported to be 18 - 22 days (Table 1) and a
great variation in the length is one of the problems in yak reproduction. The main reasons could
be silent or non-detected estrus, delayed ovulation, implantation failure, or embryonic death.
Estrus in yaks is greatly affected by the environment, and when the weather is unfavorable the
onset of estrus is delayed, and when in favorable circumstances the onset of estrus in female
yaks is advanced.
The duration of the estrus is not easily determined in the yak since the signs of estrus are not
always clear. Estimates from northwestern Sichun suggest that estrus lasts 12 - 16 hours. In a
study with 41 female yaks, 26 of them had an estrus lasting 24 hours or less and 4 yaks showed
estrus for up to 72 hours. More than 80% of these animals ovulated within 24 hours after the end
of estrus [16]. There is a tendency for the proportion of yaks with heat periods of 1 - 2 days to
increase later in the breeding season when ambiant temperature begins to decline. Purevzav and
Beshlebnov [17] recorded that among 54 Mongolian yaks, 26 were recorded on heat for only 0.5
to 6.5 hours, a further 17 females showed estrus between 6.5 and 12.5 hours, 7 females showed
estrus between 12.5 and 18.5 hours, and only 4 females showed a longer estrus duration. Most
of the yaks ovulate 12 - 24 hours after the end of estrus (10 out of 41, 24.4%) and some in 24 36 hours (2 out of 41, 4.9%) [16].
Changes in the appearance of the reproductive organs are more obvious than behavioral
changes [21-22]. The vulva becomes swollen and the vagina reddens. Mucus is discharged from
the vulva in a majority of females in estrus, but a substantial minority shows no such discharge;
vagina and cervix dilate, the female tends to raise her tail and urinates frequently.
As in other cattle, yak females on heat search out and ride other females and like to be
approached by male yaks. However, these signs are less pronounced than in Bos taurus cattle.
The most pronounced signs of estrus are following and mounting by mature bulls, swollen vulva,
reddening of vaginal mucosa and mucous discharge.
Most yaks start to show estrus in the early morning or in the evening and only rarely at other
times of day. Among 633 female yaks observed by Cai, two-thirds of the animals started to show
heat before 9:00 hours when they had started grazing, and most of the remaining third started
after 19:00 hours when grazing had ended for the day [21]. Similar observations were reported by
Lei et al., [19]. Magash [10], however, with records for 73 yaks in Mongolia, found that only 38%
came in heat between 2:00 and 8:00 hours, and 34% between 16:00 and 22:00 hours.
Table 1. Estrus cycle lenghts of yaks (days)
Source
Number of
observation
Mean
SD
Liu & Liu, [13]
1184
20.5
5.4
Liu & Liu, [14]
308
20.1
8.2
Zhang et al., [7]
53
22.5
5.4
Anim Vet Inst, Tibet
[15]
12
18.3
6.1
Yu et al., [16]
35
20.4
1.6
Katzina & Maturova,
[9]
90
19.1
(10 - 28)
Purevzav &
Beshlebnov [17]
54
20.0
4.0
Magasch [10]
74
19.8
(10 - 27)
Wang [18]
98
21.3
5 - 61
Lei [19]
18
14.9
3 - 43
Xue [20]
53
22.5
5.4
Range
6 - 47
6 - 42
8 - 45
The average duration of post-partum anestrous at Xiandong farm in Sichuan Province was found
to be 125 days, that figure, however, was subject to much variation [21]. Postpartum anestrous
period was found to be much shorter (70.5+18.5 days) for yaks in good body condition than for
those in poor body condition (122.3+11.8 days) [13]. The anestrous period following calving has
been reported to be related to month of calving: 131, 124, 90 and 75 days for females calving in
March, April, May and June, respectively, and 134, 130, 105, 89 and 37 days for each month
from March to August [19].
Magasch [8] provided results on the interval between calving and first postpartum estrus for yak
females in Mongolia, showing clearly a relationship with month of calving, the earlier the calving
the longer the interval. However, there was considerable variation in these results around the
average interval periods. Magasch reported that the service period following calving showed a
very similar seasonal pattern for the interval between calving and first postpartum estrus.
The proportion of females that come into estrus in any one season depends on the previous
calving history as well as on their body condition.
Yaks can be divided into 3 categories: Those that calved in the current year and are lactating and
nursing a calf (full lactating cow); those that calved in the previous year and are not pregnant, but
may or may not be still nursing their calves ("Yama" or "half-lactating yak"), and those that
previously had a calf, but not for at least 2 years and are not lactating ("Ganba"). In a study Yong
et al., showed that from June to mid-September "Yama" had the highest proportion of females in
estrus (112/161), "Ganba" came next (217/408) and full-lactating cows the lowest proportion
90/629 [23]. The three types of cows differ in pregnancy rate and the age of the females - with 5 6-years-old females having the best rate. By the age of 9 - 10 years the conception rate starts to
decrease. In ordinary production herds in the mountainous regions of China, in general 50 - 70%
of yak cows of suitable age show estrus in any one year, and that such yak females are mated
and calve twice in 3 years or once every two years. In the absence of pregnancy, the number of
individual estrus periods a female yak will experience in a breeding season can vary. It may also
be affected by local environmental conditions and production system, including differences in
temperature, nutrition and milking frequency. In some production systems, there are reports of
yak females capable of showing estrus up to 3 to 4 times in the same season [21], including
results reported for yak in the Tuva Republic [11] and in Mongolia [8].
Pregnancy and Parturition
Conception following mating at first estrus during breeding season is generally high. Among 68
females on heat, 53 had well-developed follicles and 15 did not probably due to uterine infections
[21]. In a trail with 265 yaks that had previously calved, 72.4% became pregnant following the first
estrus, a further 23.4% following the second, and 3.4% and 0.8% following the third and fourth
cycles respectively [24]. In a similar investigation with 342 yaks in Mongolia, it was found that
70.5% were pregnant after the first service, 19.3% conceived to a second service and 4.6% to a
third service, giving an overall pregnancy rate of 94.4% [8]. It appears from the above results that
Mongolian yaks which are not pregnant at first-service, are able to return to estrus up to three
times in the same breeding season.
In one particularly well-maintained herd of yaks on grassland in Gansu, where yaks had received
some supplementary feed in late winter and early spring, a conception rate of 93.4% was
achieved [16]. A pregnancy rate of 74.9% following insemination of 621 yaks with frozen semen
at first estrus during the breeding season has also been reported [21].
Many studies have shown that it is not uncommon to achieve pregnancy rates above 70% either
by natural service or by AI [15,25-26]. A survey showed that pregnancy rates depend much on
the production system, and have ranged from 40% of 29760 yaks to 82+6.2% of 8448 yaks [20].
However, pregnancy rates are markedly reduced for pregnancies resulting from inter-species
cross-breeding when compared to pure-breed matings. When female yaks are inseminated with
semen (or mated by a bull) of other species of cattle, incompatibilities appear to arise and
pregnancy rates decrease. The gestation length of yak is shorter than that of Bos taurus,
particularly when a pure yak calf is carried. Yaks carrying a pure-breed calf usually have a shorter
gestation length than when carrying an inter-species crossbred calf. The average gestation length
of yaks with a crossbred calf, the Pian Niu (yak x cattle hybrids) was found to be around 270 days
(273.2 days with a SD of 12.7 days for 371 cows carrying male F1 calves and 268.7 days with a
SD of 10.2 days for cows carrying female F1 calves ) [25]. For yaks with purebred calves,
Denisov reported an average gestation length of 258 days [6]; Lei et al., recorded a gestation
length of 260 days for 36 yaks (253 - 278 days) for those carrying male calves and 250 days
(226 - 283 days) for those carrying female calves [19]. Dubrovin recorded an average gestation
length of 258 days for 800 yaks in the Caucasus [27]; Katzina and Malturova recorded a gestation
length of 259 days (228 - 280 days) for yaks in the Tuva region [11], and Yu et al., reported an
average of 254 days ( 248 - 258 days, SD 2.7 days) for yaks in Gansu [16]. Joshi et al., reported
an average gestation length of 258 days in Nepal [26].
Almost all births take place during the day and only very few at night when yak cows are normally
at the herders’ campsite. When the time for parturition approaches, the female looks for a
sheltered spot, such as a depression in the ground or ditch, at a distance from the herd.
Typical behavior of the yak during labor includes lying on her side and standing up again for
delivery. Dystocia is a rare occurrence in yak with pure-breed calves. The umbilical cord is
severed by mechanical stretching as the cow gets up or the calf falls down after delivery. Yak
cows with hybrid calves, however, require help for delivery. For example, there were 28 cases of
dystocia among 681 such calvings (4.1%) over a period of 10 years in one study in Sichuan [4].
Twins are rare, and represent only about 0.5% of all births, but in exceptional breeds higher rates
have been recorded [21]. The dam generally licks the newborn calf for about 10 mins after which
the calf attempts to stand up and suck. Again, differences in behavior have been observed
between dams delivering purebred yak calves and Pian Niu calves (Table 2) [29], with some time
intervals markedly longer when a Pian Niu calf is involved rather than when pure yak calves are
born.
Table 2. Intervals between successive events at parturition according to types of
calf.
Nature of events
Interval between events
Purebred yak calf
Crossbreed calf
Appearance of calf to end of
parturition
3 min to 16 min
45 min to 107 min (with
help)
Calf out to calf being licked
0 min 2 sec to 0 min 5
sec
0 min 3 sec to 0 min 7
sec
Calf out to calf starting to stand
up
14 min 2 sec to 21 min
30 sec
60 min to 99 min 14 sec
Calf out to first sucking
15 min to 22 min
74 min to 103 min
Duration of first sucking
3 min 0 sec to 5 min 30
sec
5 min 32 sec to 11 min
21 sec
Ganbat & Magash [34] recorded the course of parturition in yak cows. The dilatation of uterine
cervix lasted 228+22.2 min, the period of expelling fetus lasted 25.2+2.6 min and the placenta
expulsion lasted 136+0.5 min. The total process of parturition lasted 339.2 min [33].
The placenta is passed within half an hour and up to 6 hours after parturition. In the period
shortly after birth, the dam is intensely protective of her calf and will attack any person coming
close. Bonding between dam and calf depends mainly on smelling and licking. Longer parturition
times and dystocia militate against such bonding and thus place Pian Niu calves at a
disadvantage versus purebred yak calves.
The calving season in Mongolia is extended over a few months. It can be estimated that a small
proportion of yak cows calved in March, probably around 25% in April, many more in May (the
peak month), and a declining number in June and July [8]. Denisov reported from Kirgizia that the
calving season for a herd of 597 yaks extended from February to December. However, only 5
calves were born in February, and 116 (19.4%), 253 (42.4%) and 113 (18.92%) respectively over
the next 3 months and tailing off rapidly thereafter [6].
When yaks are cross-bred to produce Pian Niu animals, it is common to attempt hybridization
either by natural mating or AI in the first half of the breeding season followed by the use of yak
bulls to catch cows that have not conceived and have returned to estrus. Thus, it is not
uncommon for crossbred calves to be born earlier than purebred calves.
Abortion and other causes of premature termination of pregnancies account for 5 - 10% of all
pregnancies, as observed in 971 yak females in Laqu, Tibet with an abortion rate of 5.7% [21].
Calving rate is lower when inter-species hybridization is involved. Among 1438 female yaks
carrying crossbred calves in north-western Sichuan, 20% lost their calves, and in another study
with 158 young pregnant females, 14 lost their crossbred calves before normal parturition [23].
A 90% survival rate is typical among purebred calves (1328/1470) among Jiulong yaks in
Sichuan [30], and 89.8% (1818/2025) for purebred calves at Datong Farm, Qinghai [31]. In
contrast to the greater problems experienced before and during parturition with Pian Niu calves,
crossbred calves, once delivered, have a slightly better survival rate than pure yak calves.
Results at the same locations stated above, showed survival rates for Pian Niu calves to be about
2% better.
Detailed observations [32] of 20 yak calves on a farm at an elevation of 3500 - 4100 m showed
that neonatal survival was also related to the maintenance of body temperature in the calf. The
fall in body temperature in the first hour after birth (average fall 0.38oC) was significantly
correlated (r = 0.69) with birth weight (the greater the weight, the lower the temperature loss), and
much less strongly correlated with ambient temperatures. Thus the body condition of the dam
during pregnancy affects calf survival through its effect on birth weight. The body temperature of
the calves returned to normal after 3 hours.
Exceptional yak females may live to an age of about 24 years, but 15 - 16 years is the normal
upper limit for reproductive activity. The peak reproductive ability is considered to be between the
5thand the 9thyear of life [32].
Yu and Chen studied a total of 1953 yak cows (3 - 11 years of age) in two populations differing in
calving rates. The calving rate was 78% in the high calving rate group and 50% in the other [34].
It appears that the main differences are due to milking frequency and feed supplementation
between the two populations. The high calving rate group was milked once daily and received
supplementary feed from late winter to early spring, whilst the low calving rate group was milked
twice daily and did not received any supplementary feed.
Calving interval is an important economic index in the yak industry. Wang et al., [35] observed
439 calvings from 161 yaks over a 7 year period. The average calving interval was 459+131
days, showed a tendency for gradual decrease with increase in parity.
Effects of Environment and Management on Yak Reproduction
R. C. Zhang
Department of Animal Science and Technology, Gansu Agricultural Univiersity, Lanzhou, Gansu,
China.
Summary
Yaks live in a very harsh environment, including seasonal nutritional deficiencies, cold weather
and air with low oxygen content. These conditions seriously reduce reproductive performance.
This paper summarizes recent findings of environmental effects on reproductive performance in
yaks.
Introduction
Yak production in Qinghai-Tibet Plateau is still characterized by year-round grazing without
supplementation. However, the period of grass growth lasts only about 4.5 months and only
during June does the grass adequately meet the nutritional needs of the yaks. Failure of the
pasture to provide adequate nourishment during the majority of the year profoundly limits growth,
production and reproduction.
Due to consumption and trampling, the amount of available grass declines rapidly during the nongrowing season (approximately a 50% decline each month). Concurrently, the nutritional value of
the grass also declines rapidly. Digestible protein content and overall digestibility are 11.5 and
75.4%, respectively, as the grass begins seed production but decline to 3.3 and 48.0% when
seed production is complete and the plants are mature [1-2].
Reproductive performance of yaks is influenced by many factors, particularly nutrition. Although
yaks are adapted to the cold weather, low oxygen content and rainfall in the Qinghai-Tibet
Plateau, these environmental conditions seriously affect reproduction. The severity and duration
of malnutrition is primarily related to dry matter intake, which is related to body condition at
calving. The detrimental effects of malnutrition appear to be manifested as reduced fertility; the
greater the loss of body condition, the greater the reduction in pregnancy rate. In general,
parturition and breeding (for yaks and indeed most of the livestock in the Qinghai-Tibet Plateau)
occur during the period of long days and maximal nutrient availability (summer).
Effect of Cold Weather on Reproduction During cold weather, yaks must increase their
metabolism to maintain body temperature. If the intake of natural forage does not meet their
nutritional needs, they will use stored reserves, thereby decreasing weight and body condition. In
that regard, bodyweight typically decreases 17 to 25% during the cold season. If postpartum
females become pregnant again in the same year, their body condition and fetal development will
be adversely affected due to the nutritional demands of calving, lactation and pregnancy.
Consequently, some of these yaks will abort or die. Therefore, many yaks are anestrous after
calving [3].Young yaks that are born late or are weak and lack adequate body reserves at the
start of their first cold season will have delayed puberty (normally 2 and 2.5 years for females and
males, respectively) and delayed mating (normally 3 years and 2 years for females and males,
respectively).
The preferred temperature for well-nourished yaks is about 10ºC but increases to above 18ºC if
they are malnourished. In cold weather, a 1ºC decrease in ambient temperature may result in a
2 - 5% increase in metabolic demands; yaks exposed to blizzard conditions without shelter may
die from hypothermia and exposure [1,3]. In addition, pregnant yaks may abort during very cold
weather. The effects of cold weather are exacerbated by other environmental factors such as
wind speed and humidity. For example, a doubling of wind speed causes a four-fold increase in
heat loss from the body surface.
Furthermore, in cold weather, the heat conductivity of air with a humidity of 40% is 10-fold higher
than that of dry air (0.00051 versus 0.00005 calories/cm.second.degree, respectively). Therefore,
the humid conditions resulting from yaks crowded together in poorly ventilated sheds may
increase heat loss.
Effect of Environment and Nutrition on Estrus
Seasonality of Estrus Estrus is not displayed year-round but is influenced by season and nutrition. In reports from
Datong Yak Farm, Qinghai [1,4], yaks grazing year-round (at an altitude between 3000 and 4800
m) were most likely to display estrus between June and November (Table 1).
Table 1. Seasonality of Estrus in Yaks at Datong Yak Farm, Qinghai
Province, China
Yaks
Month
No
June
July
Nº
%
70
4
5.7 25 35.7
Milking 45
-
Dry
-
Nº
-
%
-
August
No>
%
33 47.1
-
-
September October November
Nº
%
Nº
%
Nº
%
8
11.4
-
-
-
-
13
28.9
18
40
14
31.1
Dry cows were detected in estrus from June 25th to September, with the highest incidence (about
83%) in July and August. However, postpartum milking cows required a longer interval to regain
body condition and they were detected in estrus from September 5th to November, with the
highest incidence (about 40%) in October. Cows that calved late in the season usually failed to
display estrus that year. After November, pasture quality declined and most yaks became
anestrus until the following summer. However, normal spermatozoa were present in the testis and
epididymis of yak bulls during the period when the females were anestrus, suggesting that the
breeding season is determined largely by the reproductive seasonality of the female.
Diurnal Distribution of Estrus Lei studied the diurnal distribution of estrus in yaks and reported that estrus was most common in
the cool morning or evening. Furthermore, many yaks were in estrus and bred during overcast
days (in the breeding season) [5].
Table 2. Diurnal Distribution of Estrus in Yaks
Estrus
Time of Day
06:00 09:00
10:00 12:00
13:00 18:00
19:00 22:00
Number of
Yak
75
35
6
14
20
%
100
46.7
8
18.6
26.7
Duration of Estrus The duration of estrus is difficult to quantify because signs of estrus may be weak or vague. In
one report (from the Naqu district of Tibet), the average duration of estrus was 32.2 hours (range,
16 - 56). The duration of estrus was affected by ambient temperature (longer during cool weather
compared to warm weather) and age (average 23.8 and 36.2 hours in young versus mature yaks,
respectively). Liu et al. [6] reported that the duration of estrus in yaks in Shandan, Gansu was
1.6+0.8 days. In July, estrus lasted 1 or 2 days in 76.7% of yaks, while in August and September,
it lasted 1 day in 91.7% and 2 days in 98.7%.
Effects of Environment and Nutrition on Conception and Pregnancy Yaks in Datong Yak Farm that grazed year-round (without supplementary feed) had a conception
rate of 61.5% while those in Shandan (that also grazed but were supplemented during the cold
season) had a conception rate of 81.9% [1]. It appears that conception rate was positively
correlated with body condition and nutritional status of the females and that it was improved by
supplementary feeding during the cold season. Yaks living at an altitude above 4500 m started to
display estrus later in the year and had a lower conception rate than those living at lower
altitudes. Yaks in Qinghai-Tibet Plateau generally had a high conception rate if they were mated
in the cool morning. However, during hot days, heart and respiratory rates were elevated, grass
intake was decreased, and conception rates were low.
Cai et al. [2] reported that the first-service conception rate in yaks was 74.9%, but it decreased to
20.5% in those that failed to conceive and were bred a second time.
The gestation period was 250 to 260 days for female and male fetuses, respectively, with the
fetus located in the left and right uterine horns in 64.7 and 35.3% of pregnancies, respectively [7].
Xue reported that milk progesterone concentrations increased during pregnancy and that a
concentration >1.0 ng/ml between Days 16 and 24 was indicative of pregnancy (accuracy, 86.7%
[8]).
The placenta is relatively heavy (compared to the fetus), facilitating the delivery of oxygen to the
fetus. Furthermore, the relatively short gestation period and low birth weight decreases oxygen
demands on both the dam and the fetus and facilitates a rapid and easy parturition. Although the
low birth weight may be a disadvantage to the calf [3], they do have fetal hemoglobin (HbF2)
during the neonatal period. The optimal time for calving is from April to May when the
temperature is rising and the grass is beginning to grow.
Detailed observations by Ouyang et al. [9] on 20 yak calves (elevation, 3500 to 4100 m) showed
that neonatal survival was related to the maintenance of body temperature. The decline in body
temperature in the first hour after birth (average fall 0.38ºC) was inversely correlated (r = 0.69)
with birth weight (the greater the weight, the less the temperature loss) but much less strongly
correlated with ambient temperature. Therefore, body condition of dam during pregnancy affected
calf survival (through its effect on birth weight). The body temperature of the calves returned to
normal, on average, 3 hours after birth.
More detailed study of yak physiology and nutrition are needed to determine whether specific
nutrients may be limiting factors and whether specific supplements at critical times would be costeffective. There is a paucity of information on whether natural yak diets have deficiencies,
excesses or imbalances of, for example, trace minerals [10,11].
Interspecies Hybridization between Yak, Bos taurus and Bos
indicus and Reproduction of the Hybrids (Last Updated: 14-Dec-2000)
R. C. Zhang
Department of Animal Science and Technology, Gansu Agricultural University, Lanzhou, Gansu,
China.
Summary
Yak are crossed with Bos taurus and Bos indicus cattle, including common local cattle breeds (via
natural service) and via artificial insemination of frozen-thawed semen from improved breeds (e.g.
Holstein Friesian). Female hybrids reach puberty and mate a year earlier than pure yaks.
Nonpregnant F1 females usually have several estrus cycles in the same season and will
generally calve every year. Calf survival is similar to that of the pure yak, however, abortion and
dystocia are more frequent. Male hybrids are sexually active but produce no spermatozoa (the
cause is uncertain) and therefore are sterile. Sperm production does not resume until at least the
third backcross (15/16 yak or cattle), and often not until the fourth backcross. The F1 hybrids
generally grow faster and larger than the pure yak, with production of milk and meat exceeding
that of pure yak and usually exceeding that of local cattle breeds.
Introduction
Hybridization between yak and cattle of other species is recorded in ancient historical records.
The cattle originally used were local breeds (generally referred to as "yellow cattle" (Bos taurus)
in China) and both Bos taurus and Bos indicus (Zebu) cattle elsewhere. Although this practice
continues, more recently improved breeds of cattle have also been used (facilitated by AI of
frozen-thawed semen). However, due to the reproductive isolation of different animals and the
low conception rate of hybrids produced from yak and other cattle breeds, crossbreeding between
yaks and cattle is limited to only the F1 and F2 generations. The F3 generation (containing about
12.5% yak genetics) have difficulty surviving in the high-altitude areas (above 3000 m) in QinghaiTibet Plateau and therefore are seldom produced. Since AI is inevitably restricted to more
accessible areas and maintenance of improved breeds is difficult and expensive, most
crossbreeding programs use locally available cattle.
Although crossbreeding and the infertility of male hybrids have been extensively studied, there
are no clear conclusions. The ability of hybrids to produce sperm increases gradually with
successive generations. Although crossbreeding is generally restricted to the F1 and F2
generations, sperm production does not resume until at least the third backcross (15/16 yak or
cattle), and often not until the fourth backcross.
Systematic crossing of yak with other cattle has been recommended and practiced for many
years; ancient documents indicate that yak have been crossed with common cattle (Bos taurus)
for at least 3000 years. Documents from 11th century China (Zhou dynasty) suggest that crossing
of yak with cattle by the Qing people gave benefits now recognized as heterosis. From the
earliest times, the name "Pian Niu" (and other variants) has been used to describe these hybrids.
These crosses find a special niche with herdsmen, usually at a somewhat lower altitude than
typical yak country. Crossbred females are an important source of milk and dairy products. Since
males cannot be used for breeding, they are used as draught animals or are slaughtered for
meat. These hybrids are very suitable for work as they are easily tamed and have better heat
tolerance than pure yak.
Yak females are usually mated to bulls of local cattle breeds. This is regarded as the normal
hybridization; in China, the F1 is called "true Pian Niu" (or simply "Pian Niu"). The reciprocal cross
(female cattle and a yak bull) is also practiced and regarded as "counter-hybridization" (progeny
are called "false Pian Niu"). These hybrids are produced mainly in the cattle-producing, cold
areas of Gansu and Sichuan and are used mainly for draught purposes (milk production is low).
Semen from improved breeds of cattle, such as Holstein Friesian and Simmental have been used
to breed yaks but are not popular due to the lower survival of the hybrids. Recently, semen from
Hereford, Angus, Simmental, Limousin and Charolais have been used extensively to breed yak
females. In Nepal, Bos indicus (Zebu) cattle are crossed with yaks. It has been suggested that
crossing yaks with Highland cattle (from the United Kingdom) results in hybrids with superior
productivity.
Experiments were conducted in the 1920'
s and 1930'
s (at Buffalo Park, Wainwright, Canada) to
develop a meat animal for their cold northern conditions. In these experiments, a small number of
crosses were successfully made between yak males and female American bison and half-bison
(bison x cattle cross) [1].
Production Performance of Hybrids
The birth weight of hybrids produced by mating yak females and local Chinese (yellow cattle)
bulls can be as much as 50% heavier than pure yak calves [2-5]. Reports from other countries
also suggest a substantial increase in birth weight of hybrids resulting from crosses of yak
females and males of a local breed of cattle (e.g. increases of about 15% and 30 - 40% for
Buryatia [6] and Mongolia [7], respectively), with the extent of the increase dependent upon the
types of yak, breeds of local cattle used, and husbandry practices.
A number of experiments have shown that carcass composition of hybrids derived from mating
yak with both local cattle and different improved breeds did not differ from pure yak; differences in
carcass weight were detected but were attributable to differences in live weight at slaughter.
However, it is apparent from these experiments that there is potentially a very large difference
between Yaks and F1 hybrids (of improved breeds) in growth rate during the first and second
summers of life. Consequently hybrids weighed about 50% more than pure yaks when both were
slaughtered at 17 months of age. Furthermore, hybrids attained an adequate degree of finish (fat
deposition) and had a proportionately greater yield of meat than yaks of the same age.
Milk yield of hybrids of yak with local Bos taurus or Bos indicus cattle or improved Bos taurus
breeds is very dependent on the breeds used, location and conditions. In general, improved Pian
Niu produce the most milk, local Pian Niu are intermediate, and pure yak produce the least.
First crosses of yak females mated to bulls of local cattle breeds (Pian Niu), and the reciprocal
crosses, are widely used for draught, both for plowing and as pack animals. Hybrids produced by
mating yak bulls to female (local) cattle (false Pian Niu) are used mainly for plowing.
Reproduction and fertility
Female hybrids, first-cross and backcross generations, have normal fertility. Males, however, are
sterile until there have been several generations of backcrossing (to either yak or common cattle).
F1 Female Reproduction
Estrus Estrus in the F1 hybrids is seasonal and affected by climate and nutrition (similar to the yak) but
puberty occurs approximately 1 year earlier than in yak. Therefore, F1 females are usually mated
at 25 - 28 months of age (in the third warm season of their life) and they calve for the first time at
about 3 years of age. Mating a year earlier does, however, occur under more favorable conditions
and was noted (many years ago) as a potential advantage in Kazakstan [8].
During the breeding season, female Pian Niu usually display estrus from May to November, with
a peak in July and August (about 1 month earlier than pure yak under the same conditions).
Estrus can occur several times during the breeding season if the crossbred female has not been
mated or is not pregnant; this is different from the majority of pure yak cows in China (although
repeated displays of estrus in yak are more common in other countries, e.g. Mongolia).
Furthermore, signs of estrus in the F1 female are more obvious than in the pure yak.
Cai examined (by rectal palpation) 211 F1 females in estrus and reported that 185 had normally
developed follicles, a proportion slightly higher than in contemporary yak [9]. Nonetheless, in
ordinary mass mating with yak bulls, the conception rate (75%) of Pian Niu cows was somewhat
lower than that of yak at the same location [9]. However, in a specific trial conducted by Cai using
yak bulls, 155 female Pian Niu F1 had a conception rate of 70% at their first estrus of the
breeding season, but a further 25% conceived at a second estrus. With AI of frozen-thawed
Holstein Friesian semen, the overall reproductive rate of the Pian Niu was substantially better
than that of pure yak [9].
In a survey of 136 F1 females, 77 (56.6%) showed estrus, higher than the estrus rate in pure
yaks. Of the 66 F1 females that did not calve in the current year, 63 showed estrus (95.4%);
estrus was detected in 84.3% of pure yaks under the same conditions. In the current year, of the
70 female F1 Pian Niu that calved and were milked, 14 (20%) showed estrus (compared to a
36.6% estrus rate in pure yaks). Perhaps the high milk yield (and twice-daily milking) of F1
females contributed to the lower estrus rate [10]. Generally, multiparous yak cows that produced
F1 Pian Niu calves have a lower conception rate when hybridization mating is used.
Gestation Length Cai [9] reported that the gestation length in crosses of yak with ordinary local cattle as 278.0+9.7
days for 110 F1 cows with male calves and 271.3+11.1 days in 98 cows with female calves.
Denisov [8] reported an average gestation length of 282 days for F1 females mated back to
Schwyz cattle bulls and 265 days for those mated back to yak bulls.
When frozen-thawed semen from a Hartford bull was used to mate F1 females, the gestation
period was 278.6+4.12 days (n=25), and for F2 females, it was 282.6+7.8 days for female calves
and 277.3+9.6 days for male calves, for an overall average of 269.0+10.6 days (gestation is 285
days for Hartford and 255 days for yak) [10]. The same semen was used to inseminate pure yak
females and F1 Pian Niu from 1975 to 1978 and the results are listed in Table 1 [10].
Table 1. Reproductive performance of pure yak females and F1
females inseminated with frozen semen from one Hartford bull
Species Number Offspring Conception
Nº
%
Calving
Survival
Nº
Nº
%
%
F1
Female
211
F2
161
76.3
146 90.7 131 89.7
Pure
Yak
117
F1
46
39.2
31
67.4
30
96.8
Survival to 6 months of age is similar among F1, first Backcrosses (B1) and pure yak. Thus,
relative to the yak, the F1 cross has a better overall lifetime reproductive rate, due to repeated
displays of estrus (and opportunities to mate in females that fail to conceive) and a higher
probability of calving every year.
Abortion and Dystocia Due to genetic differences between yaks and other cattle breeds, there are placental
abnormalities that may result in inadequate support of fetal development. Abortion occurs more
frequently when yaks carry a hybrid fetus. In a survey of 5623 matings (to produce hybrids) in
Hongyuan County, Sichuan, from 1976 to 1982, the abortion rate was 20.7% [10]. Although the
causes of abortion have not been established, it is speculated that genetic differences, placental
underdevelopment and environmental conditions such as cold weather and low oxygen content of
the air may be involved.
Dystocia is more common with hybrid fetuses than yak fetuses. A survey from Hongyuan County
in 1979 showed that of 1144 calvings, 32.8% required assistance (5% were delivered by
laparotomy). Therefore, it is recommended that large females are used to produce hybrid calves.
Furthermore, the incidence of hydramnios was 8.2%.
The external genitalia, libido, and mating behavior of the crossbred male are normal but there are
no sperm in the seminal fluid. Therefore, the crossbred is a natural teaser bull. First and second
backcross generations are also sterile. By the third generation of backcross (15/16 yak or cattle
blood) some spermatocytes are usually present and the occasional male is fertile. Fertility is not
assured until the fourth or fifth generation of backcross [11]. In practice, however, intact males of
the backcross generations are rarely seen, since there is no good reason to produce them.
Therefore, precise data on the proportions of bulls showing normal spermatogenesis in
successive generations of backcrossing are not available.
The causes of sterility in hybrid males have been, and are still, the subject of much investigation
and speculation. It is well established that there are no spermatogonia in the seminiferous tubules
of F1 and early generations of backcrosses. Possible causes for this have been considered in the
structure of the X and Y chromosomes; these structures differ in certain respects among crosses,
pure yak and pure cattle [12]. In particular, the arm ratios differ, most notably for the Ychromosomes. In a recent review [13], it was noted that the length of the Y chromosome and the
relative lengths of the chromosomes vary among cattle breeds. Therefore, fertility of male yak
hybrids might be restored by selecting a Bos taurus breed with Y chromosomes that are of similar
length as those of the yak. A less likely explanation, advanced by Zhang [11], is that male sterility
may be due to an imbalance at many chromosome loci (including autosomal).
There are differences among hybrids, yak and ordinary cattle in the proportions of different cell
types in the anterior pituitary gland, resulting in reduced production of FSH in hybrids. However,
frequent injection of FSH and LH intra-venously in well-fed crossbred calves increased libido but
did not result in sperm production at 12 months of age [9].
Crossbreeding Policy First crosses between yak and common cattle adapt well to the conditions in which they are used,
displaying good characteristics of both parental types, including resistance to a harsh
environment and improved productivity. Backcrosses compared to cattle, however, are less well
adapted to the environment and their productivity is often lesser than in yaks, probably due to
reduced heterosis. Therefore, it is common practice to dispose of these calves immediately after
birth and subsequently milk their mothers.
Due to the infertility of F1 male hybrids, they can not be used for reciprocal crosses or to develop
new cattle breeds. Therefore, hybridization between yak and other cattle breeds are only used for
economic crosses. In order to explore heterosis of interspecies hybridization and the hybrid could
survive in the cold Qinghai-Tibet Plateau, static cross or terminal cross system is recommended
and it can be expressed as:
Terminal crosses are characterized by using small-sized common cattle breeds (body size of F1
is large and it can be used to cross with large cattle breeds so that dystocia is reduced). Ternary
crosses can be established by using two common cattle breeds and yak so that maximal
heterosis could be achieved. The F2 have 25% yak blood; if they are crossed with common
cattle, the resulting F3 generation could not survive in the cold alpine grassland (with more than 8
months of winter).
Limits to Hybridization
Poor reproductive performance in yak severely limits the number of female yaks that can be used
for crossbreeding (if the numbers of pure yaks are to be maintained or perhaps increased). In
practice, it has been found best to produce the F1 generation and then slaughter the F2
generation for meat. Since the males are sterile, only the F1 females can be backcrossed to yak
or cattle bulls. However, reduced productivity (relative to the F1) makes the backcross
generations commercially unattractive.
Reproductive Biotechnologies: Current Status in Yak
Reproduction
X. X. Zhao
Department of Veterinary Medicine, Gansu Agricultural Univiersity, Lanzhou, Gansu, China.
Summary
Novel reproductive characteristics of yak and expansion of commercial yak production have
generated considerable interest in the application of reproductive biotechnologies in yak, both for
research and increased commercial production. The objectives of this presentation are to review
the current status of estrus synchronization, superovulation, and in vitro embryo production.
Introduction
For several years, there has been considerable interest in the application of reproductive
technologies in yak, both for research and improved productivity. As commercial yak production
has expanded, reproductive technologies that are used in domestic cattle (e.g. pregnancy
diagnosis, artificial insemination, estrus synchronization, embryo transfer, and the collection and
preservation of semen) have been attempted in yak. Although similarities between Bos taurus
cattle and yak have facilitated the direct application of these technologies to the latter, critical
differences in biology and husbandry have limited application and success.
Reproductive studies in yak have shown that anatomically and physiologically they are generally
similar to cattle. One notable difference is that yak are apparently seasonally polyestrus.
Depending on the report, yak cycle from late summer to late fall or from July to September; the
short breeding season is dictated by intense breeding activity during the early part of the breeding
season and subsequently the absence of conception due to declines in both temperature and
feed supplies. In addition, yaks have a relatively small uterus and ovaries (comparable in size to
young beef heifers). Consequently, ovarian follicles and corpora lutea are smaller and more
difficult to detect by rectal palpation than they are in cattle [1,2]. Furthermore, delayed maturity,
silent estrus, low conception rates, and prolonged postpartum anestrus in yaks seriously limit
both reproductive performance and the application of reproductive technologies that are widely
used in cattle.
Estrus Synchronization
Estrus synchronization in the yak has been effectively accomplished with products commonly
used in cattle, including PGF2α and progesterone. Synchronization of estrus is most effective
during the breeding season but estrus can be induced outside the breeding season. Regardless
of the method used, considerable effort must be made to minimize stress (e.g. due to handling) or
fertility will be adversely affected.
When PGF2α is used, the animal must be cycling and have a responsive corpus luteum.
Following a single injection of PGF2α (during the ovulatory season), approximately 50 % of yaks
will be detected in estrus (similar to cattle), but the interval from treatment to estrus is usually
slightly longer than in cattle. A second injection of PGF2α 11 days after the first will effectively
synchronize the group. In one study [3], 135 yaks were injected twice-daily with two PGF2α
preparations. Most yaks (55 - 70 %) were detected in estrus 3 days after treatment. However,
there was a seasonal effect on the proportion in estrus, with 50.0 % and 46.2 % detected in
estrus following administration of Oestrophan and Enzaprost, respectively, in July and a better
response rate (61.5 % and 58.3 %) in August. Following two treatments at 10-day intervals, the
estrus synchronization rate 86.2 % and 90.0 % and the conception rate following insemination
with frozen-thawed Holstein semen was 77.9 and 78.9 % (compared to 85.7 % in a control
group). Overall, these results were similar with those reported in cattle [3,4].
A steroid preparation ("Three-in-One" containing testosterone propionate, progesterone, and
estradiol benzoate in concentrations of 25, 12.5 and 1.5 mg/ml, respectively) has been used to
synchronize estrus in yaks. In one study [6], 66/93 (71.0 %) of yaks treated with this preparation
were detected in estrus within 5 days, compared to 13/90 (14.4 %) in control group, with
conception rates of 22/71 (31.0 %) and 21/78 (27.0 %), respectively. In a subsequent study [7],
the same preparation was given at the following doses: 1.25 ml and 0.75 ml/100 kg body weight.
In this experiment, 18, 12 and 6 yak cows, respectively, were detected in estrus 96 - 120 hours
after treatment and a total of 36, 24 and 12 (48/150 = 48.0 %) were detected in estrus 24 - 120
hours after treatment. In another study [5], 11/17 (64.7 %) of females were detected in estrus
following treatment with this preparation, compared to 15/26 (57.7 %) that were detected in estrus
within 2 - 4 days after treatment with PGF2α.
Superovulation
Superovulation has been attempted in yak females using a variety of treatments and under
varying conditions. Satisfactory responses have not been consistently achieved, thereby limiting
the use of embryo transfer in this species. In a small trial [8], 3 barren females (4 - 8 years old)
were given a CIDR (containing 1.2 g progesterone) for 12 days, injected with Lutalyse (PGF2α)
on the third day after the first injection of FSH and inseminated 24 - 36 hours after the last
injection of FSH. The average number of luteal glands and follicles were 5.0 + 0.6 and 1.3 + 0.9,
respectively, but embryo recovery was not reported.
Davaa et al. [9] used FSH and PMSG to induce superovulation in yak cows. Estrus was detected
34.1 + 0.52 hours after the prostaglandin treatment. The average number of ovarian follicles was
5.4 + 0.65 and 4.5 + 0.43 of them ovulated (2.6 + 0.30 and 1.9 + 0.20 ovulated from the right and
left ovaries, respectively).
In Vitro Embryo Production (IVP)
Difficulties and inconsistencies associated with in vivo embryo production in yaks may be
overcome with in vitro maturation and fertilization techniques. Potential advantages include:
1. circumvention of the need to synchronize ovulation for AI;
2. the potential for producing more embryos that can usually be collected from hormonally
stimulated donors;
3. the ability to make use of animals with certain types of infertility (e.g. endometritis or
oviductal occlusions);
4. a reduction in the numbers of viable sperm needed;
5. using sperm microinjection techniques, the potential for using nonmotile, nonviable
sperm, or epididymal-derived sperm for assisted fertilization;
6. the potential for salvaging genetic material from female animals after death; and
7. the possible utilization of pre-pubertal or pregnant animals as oocyte donors.
Despite success in other species, there are no reports of yak calves being produced from
following in vitro maturation and fertilization of oocytes collected from nonstimulated females.
However, it is noteworthy that these embryos are capable of developing to the blastula phase in
vitro.
Using standard procedures, Chen et al. [11] studied the efficiency of IVP methods in yak and
local breeds [10]. Ovaries (8 and 50 from yak and local cattle, respectively) were collected after
slaughter, washed with saline or PBS, maintained at approximately 30 0C, and transported to the
laboratory within 3 hours. Ovaries were rinsed with 75 % alcohol, washed with saline and the
follicles were aspirated with a syringe. The aspiration media was TCM-199 with 25 mM HEPES
and 2 % calf serum. It was noteworthy that an average of 7.0 and 2.7 oocytes were obtained per
ovary from yak and local breed cows, respectively.
Only oocytes with complete granular cells were selected for maturation culture; they were washed
three times with maturation media and cultured for 24 hours. The addition of serum (15 %), FSH,
LH, and estradiol to the maturation media enhanced the maturation rate.
Sperm quality and the induction of sperm capacitation in vitro also limit IVP in yak. Only recently
have morulae and blastocysts been produced following in vitro maturation (IVM) and in vitro
fertilization (IVF) of yak oocytes. In certain circumstances, epididymal sperm for IVF may obviate
the need for supplemental treatments (e.g. calcium ionophore) to induce capacitation in vitro.
Cryoprotectants appropriate for freezing yak semen may not be effective for epididymal sperm.
An alternative to cryopreserving epididymal sperm may be the direct refrigeration of testes, which
for certain animals, can maintain sperm viability for up to 5 days [10].
Chen et al. [11] used epididymal sperm from yaks to study sperm capacitation conditions. Frozen
(in a pellet) semen from local breed was included as a comparison. Yak testes were collected
soon after slaughter; the cauda epididymis was excised, minced, and put into dishes containing
PBS. Sperm samples with acceptable motility and density were sealed in a tube and kept in a
fridge (4 0C). Samples were subsequently removed and mixed with 1 ml of BO media containing
10 mM caffeine and 3.6 IU heparin. Motility was assessed and the semen was subsequently
washed twice and centrifuged at 350 G for 5 minutes; the sediment was mixed with the BO
medium with 0.6 % BSA to adjust sperm concentration and 100 ul droplet was made, covered
with mineral oil, and put into an incubator containing carbon dioxide.
Following culture in maturation media for 24 hours, oocytes were aspirated, put into BO media
(with 0.3 % BSA) and washed three times. Thereafter, 10 - 15 oocytes were put in one semen
droplet, co-cultured for 6 hours, and washed three times (with culture media) to remove extra
sperm. Following this washing, 10 - 15 ova were put in each 100 ul culture droplet, covered with
mineral oil, and cultured for a further 48 hours. At that time, cleaved ova were counted (and
separated from non-cleaved ova), with 2 - 8 cell embryos cultured up to day 8 (and the number
that formed blastocysts was determined). Cleavage and blastocyst rates for yak IVF were 35/56
(62.5 %) and 9/56 (16.1 %), respectively, indicating that these procedures, derived from those
used in cattle, produced acceptable results that were at least as good as those obtained with IVP
using oocytes and semen from local cattle.
Luo et al. [12] obtained ovaries from 4 yaks after slaughter. It is noteworthy that one of these yaks
was 40-days pregnant. Ovaries were washed with 0.9 % saline, kept in a vacuum bottle (at 25 30 0C) and taken to the laboratory within 5 hours after collection. Oocytes were aspirated,
washed 3 times with TCM-199 containing 5 % calf serum, and cultured at 38.5 0C, 5 % CO2 and
100 % humidity for 20 hours. Frozen-thawed semen from wild yaks was diluted to suspension
with BO media containing 5 mM caffeine. The solution was centrifuged twice for 5 mins at 1800 G
and the semen concentrations adjusted to 20 million per ml. Capacitation was induced by
incubation in Tyrod’s media (with 20 ug/ml heparin) for 15 minutes. Following 20 hours of culture,
oocytes were washed with BO media (containing 0.5 % BSA and 10 ug heparin). Semen droplets
(50 ul) were made in Petri dishes, covered with sterilized paraffin oil, and equilibrated in a carbon
dioxide incubator; mature oocytes were moved into droplet after 1 - 2 hours. For insemination,
capacitated semen suspensions were flushed into semen droplets (final concentration, 4 million
cells/ml) and the Petri dishes were immediately put into the incubator. After 5 hours of incubation,
fertilized eggs were washed with TCM-199, incubated for 48 hours with TCM-199 (38.5 0C, 5 %
carbon dioxide), the incubation medium was changed and they were incubated (same conditions)
for an additional 72 hours. In this study, 25 oocytes were aspirated from 8 ovaries and after
incubation for 20 hours, 20 oocytes developed to meiosis II. Fertilization and cleavage rates were
12/20 (60 %) and 8/20 (40 %), respectively. Although 2 normal morale were transferred into the
uterus of a Holstein-Friesian cow, she was subsequently diagnosed nonpregnant. It has been
reported that Y-bearing bovine spermatozoa may be distinguished by fluorescent in situ
hybridization (FISH), using a specific DNA probe; this technique provides an in vitro verification
for experiments producing sex-oriented or sex-specific semen.
It was reported that a bovine Y-specific probe produced a visible signal in yak spermatozoa;
specific binding of the probe to the corresponding sex chromosome was subsequently confirmed
on a chromosome preparation [13].
Limitations and Associated Problems
Under ideal conditions, the utilization of reproductive technologies in yak can produce results
similar to those in cattle. However, yak behavior is very different from that of cattle. In that regard,
yaks are often difficult (indeed dangerous) to handle, with considerable potential for harm to the
handlers as well as the animals.
Research progress has been made is several technologies but success in others has been
limited. Although the study of reproductive events in yak has generally lagged far behind that of
domestic livestock, recent interest and investigations have in part corrected this deficit. Although
traditional approaches to embryo transfer technologies have resulted in limited success, the
demonstration that IVP (utilizing techniques developed for cattle), can be very successful yaks
provides great optimism. Notwithstanding, there is still much to be learned before these
technologies are commonplace in yak. In that regard, short-term preservation of yak ovaries
and/or oocytes and cryopreservation of oocytes and embryos would be of considerable benefit.
Remarkably, many techniques developed and proven in cattle have been adapted to yak with
only a few, minor modifications. The success of IVP techniques in yak suggests that there are
many opportunities for similar approaches in other animals. In that regard, IVM and IVF utilizing
epididymal spermatozoa and IVC with oviduct cell co-culture are likely to be of use in many
different species in the near future. It is expected that this will promote assisted reproduction in
exotic and endangered species and facilitate opportunities for the international movement of
gametes.
Semen Characteristics and Artificial Insemination in Yak
Z. W. Zhang
Department of Animal Science and Technology, Gansu Agricultural University, Lanzhou, Gansu,
China.
Summary
The major achievements in scientific research (since 1980) concerning artificial insemination (AI)
in yaks are summarised. Semen characteristics and morphological characteristics of yak
spermatozoa are described. Semen collection in yaks, including training bulls for restraint and
ejaculation into an artificial vagina, are included. Estrus detection, when, where and how to
inseminate are all described. Protocols for freezing and thawing semen are also included.
Semen Characteristics
Ejaculates of adult yak bulls are milky-white in colour, pH 6.4 - 6.7 and a volume of 2 - 5 ml.
Spermatozoa are present at a concentration of 7.5 - 16.0 × 108/ml, with 70 - 85% progressively
motile, 5.1 - 10.0% morphologically abnormal, and 71.8% with intact acrosomes. The general
survival time of spermatozoa and the survival index are 54 hours and 18.72 hours, respectively,
at 15 degrees Celsius. The resistance index of sperm is 5899 and the viscosity of semen is 1.17
centipoise, lower than semen from Bos taurus bulls (1.92 centipoise). The total nitrogen content
of yak semen is 1334 mg/100ml, higher than for Bos taurus semen (877 mg/100ml) [1-3].
The morphology of yak sperm is similar to that of other cattle [4,5]. The reported dimensions are:
sperm head 7.74 - 8.1 m long and 4.1 m thick at its widest point ; midpiece 14.47 m long; and
principal piece 49.67 m.
Artificial Insemination
Semen Collection Methods of semen collection with an artificial vagina have been established in yaks [1,4,7-9] (Fig.
1). Yak bulls can readily be trained to provide semen for AI. However, due to a lack of information
regarding the genetic merit of young yak bulls, most of the semen is collected from older, proven
bulls that are considered superior.
Figure 1. Semen collection from a yak bull by artificial vagina.
The training of adult bulls is an important aspect of AI practice. Under natural conditions, bulls
remain alone or in groups of 3 - 5 in the high mountains for extended intervals (in the nonbreeding season) and they join the female herds only during the breeding season. The first step
in training is trying to eliminate the bull’s hostility. Initially, the bull is confined in a pen until it is
accustomed to being restrained, fed in a fixed placed and having the herdsman approach. To
obtain acceptance by the bull, the herdsman will initially tempt the bull in the pen with grass. Later
he strokes and scratches the bull from the front to the rear of the body, and from the back to the
abdomen. Since Yaks have a great aversion to being touched on the head, the herdsman avoids
this area. When the bull is more accustomed to being handled, the herdsman will start to stroke
the scrotum and testes of the bull, to pull on its sheath, and to lead a haltered bull to its feed [9].
Training for semen collection follows by introducing the bull to a female in estrus (whilst she is
restrained in a crate) and allowing him to mate. Thereafter, the sheath is held to guide the penis
into an artificial vagina (internal temperature, 39 - 42 degrees Celsius). Eventually, a dummy cow
can be used for the bulls to mount. Yak bulls should be handled in a gentle manner and their
surroundings should be quiet and familiar [9].
Semen can be collected twice weekly. The volume and the quality of the ejaculate are affected by
bull age and season [16]. The average volume obtained by artificial vagina is 4.8 ml, with
volumes ranging from 2.6, 4.0, 4.4, 5.5, 7.6, and 6.0 ml collected from bulls that were 2 - 8 years
old [2]. The highest ejaculate volume (6.3 ml) was obtained in October [2].
Semen Evaluation Semen evaluation should be rapid but careful, so that samples can be processed to preserve
quality and fertility. Inferior samples should be discarded. Although no single test is an accurate
predictor of the fertility of individual ejaculates, when several tests are combined, ejaculates with
a higher fertility potential can be selected. The major criteria considered include ejaculate volume,
sperm motility, sperm concentration, proportion of abnormal sperm, proportion of sperm with an
intact acrosome, general survival time, and survival index of sperm.
Insemination
Estrus Detection Ovulation occurs approximately 12 hours after the end of estrus. Estrus behavior is more
commonly displayed at cooler temperatures (i.e. in the morning and evening), especially if the
weather includes rain and wind. Signs of estrus are subtle and therefore close observation,
especially in the morning and evening, is important. If the females stand to be mounted by teaser
bulls, they should be inseminated [10-13].
Insemination The rectovaginal method is used for insemination (Fig. 2). In the yak female, the cervix is near the
vaginal orifice (22.5+0.6cm), and the uterus has limited mobility. Therefore, the rectovaginal
technique is even simpler and easier than in other cattle [10].
Figure 2. Insemination of female yaks
Dosage - regardless of the type of semen that is used (undiluted or diluted, pellet or straw), it is
generally recommended that females are inseminated twice, 12 hours apart, with 10 million
progressively motile spermatozoa used (for each insemination).
Optimum time for insemination - ovulation occurs about 12 hours after the end of estrus in yaks
and the optimum time to inseminate is approximately 10 hours before ovulation. The first
insemination is generally performed 24 hours after the beginning of estrus and repeated 12 hours
later. Xie [14] reported a conception rate of 82.3% (296/372) following two inseminations, the first
at the end of estrus and the second 12 hours later. However, in that report, when the females
were inseminated only once (8 hours after the end of estrus), the conception rate was 81.8%
(203/248), indicating that a single, appropriately timed insemination can result in fertility
comparable to two inseminations.
Insemination site - Xie [15], using the rectovaginal method of insemination, semen was deposited
at different sites and fertility was compared. In that study, conception rates were 44.0%
(113/302), 52.0% (258/496), 69.6% (162/230), and 50.8% (93/118) when semen was deposited
intracervical (1 - 3 cm beyond the external os), intracervical (3 - 5cm beyond the external os), in
the uterine body or in the uterine horn, respectively. Therefore, deposition of semen into the
uterine body resulted in the best fertility. It is noteworthy that the cervix is approximately 5 cm
long, has rings (similar to other cattle) and the uterine body is rather short (approximately 2 cm).
Frozen Semen
Advances in sperm cryopreservation have played an integral role in cryobiology. Mechanisms of
cryoprotection or cryodamage at the molecular and macromolecular levels have been studied
with spermatozoa, with ongoing studies to make further improvements. Du [7], Li [8], Zhang [1],
and Guo [21] reported methods for freezing semen (in pellets) collected from domestic and wild
yaks, respectively.
Extenders
Extenders should be prepared aseptically and used within a week (if fresh) or frozen. Both egg
yolk and yak milk are used to protect against cold shock and glycerol is added as a
cryoprotectant. Penicillin and streptomycin (or other combinations of antibiotics) are added to
inhibit bacterial growth. Extenders used are listed in Table 1.
Table 1. Composition of Extender for Yak Frozen Semen
Authors
Compositions (1)
Basic
Solution
(ml)
Egg
Yolk
(ml)
Glycerol
(ml)
PosThawing
Motility
Conception
Rate (%)
Remarks
Du [3,7]
12% sucrose
75
20
5
0.55+0.14
0.36+0.04
13/20 (65)
39/52 (5)
One-step
dilution
Li [8]
12% sucrose
75
20
5
0.43
49/61 (80.3)
Two-step
dilution
Guo [17]
12% sucrose
75
20
5
0.4+0.11
Zhang
[1]
12% sucrose
75
20
5
0.4 - 0.5
Du [7]
Skimmed
milk 80
20
3
0.55+0.07
Zhang
[1]
3.97%
sodium
citrate
dihydrate
12% lactose
75
20
20
7
5
0.4 - 0.5
0.4 - 0.5
Guo [19]
7.5%
glucose 75
11% lactose
75
20
20
5
5
0.38+0.09
0.4+0.1
(1) All the extenders contain penicillin 1000 g/ml.
One-step
dilution
84/114 (73.7)
One-step
dilution
Diluting, Cooling and Balancing
Fresh semen can be diluted by either of the following two methods. Regardless of the method
used, make sure that semen and extender are properly mixed.
One-step dilution - the semen and the extender should be at the same temperature (32 - 35
degrees Celsius). The ratio of semen to extender dependends on sperm concentration and
motility but generally ranges from 1:2 - 1:8 (insure a minimum of 10 million progressively motile
spermatozoa per pellet). Put the diluted semen in a container, wrap the container in 8 - 16 layers
of gauze, and place into a refrigerator (temperature, 4 - 5 degrees Celsius) for 3 - 4 hours [1,3,7].
Two-step dilution - fresh semen is initially diluted with Extender I (does not contain glycerol) at a
temperature of 32 - 35 degrees Celsius and left at room temperature (approximately 15 - 20
degrees Celsius) for 1 - 2 hours. Then, Extender II (containing glycerol) at room temperature (or
at 0 - 4 degrees Celsius) is added and the mixture put into a cold environment (0 - 4 degrees
Celsius) for 3 - 4 hours [1].
Freezing
Chilled semen (prior to freezing) should have a minimum of 60% progressively motile
spermatozoa. During the preparation of pellets, keep the extended semen cool (place the flask on
ice blocks if necessary). For initial freezing, a fluon plate, copper gauze or aluminium plate is
placed 1 - 2 cm above liquid nitrogen, the surface free of frost, and the temperature between -80
and -130 degrees Celsius. Each 1 ml of extended semen should yield 10+1 pellets. Complete
100 pellets within 3 min; plunge the pellets in the liquid nitrogen when the last pellet turns from
yellow to white. On completion, calculate the number of prepared pellets, check, pack and
register.
Thawing of Frozen Semen
The following three regimens are frequently used. The first is 2.9% sodium citrate solution, the
second is glucose-sodium citrate solution (5 g of glucose and 0.5 g of sodium citrate dissolved in
100 ml of twice-distilled water) and the third is fresh de-fatted milk solution. Fresh milk is boiled
and cooled, the cream is skimmed off, and the milk filtered through four layers of gauze [17,18].
The inseminator prepares the aliquots by adding thawing solution into sterilised test tubes (an
insemination dosage is 1 pellet of semen and 1.5 - 2.0 ml of thawing solution). Then, the tubes
are put into a beaker (or a cup) filled with boiling water. The temperatures of the thawing solution
should be monitored with a thermometer; when it reaches 38 - 42 degrees Celsius, a semen
pellet (held with a bamboo clip or metal forceps) is placed into the tube, the tube is gently
agitated, and 3 - 5 seconds later the tube is placed into a water-bath containing water at 30
degrees Celsius. Sperm motility should be rapidly examined and if it exceeds 30%, the sperm
should be inseminated as soon as possible.
Packaging, Labelling and Storage
One hundred frozen sperm pellets are packed in a sterilised bottle made of polythene, the bottle
must be labelled (bull identification, freezing date, quantity, lot number, producer, etc), and stored
in liquid nitrogen [20]. The application of AI, especially with frozen semen, can dramatically
hasten genetic progress and have substantial economic benefits. For example, experiments
conducted on Waqie farm (Sichuan Province) showed that high milk production is achieved in F1
hybrids (Yak females and Holstein semen). The meat yield of crossbreeds also increased
substantially. Birth weights of F1 males and females were 73.7 and 76.3%, respectively, greater
than that of purebred yaks, and the body weight at 18 months of age (229 Kg) surpassed the
average weight of mature yaks (5 years, 222 Kg). Zhang [9] reported that until September 1997,
40 stud yak bulls were producing 400,000 semen pellets frozen which amounted to about 2 000
000 RMB Yuan annually. Frozen semen could be used to inseminate 200,000 cows with a
conception rate of 80% (35% higher than that achieved with natural service), resulting in an
additional 70,000 calves and 1.1 x 106 Kg butter produced compared to traditional mating and
production systems.
These improvements would result in a total increase of 27 million RMB yuans annually.
Although AI has not been widely adopted on a widespread basis in the vast area in which Yaks
are raised, there is considerable optimism that AI, and in particular the use of frozen semen, will
be more widely used in the future.
Male Reproductive Physiology
P. Yan
Lanzhou Institute of Animal Science and Veterinary Medicine, Chinese Academy of Agriculture,
Xiaoxihu, Lanzhou, China.
Summary
A comprehensive description of reproductive physiology of yak bulls has been compiled.
Spermatogenesis in the yak is similar to that in other mammals. Distinctive characteristics of
spermatogenesis in the yak and fine structures of yak spermatozoa are described.
Introduction
Yaks, a unique breed of cattle, are adapted to the alpine grassland on the Qinghai-Tibet Plateau
at an altitude exceeding 3000 m. Yaks are raised under extensive conditions, continuously
grazing pastures of limited nutritional value. The majority of the breeding is natural service with
artificial insemination limited to a few specific regions.
Spermatogenesis in the Yak Bull
Spermatogenesis in the yak, a lengthy but precisely controlled chronological process in which
spermatozoa are produced in the seminiferous tubules, is similar to that in other species.
Spermatogenesis is divided into three major divisions: spermatocytogenesis, meiosis and
spermiogenesis [1]. Stem cell spermatogonia divide by mitosis to maintain their population and to
produce primary spermatocytes; the latter cells undergo meiosis to produce haploid spermatids
that differentiate into spermatozoa.
Spermatogenetic Cycle and the Seminiferous Epithelium Cycle
The spermatogenetic cycle includes all the events that occur between two appearances of the
same developmental stage (steps) [2]. These stages are defined by the morphologic appearance
of the germ cells in PAS-stained sections. Xia et al., [14] reported that the spermatogenetic cycle
and the seminiferous epithelium phase of yak bulls are very similar to that in cattle, as reported by
Berndtson et al., [4]. It appears that the VII phase and the XI phase are very long, yet the IX, X,
and XII phase are rather shorter [3,4].
Qualitative and Quantitative Aspects of Spermatogenesis
Oravant et al., [5] and Ekstedt et al., [6] suggested that spermatogonia in bulls undergo five or six
mitotic divisions, during which their structure and nuclear size gradually change. Hochereau de
Reviers [7] found six mitotic divisions and thought that there are two generations of B
spermatogonia. Similarly, there are two types of B spermatogonia in yaks; A spermatogonia
divide to form B1 spermatogonia that subsequently divide to form B2 spermatogonia. During
mitotic division of the A spermatogonia, some become quiescent and others continue to divide. In
theory, after six mitotic divisions, there would be 26 (= 64) spermatids. However, due to
degeneration, cell numbers do not increase by the theoretical value; 26.7% of A spermatogonia
degenerate when they differentiate into the intermediate type. One A spermatogonium produces
26.95 round spermatids (42.1% of the theoretical yield) in the Yak [3] and 27.23 round spermatids
in the bull.
Spermatogonia undergo mitosis to form spermatocytes. The numbers of primary spermatocytes
remain relatively stable at the stage of proleptonema, leptonema, zygotene, and the long
pachytene period. There are an average of 30.8 secondary spermatocytes in each seminiferous
tubule cross section in yaks, close to double the number of primary spermatocytes
(15.9 × 2 = 31.8), indicating that few cells at this stage are lost by degeneration.
However, these form 55.4 spermatids, indicating that losses during meiosis are approximately
10% of the theoretical value (30.8 × 2 = 61.6). However, the number of Sertoli cells remains
relatively constant throughout the seminiferous epithelium cycle [8,9].
Xia [8] concluded that the numbers of both Sertoli cells and all kinds of germ cells in yak testes
are approximately 80% of those in cattle; he attributed this to breed differences and the effects of
nutrition. The rate of seminiferous cell production in yaks is similar to that in cattle during
spermatogenesis; however, the proliferation rate in yak is about 10% lower than in cattle.
The Quantitative Histology of Testes
Yak testes are anatomically and histologically similar to those of other bovine bulls except that
they are smaller (approximately 300 versus 550 - 650 g, respectively). The yak scrotum is small
with abundant hair, apparently an adaptation to the cold environment [10].
The weight and volume of the testes are closely related to their ability to produce sperm. Intrinsic
factors affecting the weight and volume of testes include diameter of the seminiferous tubule,
height of the seminiferous epithelium, the amount of interstitial tissue, and the level of
spermatogenesis. Environmental factors, age, and hormones also affect testicular weight and
volume. The height of the seminiferous epithelium and volume density of seminiferous tubules
and seminiferous epithelium increase with age, but the volume density of the lumen of the
seminiferous tubule and interstitial tissue decrease gradually with age. At 24 months of age, the
volume density of seminiferous tubule of yak is 0.786 µm3/µm3, the volume density of
seminiferous epithelium is 0.677 µm3/µm3, the height of seminiferous epithelium is 85.66 µm, and
the volume percentage of seminiferous tubules is 78.6%, similar to that in mature males of
common cattle breeds (79.4%) [11].
Daily sperm production per gram of testicular parenchyma, a measure of the efficiency of
spermatogenesis, is useful for comparisons. Yaks have an efficiency of 12.94 × 106/g, similar to
other cattle (e.g., 13.00 × 106/g and 12 × 106/g in Charolais and Holstein bulls, respectively). Total
daily sperm production in yaks is 4.66 × 109, less than that of Bos taurus bulls (e.g., 8.9 × 109 and
7.5 × 109 in Charolais and Holstein bulls, respectively) but greater than Nile Zebu bulls (2.6 × 109).
The difference is due to the different index of testes. Furthermore, compared to the Yak, testicular
weight is 2.15, 2.01 and 0.65 times higher in Charolais, Holstein and Nile Zebu bulls, respectively.
The height of the seminiferous epithelium is 61 and 102 µm in Zebu and Bos taurus bulls,
respectively [11-13].
Sperm Morphology
The yak sperm consists of a head, neck and tail that are very similar to those of other cattle
breeds. The tail is composed of a midpiece, principal piece and endpiece.
In yaks, the head of a sperm has a reverse oval shape; the front two-thirds are enclosed by the
acrosome, the final one-third lies in the post nuclear cap, and the nucleus is located in the middle.
The nucleus appears as a reverse flattened oval, enclosed in the nuclear membrane. The
acrosome is a sheath that covers the front two-thirds of the sperm'
s head. It is composed of outer
and inner acrosomal membranes and the acrosomal inclusion (moderately dense and uniform),
located within the inner acrosomal membrane. The outer acrosomal membrane protrudes to form
an acrosome ridge along the upper aspect of the sperm head. The inner acrosomal membrane
coils and projects a sharp tip (containing moderately dense material), similar to the perforatorium
of avian and rodent sperm. The inner and outer acrosomal membranes come together to form the
equatorial segment (ES); this is usually visible by light microscopy in Giemsa-stained sperm and
is even more distinct in abnormal sperm that lack an acrosome. The postacrosomal sheath
covers the region of the sperm head below the equator [14].
The plasmalemma (protoplasmic membrane) completely covers the sperm head and tail. The
plasmalemma covering the anterior of the sperm head (in apposition with the outer acrosomal
membrane) is relatively labile while that below the equator is more stable.
The middle of the lower part of the sperm'
s head is depressed to form an implantation fossa that
accommodates the implantation plate of the sperm tail. The neck consists of nine outer-segment,
cylinder-shaped implantation plates. These nine plates are combined closely into the joint lump;
the front of the joint lump sticks out and forms a hemi-spherical structure that inserts into the
implantation fossa. In the center of the implantation fossa is the near centriole. Although the form
and size of the section of the nine implantation plates vary, they become more uniform distally.
The middle piece of the tail is composed of an axial filament that is covered by a mitochondrial
sheath. The axial filament consists of 9 thick fibrils, 9 microtubule doublets and 2 center
microtubules. The mitochondrial sheath is comprised of a mitochondrial helix covered by a
protoplasmic membrane.
The principal piece (sperm tail) is made up by the mid-axial filament and fibrils sheath. The midaxial filament continues from the mid piece but only the third and the eighth strand fibril elongate
distally; many cross ribs are joined from two sides to form the fibrils sheath (in lieu of the
mitochondrial sheath of the mid piece). The fibrils sheath becomes slim gradually, and the
microtubule doublet varies in the same way.
The telopiece of sperm has lost the fibrils sheath and the thick fiber; the only structure remaining
is the mid axial-filament covered by a protoplasmic membrane.
Compared to sperm from Bos taurus bulls, the dimensions for yak sperm are 9.50 vs. 8.32 µm,
the length of the midpiece is 14.20 vs.14.40 µm (P<0.05), and the length of the principal piece is
51.83 vs. 47.5 µm (P<0.01) [14].
Sexual Activity
Yak bulls start mounting around 6 months of age; over the next year, this behavior continues and
intensifies, including seeking and mounting yak females. Many studies in Yaks suggest that
spermatogonia begin to differentiate at 12 months of age and that sperm are present in the
epididymis by 18 months of age. However, the LDH-x band, a marker characteristic of sexual
maturity in mammals, is not present until 24 months of age.
Bulls spend winter and early spring alone, joining the herd only during the breeding season. Bulls
can detect the scent of an estrus female from a distance of several kilometers. There is
considerable fighting among bulls, with the strongest, most dominant bulls getting the majority of
the opportunities to mate. Young bulls do not usually win a place in the competition for mates until
they are approximately 4 years old (after some fighting experience). Old feeble bulls do not have
mating opportunities and leave the herd. The competition for mates, to the extent that it
introduces an additional element of natural selection, provides the yak with some advantages in
surviving in a harsh environment. Furthermore, by ensuring that old bulls are generally replaced
before their daughters have reached breeding age, this competition for dominance may also
reduce the degree of inbreeding. However, with the exception of human intervention, there is
nothing to prevent bulls from mating their siblings or succeeding their sires in the herd.
Bulls that have won a mating position in the herd usually mate several times a day. During
mating, bulls will not attack other bulls, unless strongly provoked.
The interval during which Yak bulls are suitable for mating is breed dependent. The Tibet Mendui
yak is suitable for breeding between 3.5 and 8 years (ideally 4.5 to 6.5 years) with few used after
8 years. Gannan Maqu bulls are used from 3.5 to 14 years and Qinghai Haibei bulls from 4 to 10
years. In Sichuan Jiulong, the period is from 4 to 12 years (with best results from 6 to 10 years).
According to the studies on semen collection and semen quality evaluation conducted for many
years in Datong Yak Farm, Qinghai, Yak bulls reach their reproductive peak at 7 years and
gradually decline thereafter, with the best results between 4 and 8 years of age.
A sexually productive life expectancy of not more than 10 years for a yak bull is reinforced by
results from an AI stud of 38 yak bulls in Tibet (altitude, 4300 m) where ejaculate volume,
concentration and motility of sperm rose steadily from the age of 3 to 9 years and subsequently
declined [15].
Reproduction and Conservation of Wild Yaks
Z. L. Lu
Lanzhou Institute of Animal Science and Veterinary Medicine, Chinese Academy of Agriculture,
Lanzhou, China.
Summary
Wild yaks, the closest ancestor to domestic yak, are perhaps the most threatened animals in the
Qinghai-Tibet plateau. By the 1970’s, wild yaks were on the verge of extinction, but due to
protective measures by the Chinese government some wild herds are now reported at elevations
between 4000 and 4500 m. Wild yaks and other wild ungulates of the Tibetan Plateau are gaining
increasing attention from international conservation organizations; they are listed on Appendix 1
of CITES and are the first class of "key" species of wildlife protected by Chinese legislation.
Conserving wild yaks and their habitat requires more knowledge of their reproduction, habitat and
grazing. Information on reproduction in the wild yak is scanty and most is focused on the male.
This review summarizes the available literature to provide basic knowledge to promote
conservation of these animals.
Introduction
Wild yaks (Bos mutus, Prez, 1876), the closest ancestor of the domestic yak, are found in the
central and eastern cold pastures of Qinghai-Tibet Plateau. Herds existed on the cold pastures of
western Sichuan, Qinghai and Gansu provinces. Male wild yaks could be seen mingling and
mating with the domestic female yaks. A few individuals with hair color characteristics of wild yaks
are present in domestic herds. However, excessive hunting of wild yaks drove them from the
plateau to high altitudes (above 4500 m) and right to the tops of the mountains (6000 m). By the
1970’s, wild yaks were on the verge of extinction. Some were present in China’s Kunlun
Mountains, but due to protective measures by the Chinese government, some wild herds are now
reported at elevations between 4000 and 4500 m [1,2].
Other than a paper by Schafer [3], the literature on the wild yak consists of explorer’s records [46] and brief comments when discussing other species [7,8]. A report on the Arjin Mountain
Reserve in Xinjiang [9] provided some important information on the ecology of wild yaks and,
more recently, Miller et al. [10] and Lu and Li [11] reported more important data from field
investigations.
The number of wild yaks has declined considerably in recent decades due to widespread hunting.
Explorer’s accounts from a century ago describe seeing enormous herds of wild yak in the
eastern Kunlun Mountains, on the upper reaches of the Yangtze River and near the headwaters
of the Yellow River in Qinghai Province. The construction of roads and government policies
favoring shooting of wildlife for food from 1950 to the1960'
s substantially reduced the numbers of
wild yak.
Wild yaks and other wild ungulates of the Tibetan Plateau are gaining increasing attention from
international conservation organizations; they are listed on Appendix 1 of CITES and are the first
class of "key" species of wildlife protected by Chinese legislation (PRC) [12]. Conserving wild
yaks and their habitat requires more knowledge of their reproduction, habitat and grazing.
Evolution and Historical Range of Wild Yaks
Yaks combine the features of the genus Bos and genus Bison and occupy a somewhat
intermediate position between them. Wild yaks are descendent of the Pleistocene Trans-Baikal
Poephaqus baicalensis that disappeared in Mongolia and the Trans-Baikal area under the
influence of man. The historic range of wild yak extended throughout the Tibetan Plateau,
northwestern Mongolia and into the Lake Baikal region of Russia.
The taxonomic classification of wild yak is still disputed. At one time, wild yaks were considered a
separate species (Bos mutus) from domestic yaks (Bos grunniens). Olsen [13] argues that yaks
should be classified as Peophaqus grunniens. However, based on genetic and ecological studies,
we believe that the wild yak (which has been naturally selected over many generations) is a
dominant wild species, while domestic yak (which survived with a low nutritional level) is a
deterioration species. Therefore, we maintain that wild yak should be classified as a separate
species Bos mutus [14,15].
Distribution Status
Wild yaks are presently found in the area surrounded by the Kunlun and Aerjin Mountains with a
total surface area of 1.400 000 Km2 and the lowest altitude of 4.000 m (above sea level). The
average annual temperature is about -8oC and the growing season for grass lasts about 100
days. Wild yaks have to travel about 200 - 300 km for grazing every day.
In recent years, the wild yak population has been increasing as wildlife conservation laws were
implemented. A herd of wild yak (n= 230) was found in Wutumeiren in the Geermu region.
According to a survey from Qinghai, Tibet and Gansu [15], there are about 20.000 - 40.000 wild
yaks in China. An accurate estimate of the wild yak population on the Tibetan Plateau is
unavailable, but based on the report by the investigations of Miller et al. [10], there are about
15.000 wild yaks in the Plateau. Therefore, the status of wild yak on the Tibetan plateau is
regarded as threatened. Despite being classified as a Class I protected species in China, wild
yaks continue to be hunted and are probably the most threatened wild animal in Tibet.
Types of Wild Yak
Yaks were domesticated at least 4.000 years ago. Wild yaks are classified as Qilian mountain
type or Kunlun mountain type, according to their body conformation, horn shapes and the natural
characteristics of the regions in which they live.
Qilian Mountain Type - The Qilian mountain type wild yak (called "Gaxi" by the nomads of Tibet)
is found mainly on the alpine meadow in the west Qilian Mountain and the east part of the Aerjin
Mountains. These yaks are not fierce and tough and generally do not attack people or other
animals [2]. Bulls are 160 - 170 cm at the withers, chest girth 210 cm, live weight 500 - 600 Kg
with a prominent hump, long legs, long face, small muzzle, short ears, and no dewlap. Females
are smaller than males; both have horns, but the male’s are bigger. The distance at the base of
the horns is more than 70 cm with a cllipsoid and round scur. The horn grows outward and curves
backward. The brisket, belly, rib, sides, legs, and hump are all covered by long hair. The hair is
brown-black, and the nose ring, eye ring and back line are gray-white. The tail is long, fluffy and
broom-like.
Kunlun Mountain Type - The Kunlun mountain type wild yak, called "Hengde" (snow hill wild
cattle) by Tibetan nomads, is found mainly on the alpine meadows of upper reaches of the
Yaluzangbu river, the Kunlun Mountain and the northern part of Tibet. These yaks are very
aggressive and may attack people or other animals. They are bigger than the Qilian type. Adult
males are 205 cm at the withers with a 270 cm chest girth, trunk length of 240 cm and live weight
of about 1200 Kg. The shoulder has a prominent, tuberous projection and the legs are stocky.
The face is short but the forehead is wide and prominent. On adult males, the scur is thick with a
circumference > 50 cm (the nomads use these horns for storing milk). The distance at the horn
base is up to 100 cm and the horn grows openly. Hair color is black or black brown and the back
line is rather clear. The nose and eye rings are gray-white. Although the face does not have hair,
the hair on the top of the head is long. Furthermore, there is long hair on the shoulder, rib, and
legs (in bulls the hair may touch the ground).
Reproductive Characteristics
Information on the reproduction of wild yaks is scanty. Most of the available data are studies on
crossbreeding between domestic and wild yaks and focused on male reproduction, especially
semen characteristics and the productivity of hybrids.
Sperm Concentration
Fresh semen of the wild yak bull contains 2.13x1010 spermatozoa per ml, much higher than those
of the domestic yak (1.10 x1010) and yellow cattle (6x109). Wild yak semen was diluted (ratio 1:3
to 1:6) and 1.5 - 1.7x106 spermatozoa per pellet were frozen. From 1984 to 1989, frozen-thawed
semen was used to inseminate domestic yaks with a conception rate of 88.9 %. In addition, 771
local yellow cattle were inseminated with the frozen wild yak semen and the calving rate was
71.85 % [15].
Sperm Motility
The motility of wild yak semen averages 63 and 39 % for fresh and frozen-thawed semen,
respectively. Fresh semen (diluted with 7% glucose) survives for 57 hours at 0 - 4oC. Frozen
semen thawed at 37oC subsequently survives 12 hours. The resistance coefficient of the sperm of
wild yak, domestic yak and domestic cattle is 144.000, 12.750 and 6.000, respectively, indicating
that wild yak sperm have exuberant motility [15].
Morphology of Spermatozoa
For wild yaks, the percentage of defective sperm for fresh and frozen-thawed semen is 6.3 and
9.2 %, respectively. Post-thaw, 87.5 % of wild yak spermatozoa have intact acrosomes
(approximately twice as high as domestic cattle). Careful freezing technique (and extender
composition) are important to maintain acrosome integrity, otherwise conception rate will
decrease. Semen characteristics include: milky-white and slightly yellow; specific gravity, 1.055;
osmotic pressure, 0.65; and pH 6.60 (none of these are significantly different from the domestic
yak). The moving viscosity of wild yak semen is 1.169 centipoise (compared to 1.94 4.1centipoise for domestic cattle). The low moving viscosity of wild yak semen enable the
spermatozoa to move easier, faster and with less energy, thereby prolonging their survival time
and increasing the chance of fertilization. The total nitrogen of wild yak bull semen
(1437.7mg/100ml, approximately twice as high as in ordinary cattle) supports sperm metabolism
[15].
Ultrastructure of the Sperm
Yak spermatozoa are a typical flagellar type. The length and width of the head and middle piece
of wild yak sperm are not significantly different from those of domestic yak, but the principal piece
is significantly longer (Table 1) [15].
Artificial Control of Estrus and Ovulation in Female Yaks
Z. P. Liu
Department of Veterinary Medicine, Gansu Agricultural Univiersity, Lanzhou, Gansu, China.
Summary
Artificial control of the estrus cycle has provided an efficient means of increasing the reproductive
capacity of yak by obviating the need for frequent visual inspection. This review describes
hormone treatments for induction of estrus and ovulation, hormonal changes after estrus
induction, and hormone treatments for advancement of puberty in female yaks.
Introduction
Yaks are seasonal breeders and the breeding season lasts from July to October. If the females
do not show obvious estrus in a particular breeding season, they will express estrus in the next
breeding season or in some cases not until several breeding season later. The observed estrus
rate of yaks is rather low, and the postpartum period is long. Most females deliver calves once
every two years, some of them only once every three years. Yak heifers attain puberty much later
than cattle and usually first get pregnant at 3 to 4 years old [1]. Silent estrus constitutes the single
largest factor responsible for poor reproductive efficiency in yaks. Aside from seasonal factors
affecting the display of estrus, the most important contributing factor is an inability to detect
estrus. Application of reproductive biotechnologies in the yak requires a reliable method for estrus
control of female and such a method is also essential in artificial insemination or embryo transfer
programmers. The possibility of inducing estrus and ovulation in acyclic females and of
synchronizing estrus and ovulation in groups of females offers an opportunity to increase the
efficiency of yak production and to makes artificial insemination more convenient. A reliable
method for estrus control would benefit not only the use of artificial insemination but would also
be important for establishing embryo transfer programmers.
As in other livestock animals, exogenous gonadotropins in various treatment regimens have been
used for estrus induction in the yak [2-4]. The effects of these treatment are, however,
considerably variable. There is only limited information from field studies on such protocols. There
remains a considerable need for more studies on methods to induce estrus and ovulation during
acyclic periods in females yaks, either by the administering of exogenous hormones and/or by
manipulating environmental factors that suppress ovarian activity.
Hormonal Induction of Estrus and Ovulation
The endocrine changes occurring during the estrus cycle involve integrated interactions between
hormones released by the hypothalamus, pituitary, ovaries and uterus. Each estrous cycle can be
broadly divided into a follicular phase and a luteal phase, with each phase having a
developmental period preceding the principal functional period [5]. In order to induce ovulation in
anestrous yaks, a single follicle or group of follicles must be stimulated to develop to a state of
maturity so that subsequent administration of luteinizing hormone (LH) or a hormone with LH-like
properties (e.g., human chorionic gonadotropin, hCG) will cause ovulation. This approach to
artificial control of the estrous cycle has provided an efficient means of increasing the
reproductive capacity of yak by obviating the need for frequent visual inspection. To induce estrus
using exogenous hormones treatment in female yaks, the following four points are important to
consider:
1. determining the correct time of treatment,
2. achieving some degree of uniformity of the number of days from treatment to the
appearance of estrus,
3. induction of estrus sufficient to permit fertile mating, and
4. efficacy with a high level of fertility.
Gonadotropins and steroids are used for estrus-induction. Either follicle stimulating hormone
(FSH), in the presence of LH, or equine chorionic gonadotropin (eCG) will stimulate follicular
growth. However, eCG is preferred since it has a longer metabolic half-life and is more readily
available. As mentioned above, results are variable. Shui et al. [2] used a "three-in-one hormone
preparation" containing progesterone, estradiol and testosterone to induce estrus in yak.
However, the pregnancy rate was only 30.9 % .
GnRH and GnRH-agonists are used for estrus induction. In addition to treatment with
gonadotropins, it has been possible to stimulate endogenous FSH and LH release and estrus,
with subsequent spontaneous ovulation, by administering GnRH, as observed in cattle. The
estrus-rate in non-milking yak-cows was significantly increased after gonadotropin-releasing
hormone (GnRH) administration during the breeding season compared to control cows [6]. The
results suggested that a GnRH-agonist might be effective in yaks. A single injection of luteinizing
hormone releasing hormone-A2 (LRH-A2 ,100 µg, Ningbo Hormonal Factory, China) caused
estrus in over 80% of treated anestrus yaks within 7 days, and 73.64% of these conceived [7].
Prostaglandin-F and GnRH-agonist - In non-milking yak-cows, a single injection of a combination
of prostaglandin F2 α (2 mg, Shanghai Wuxing Pharmaceutical Factory, Shanghai, China) and
GnRH (100 µg, Ningbo Hormonal Factory, Ningbo, China) induced estrus in 78.9% of the treated
animals within 7 days, and 46.7% of these conceived after artificial insemination. Compared to
control cattle, the estrus and fertility rates were increased 12.3% and 13.3%, respectively [7]. The
efficacy of PGF2α preparations to induce the regression of corpus luteum and inhibit the synthesis
of progesterone in yaks appears to be the same as in cattle. Estrus rates in yaks treated with the
PGF2α preparations Oestrophan (made in Czechloslovakia) and Enzaprost, twice at 10 day
intervals, were 82.6% and 90% and the conception rates were 77.9% and 78.9% , respectively.
Such PGF treatment was more effective than a single treatment [6].
Yu and Liu [3] used various exogenous hormones (LRH-A3, FSH, eCG and hCG) to induce estrus
in 180 female yaks of different ages. The animals included 80 milking yak-cows that had calved
and had their calves with them for milking during the experiment year, 80 non-milking cows that
had not calved guring the experiment year; and 20 heifers. The experiment was carried out at the
beginning of the breeding season. Female yaks in estrus were mated by natural service.
Pregnancy diagnosis was performed through rectal examination 30 days after mating and was
confirmed by delivery. After a single intramuscular injection of LRH-A3 (100 µg, Ningbo Hormonal
Factory, Ningbo, China), the non-milking yak-cows showed the best response with an estrus rate
of 95%, and yak-heifers responded better than milking yak-cows, with an estrus rate of 80%. The
results indicated that the estrus rates were 82% and 88% respectively in the non-milking yakcows after eCG (800 IU, Daqingshan Pharmaceutical Factory, Inner Mongolia Autonomous
Region, China) or hCG injection (2500 IU, Ningbo Hormonal Factory, Ningbo, China). The estrus
rate in the milking yak-cows reached 70% only after a combined treatment of an intramuscular
injection of FSH (100 IU, Ningbo Hormonal Factory, Ningbo, China) on day 0 and LRH-A3 on day
2. Estrus in treated yaks occurred within 1 to 10 days after normal treatment, with the non-milking
yak-cows and heifers showing estrus earlier than milking cows [3]. While there is considerable
variation in fertility after estrus induction in female yaks in this study, pregnancy rates were 86%
in non-milking yak-cows and 73% in milking yak-cows [3].
Liu [9] reported the use of a potent GnRH-agonist LRH-A3 (100µg, Ningbo Hormonal Factory,
Ningbo, China,), eCG (800 IU, Daqingshan Pharmaceutical Factory, Inner Mongolia Autonomous
Region, China) or hCG (2500 IU, Ningbo Hormonal Factory, Ningbo, China), alone or in
combination with FSH (100 IU, Ningbo Hormonal Factory, Ningbo, China) administrated 2 days
before the beginning of the breeding season.
This treatment was successful in inducing estrus in yaks at the onset of the breeding season and
increased the observed estrus rate by 30% [9]. In conclusion, a treatment regimen using LRH-A3,
eCG or hCG, alone or in combination with FSH during the breeding season, was successful to
induce ovulatory estrus in female yaks.
Hormonal Changes after Estrus Induction
The endocrinological changes associated with the induction of estrus using LRH-A3, eCG or hCG
in yaks have been investigated. Injection of LRH-A3, eCG or hCG alone elicited a peak release of
LH within a short time and then returned to the baseline. However, LH showed a second peak
where the amplitude and the time depended on category of the exogenous hormones and
reproductive state of yaks. The profiles of 17β-E2 were similar to those of elicited by the second
LH release, but not simultaneous, the first estradiol levels peaked before the second LH release
[9].
In the early postpartum period, yak-cows were not able to response to hormonal treatments,
presumably due to a failure of the hypothalamo-pituitary axis response to the treatment. Delayed
postpartum estrus and mating is the main factor determining reproductive capability in the year
following calving. Injection of eCG (100 IU, Intervet, Holland) in postpartum yaks resulted in an
elevation of progesterone level for a duration of 7 to 10 days which is shorter than that of the
normal estrous cycle [4]. Hormone treatment may cause changes in follicles that affect
subsequent progesterone production. In one study, after the treatment with LRH-A3, eCG or hCG,
the lactate dehydrogenase (LDH) activity in the follicular fluid, the distribution of LDH isozymes,
and the percentages of A and B type of LDH changed significantly; and the ratio of LDH2 vs LDH1
was reduced [9].
The in vitro effects of hormones on yak pituitary cultures have also been studied. Addition of
LRH-A3, eCG and hCG in the culture media of pituitary tissues could increase LH and FSH
production significantly. The amount of LH and FSH released was positively correlated to the
dose of LRH-A3. LH release was not correlated to eCG. FSH release was the highest when 80 IU
of eCG were added to the media. There were no correlations between the amounts of LH or FSH
released and the doses of eCG and hCG added [10].
Control of Puberty
Puberty covers the period during which the functional hypothalamic-pituitary-gonadal relationship
and interactions are being established. Yaks mature late and sexual maturity may be correlated
not only with absolute age and body condition but also with other factors, such as nutrition and
climate affecting the onset of the first breeding season. In an attempt to improve productivity, yakheifers were injected intramuscularly with 100 µg of LRH-A3 (Ningbo Hormonal Factory, Ningbo,
China ) at the beginning of the breeding season. Eighty percent of treated heifers showed estrus
within 15 days after treatment while only 50% of control heifers showed estrus. Sixty percent of
treated heifers were pregnant by the end of the breeding season, while only 40% of control
heifers had conceived [3]. The concentration of plasma LH showed two peaks, one occurred from
10 to 4 days before the first estrus, and the second on the day of estrus. LH then decreased to
baseline levels after estrus [11].
When both ovaries were removed in normal yaks before puberty, the concentration and release
frequency of LH increased significantly, while its amplitude decreased. The LH concentrations
remained high similar to levels in ovariectomied yak heifers treated with progesterone (20 - 30
mg, Suzhou Hormonal Factory, China) and estradiol-benzoate (2.5 - 3 mg, Shanghai Hormonal
Factory, China), returning to levels before ovariectomy when animals showed signs of estrus [12].
Conclusion
Treatment with exogenous hormones is a useful method for reducing the interval from parturition
or puberty to conception in female yaks, and for improving reproductive efficiency during the
breeding season. Artificial control of estrus can reduce management problems associated with
daily estrus detection in large or small herds, especially in the presence of suckled calves and
anoestrous cows. The current knowledge of basic patterns of follicle development in yaks is
insufficient to develop and apply protocols for induction of estrus. The induction of estrus would
be a useful tool to study the process and patterns of follicular dynamics in yaks. The data
obtained from such basic studies may then be used to develop test models for enhancing
reproductive efficiency.
Reproductive Endocrinology of the Female Yak
S. J. Yu
Department of Veterinary Medicine, Gansu Agricultural University, Lanzhou, Gansu, China.
Summary
In reviewing the literature, this paper assesses the current level of our understanding of the
hormonal control of puberty, estrous cycle, pregnancy, parturition and postpartum period in the
yak in order to provide basic information for improving reproduction in yaks.
Introduction
Endocrinology and reproductive physiology are rapidly growing areas in the broad field of animal
physiology. A good understanding of endocrine mechanisms which control reproduction is
important in all animals. The endocrinology of reproductive cycles has been studied in different
mammalian species. However, information pertaining to reproductive hormones in yaks remains
obscure at present. Since 1985, the author and his co-workers have engaged in the study of
reproduction in yaks, with emphasis on reproductive hormones. Research has progressed slowly
and much remains to be studied, given that the environment in which yaks live is extremely
harsh. The aim of this review, therefore, is to assess our current level of understanding about
reproductive endocrinology in an effort to provide basic information for improving yak
reproduction.
Before and at the Onset of Puberty
Fifteen yak heifers were used to monitor plasma progesterone concentrations before and at the
onset of puberty [1]. Heifers were divided into three age groups: group I (10 - 14 months, n=5),
group II (20 - 24 months, n=5) and group III (30 - 36 months, n=5). Yak heifers in Group I were
found to have two different progesterone profiles: inactive ovary profile (IO) and low progesterone
with short-length cycle profile (LPSC) (Fig. 1). Yaks in group II had three profiles: IO, LPSC and
low progesterone with normal-length cycle profile (LPNC) (Fig. 2).
Figure 1. Patterns of plasma progesterone before puberty in Group I
yak heifers (n=5). Time= each intervals represents 4 days (see text).
Figure 2. Patterns of plasma progesterone before puberty in Group II
yak heifers (n=5). Time= each intervals represents 4 days (see text). -
Yaks in group III had three profiles: LPSC, LPNC and a normal estrous cycle profile (Fig. 3). In
the present study, it was observed that yak heifers have one, two or more brief rises in circulating
progesterone. These increases, however, were not followed by a normal luteal phase except in
two yak heifers that showed estrus. This finding is different from literature reports for other cattle.
Figure 3. Patterns of plasma progesterone before and at the onset of
puberty in Group III yak heifers (n=5). O1 equals day of estrus (n=14).
O2 equals day of estrus (n=15). Time= each intervals represents 4
days (see text).
During the Estrous Cycle At Estrus
Yu and Chen [2] measured concentrations of luteinizing hormone (LH), estradiol-17β and
progesterone in peripheral plasma in six yak cows at estrus (Fig. 4).
Figure 4. Mean concentrations (+SD) of LH (n=6), oestradiol-17β (n=5)
and progesterone (n=5) in peripheral plasma taken at 1 hour intervals
around the time of estrus
The LH peak occurred 12 - 15 hours after the onset of estrus, reaching its maximum level at 14
hours. Estradiol-17β concentrations were high at the onset of estrus, continued to increase and
reached a peak value about 2 hours after the LH surge decreasing thereafter. Progesterone
concentrations were low at the onset of estrus, rising markedly at 16 hours and peaking 18 hours
after the onset of estrus (4 hours after the LH surge). Progesterone concentrations decreased
rapidly starting around 19 hours reaching basal levels 20 hours after the onset of estrus.
Overall, the patterns and temporal relationships of LH, estradiol-17β and progesterone at estrus
in the yak were similar to those reported for other domestic ruminants, suggesting that
mechanisms for the control of ovulation are similar.
Hormone Concentrations during the Normal Estrous Cycle
Concentrations of estradiol-17β and progesterone in plasma were measured in six normally
cycling yaks [3] (Fig. 5 and Fig. 6). There were three peaks of estradiol-17β in plasma and milk on
the day of estrus, and on days 5 and 14 of the cycle. Progesterone levels in plasma and milk
were low during estrus but peaks were observed on day 15. As measured in our laboratory,
estradiol-17β and progesterone concentrations in milk were about 4 or 5 times higher than
plasma concentrations.
Figure 5. Progesterone concentrations (P4) in plasma and milk just
before the breeding season (n=9) and during the normal cycle (n=6) in
yaks
Figure 6. Estradiol-17β concentrations in plasma and milk just before
the breeding season (n=9) and during the normal cycle (n=6) in yaks.
Hormone Concentrations During a Short Cycle
The pattern of both oestradiol-17β and progesterone concentrations during the short cycle were
similar to those of the normal cycle, however the values were lower [3] (Fig. 7 and Fig. 8).
Figure 7. Progesterone concentrations in plasma and milk during a
short estrous cycle in three yaks
Figure 8. Estradiol-17β concentrations in plasma and milk during a
short estrous cycle in three yaks.
Hormone Concentrations During Pregnancy and Periparturient Period
Progesterone profiles were similar for pregnant and non-pregnant yaks 14 days after estrus,
however, in pregnant yaks concentrations were significantly higher on Day 19 and tended to
increase gradually thereafter (Fig. 9 and Fig. 10).
Figure 9. Progesterone concentrations in plasma and milk during the
first month of pregnancy (n=12).
Figure 10. Estradiol-17β concentrations in plasma and milk during the
first month of pregnancy (n=12).
Plasma progesterone concentrations decreased rapidly on Day 120, then increased to reach a
maximum level on Day 210. Concentrations decreased again 20 days before parturition reaching
basal levels at parturition. Estradiol-17β levels in plasma and milk increased gradually from Day
23 after conception, decreased abruptly on Day 60, and increased again to reach a maximum
level at parturition. Estradiol concentrations decreased again after parturition to reach similar
levels as measured during mating (Fig. 11) [4].
Figure 11. Estradiol-17β concentrations in plasma and during
gestation and the periparturient period in yaks (n=8).
Table 1 shows progesterone (P4) and estradiol-17β (E2) concentrations and P4/E2 ratios measured
in eight yak cows during the periparturient period [4]. The progesterone to estradiol-17β ratio was
very high (265.48) on Day 234 of pregnancy declining abruptly at parturition (Day 0; 0.66). The P4
to E2 ratio increased rapidly again after parturition (reaching 16.7 one day later) maintaining this
level for 20 days following parturition. The correlation between plasma progesterone and plasma
estradiol-17β within 20 days before parturition was significant (r = -0.8446).
In general, progesterone and estradiol-17β patterns during pregnancy and the periparturient
period are similar to those of the dairy cow [5-8]. However, levels of plasma progesterone in yaks
declined on Day 90 and increased again on Day 150, similarly to progesterone concentrations in
the guinea-pig [9]. This change in progesterone secretion may be due to decreased secretory
function of the pregnant corpus luteum by Day 90; the function of the corpus luteum of pregnancy
being enhanced by the placenta, as in sheep and guinea-pigs on Day 150. This mechanism
needs further study.
The estradiol-17β peak in dairy cows occurs on the day before parturition [10,11] or on the day of
parturition [12]. The results of our study in yaks agree with the latter report. The sharp rise of
estradiol-17β at parturition may be caused by the pronounced increase in uterine blood flow, as in
dairy cows, which may benefit parturition. At parturition, in yaks progesterone levels decline
abruptly and the ratio of P4/E2 switches markedly. This is obviously different from the horse [13]
and guinea-pig [14], but similar to sheep [15], dairy cows [6,7] and rats [17]. The `progesteroneblock'hypothesis may be used to explain the mechanism regulating the onset of parturition in
yaks.
Table 1. Ratio of P4 to E2 in Plasma of Eight Yak Cows
during the Periparturient Period
Periparturient
Period
(Days or Hours) (No
Animals)
Concentrations (ng/ml)
P4
E2
P4/E2Ratio
234 (D) (5)
11.15
0.042
265.48
239 (D) (6)
8.34
0.059
141.36
244 (D) (6)
6.47
0.068
95.15
249 (D) (5)
5.70
0.09
63.33
252 (D) (8)
4.07
0.142
28.66
253 (D) (8)
1.175
0.252
6.94
0 (D)* (8)
0.34
0.515
0.66
3 (H) (6)
0.32
0.281
1.14
7 (H) (7)
0.5
0.098
5.68
15 (H) (7)
0.5
0.032
15.63
253 (D) (8)
1.175
0.252
6.94
1 (D) (8)
0.3
0.018
16.67
3 (D) (8)
0.26
0.017
15.29
5 (D) (8)
0.31
0.019
16.32
10 (D) (8)
0.41
0.016
25.53
15 (D) (8)
0.25
0.011
22.73
20 (D) (8)
0.24
0.018
13.33
*= parturition; D= day; H= hour; P4= progesterone; E2= oestradiol-17β.
Hormone Concentrations During the Postpartum Period
Milk progesterone concentrations in 15 suckled yak cows were determined to monitor postpartum
ovarian activity [18]. Yak cows were classified into four groups (Table 2). Type I (normal): 6 cows
showed cyclic changes in milk progesterone concentrations within 40 days postpartum. Type II (a
short progesterone rise): 2 cows had a short rise in milk progesterone concentration within 20
days postpartum, then progesterone concentrations remained low (<0.5 ng/ml) until 90 days
postpartum. Type III (cycle ceased in the presence of low progesterone concentrations): 2 cows
had milk progesterone concentrations below 0.5 ng/ml until 90 days postpartum. Type IV (cycle
ceased in the presence of high progesterone concentrations): 5 cows had milk progesterone
concentrations above 1.0 ng/ml from Day 20 until 90 days postpartum.
Among cows classified as type II and III, all four cows had inactive ovaries as determined by
rectal palpation. Type IV cows were diagnosed as bearing a persistent corpus luteum as
determined by rectal palpation.
Table 2. Classification of Yak Cows According to Type of
Postpartum Ovarian Activity, as Determined by Milk
Progesterone Concentrations and Examination by Rectal
Palpation.
Pattern of Milk
Progesterone
Concentrations
No of
Cows (%)
Ovarian State
(Rectal
Palpation)
Type I
Normal
6 (40)
Normal
Type
II
A brief progesterone rise
2 (13.3)
Inactive ovary
Type
III
Cycle ceased with low
progesterone
2 (13.3)
Inactive ovary
Type
IV
Cycle ceased with high
progesterone
5 (33.3)
Persistent
corpus luteum
Total
15 (100)
Table 3 [19] shows that 66.7% of cows presented luteal activity within 40 days postpartum as
determined by progesterone concentrations. However, only 13.3% had shown estrus at the time
of estrus observation. Since estrus observations were carried out accurately and by experienced
herdsmen and researchers, our results suggest that a high proportion of estrus periods were of
low intensity and /or of short duration.
Table 3. Time from Parturition to Commencement of Ovarian Activity as
Determined by Milk Progesterone Concentrations, Examination by Rectal
Palpation and Observation for Signs of Estrus in Yak Cows.
Starting Ovarian Activity by
Days
Postpartum
Plasma Progesterone Concentrations
and Rectal Palpation
Signs of Estrus
No cows (%)
Cumulative (%)
No cows
(%)
Cumulative
(%)
<20
4 (26.7)
26.7
0
0
21 - 30
3 (20)
46.7
1 (6.7)
6.7
31 - 40
3 (20)
66.7
1 (6.7)
13.3
>90
5 (33.3)
100
13 (86.7)
100
Total
15 (100)
100
15 (100)
100
Results indicated that ovarian activity postpartum was re-established earlier in yak cows than in
suckled buffaloes, and similarly to dairy cows. Milk progesterone profiles assessed by RIA can be
used to monitor postpartum ovarian activity. Progesterone concentrations measured in milk may
be helpful in the early detection of ovarian dysfunction in yak cows.
Ovarian Follicle Activity in Yak versus Cattle and Buffalo
Y. Q. Tian and X. X. Zhao
Department of Veterinary Medicine, Gansu Agricultural University, Lanzhou, Gansu, China.
Summary
A comparison of the ovarian follicular system in cattle, buffalo and yak yields potentially useful
information in developing methods for the improvement of reproduction efficiency in yaks. The
ovaries in yaks were generally smaller than in both cattle and buffalo. The histological structures
of the yak ovaries and follicular system resembled in all aspects those described for cattle and
buffaloes. In yak, the average number of primordial follicles was markedly lower than in cattle, but
higher than in buffalo. The number of growing follicles was higher than in cattle and buffalo. The
number of Graafian follicles was lower than in cattle, but generally similar to that in buffalo. In
conclusion, the number of follicles in each developmental phase in yaks is lower than in cattle,
and higher than in buffalo. The possible roles of genetic and environmental factors in determining
follicle status is discussed.
Introduction
Yaks play a prominent role in the mountainous livestock production, particularly in the highland of
Asia, and factors affecting productivity are of paramount importance to agricultural economics in
this region of the world. Both yak and buffalos are adaptable to the extreme ecological conditions.
Due to constriction of nutritional and ecological condition, their reproduction performance is
known to suffer from a number of inherent problems that include late maturity, poor estrus
expression, distinct seasonal reproductive patterns, prolonged intercalving intervals and low
conception rates. Their fertility performance and reproductive efficiency show a distinct influence
of time of year. The review made a comprehensive comparison of form and functions of ovaries
among cattle, buffaloes and yaks with view to applying reproductive technology in cattle to yak
and buffaloes and promoting the development of yak and buffalo production. All means are
reported as mean +/- standard deviation.
Ovarian Anatomy
In cattle the growth of ovaries is the most rapid from birth to puberty. The weights of both ovaries
are almost the same at birth. At sexual maturity the right ovary is heavier. Settergren [1]
demonstrated that the left ovary was generally heavier, but the difference between the right and
left ovary was not significant. The length, width and thickness of the ovaries were almost the
same for the right (25.1, 18.4 and 15.3 mm) and the left ovary (25.7, 19.1 and 16.1 mm) [1].
In buffaloes the ovaries are smaller than in cattle. However, there is considerable change in
ovarian weight and dimensions during the different phases of the sexual cycles. Maximum size
and weight were observed when a fully developed corpus luteum existed in the ovary The
minimum and maximum average weights being 2.9 g and 6.1 g in buffaloes and 3.9 g and 9.9 g in
cattle [2,3]. Danell reported that the average weight of the left and right ovaries in 30 normally
cycling buffaloes were 3.4+1.3 g and 3.6+1.5 g, whereas in non-cycling animals the weights were
2.5+1.2 g and 2.5+0.9 g, respectively [4].
In yaks, ovarian weight and size increased with age. The average ovarian weight increased from
0.32 g in 1-month-old calves to 2.12 g in 7-years-old yak cows. The average length, width and
thickness of ovaries increased from 1.18, 0.70 and 0.44 cm to 2.15, 1.66, 1.10 cm. No significant
differences were found between the weight and size of yak ovaries within the same age group [5].
The results indicate that the weight and size of yak ovaries are less than those of cattle, but
similar to those of buffalos.
Ovarian Histology in Yaks
The histological structure of yak ovaries resembled in all aspects the description of cattle ovaries
[1,6-8] and buffaloes [3,4-9]. The covering epithelium, is about 5 - 10 µm and usually consists of
simple columnar and cubical, or sometimes squamous epithelial cells, although stratified
epithelial cells are sometimes observed in neonates. The nuclei of the epithelial cells are round,
oval or flat and often rich in chromatin. The covering epithelium of ovaries is invaginated into the
cortical stroma to form tubule-like structures lined with columnar or sometimes cubical cells in the
neonates and heifers, and sometimes in cows too [10].
The tunica albuginea consists of connective tissue fibers. The cortex constitutes the main part of
the ovary. The stroma consists mainly of connective tissue cells and interstitial epithelial cells.
The cortex was well supplied with various types of blood vessels. The medulla consists of loosen
connective tissue and cells and is well supplied with blood vessels. The rete ovarii is situated in
the medial part of the ovary, closed to the attached border, and sometimes continues into the
mesovarium. It consists of cords and ductuli. The lining is composed of epithelial cells with oval
nuclei, usually forming a single layer.
Follicular System
The morphological features and distribution of ovarian follicles in the yak is fundamentally similar
to those of cattle and buffaloes [1-4,6,9]. In cattle and buffalo, the cortex contains the primordial
follicles, which usually contain one oocyte but sometimes two, rarely three. The growing follicles
with two or three layers of follicular epithelium are situated deeper into the cortex than primordial
follicles. The connective tissue around the growing follicles forms the theca that later
differentiates into theca interna and theca externa.
Primordial follicles in yak are distributed in the peripheral part of the cortex, frequently rather
close to the tunica albuginea. Primordial follicles usually contain one, but occasionally two
oocytes surrounded by a single layer of squamous epithelial cells. In newborn calves, primordial
follicles are generally distributed in groups, usually more than 10 grouped together [10].
The growing follicles are usually deeper into the cortex of the ovary with two or more layers of
polygonal or cubical epithelial cells. The size of growing follicles is 80 - 120 µm in diameter and
their oocytes are 40 - 45 µm, with nuclei of about 15 µm. When the oocyte is surrounded by 4 - 5
layers of epithelial cells, the surrounding connective tissue starts to form what will become the
theca layers and small pools of fluid starts to form among the epithelial cells.
The young Graafian follicles, characterized by the antrum formation, are surrounded by three or
four layers of granulosa cells resting on a basement membrane. In general, when follicles enlarge
to 300 - 500 µm, one complete antrum forms. At this time, the oocytes are 60 - 90 µm with nuclei
of 35 - 40 µm. The granulosa cells next to the basement membrane are larger than other cells,
with regularly arranged nuclei. The theca layer consists of two parts: the theca externa and theca
interna, as in cattle and buffalo. Blood vessels of the theca externa supply capillaries to the theca
interna. The theca externa has indistinct boundaries [10].
Atresia of Follicles
Obliterative atresia with primary follicular wall degeneration in Graafian follicles is described in
cattle [11] and divided into three degrees. The same type of atresia is by far the most common in
buffalo heifers. Danell [4] described two degrees of atresia in buffalos. The first degree of atresia
was characterized by a number of pyknotic nuclei in the liquor folliculi and in the granulosa layer
of the follicular wall. The beginning of the second degree of atresia is characterized by changes in
the granulosa layer, alone, with few or no pyknotic nuclei in the antral fluid. Later, theca
connective tissue cells are observed in the antrum. In the theca interna the epithelial cells
disappear and the connective tissue cells predominate.
Pyknotic nuclei are observed in the cumulus. Later, the cumulus disappears and only the naked
oocyte remains and an ingrowth of connective tissue into the lumen takes place. The follicle
finally becomes a corpus atreticum.
In a study on follicular population in cycling and non-cycling buffalo heifers, Danell observed that
average number of follicles over 2.00 mm in diameter to be 16.8 and 23.6, respectively, and did
not significantly differ. However, there were twice as many atretic follicles as normal ones (31.7
vs. 14.6, respectively) in cycling animals. The average atresia frequency for all animals was
76.6% [4]. A similar value of 82%, was obtained following histological examination of follicular
atresia at random stages of reproduction in ovaries from swamp buffaloes obtained at slaughter
[12].
The atresia of follicles in the yak can occur at any stage of follicular development, including
Graafian follicles late in development. The atresia of Graafian follicles can be divided into the
early, definitive and late stages. Early atresia appears in three forms: loosening and sloughing of
granulosa cells lining the antrum; disappearance of the membrana propria and loss of orientation
of the basal layer of the granulosum and shortening and rounding of the theca interna cells. The
definitive stage of atresia in yaks , which is also described in dairy cows, is characterized by
collapsing, contraction or cystic appearance. Collapsing atresia constitutes about 15% of the total
follicular atresia. Contracting atresia is the most common type of follicular regression, it is
observed as the only type of atresia in many follicles, and comprises more than 80% of all the
definitive atresia. Cystic follicular atresia occurs infrequently, comprising less than 5% of all the
definite atresia. Late-stage atresia is the final stage of regression common to all types of atretic
vesicular follicles. The size is reduced and cell layers are disorganized, the antrum becomes
gradually filled with fibrous granulosa remnants and the theca layers become hyalinized [10].
Follicular Development
The numbers of primordial follicles in the ovary is lower in buffaloes and yaks than in cattle. Left
(49.3%) and right (50.7%) ovaries have a similar number of primordial follicles [4]. The population
of primordial follicles is estimated to be about 19 000 in Nili-Ravi buffaloes [13] and 12 000 in
Sarti Buffalo [4], 32 870 in 2-year-old yaks [10], compared to between 60 000 and 100 000 in the
cow. This suggests a reason for the lower reproductive potential of buffaloes and yaks. In the
cow, the number of ovulations following gonadotrophin treatment is related to the number of
healthy follicles over 1.7 mm in diameter present before initiation of treatment [14]. Since the
number of such follicles has been found to range from one to five in buffaloes, in comparison with
a range of 17 - 32 in cows, the low follicular population may contribute to the low superovulatory
response in buffaloes [15].
According to Cui [10], the number of primordial follicles in yaks is much lower than in cattle, but
higher than in buffalos. The average total number of primordial follicles per ovary pair in 1-monthold calves ,1-year and 2-year-old heifers and 7 to 10 year old yak cows were 53.5+6.3, 32.9+4.5,
22.9+2.8 and 9.5+1.2 respectively. On average, there were about the same number of primordial
follicles in the left and the right ovaries among each age group. However, a great difference
existed between the two ovaries in an individual animal. Although the number of primordial
follicles appears to be considerably lower in buffaloes and yaks than in cattle, the effect of age on
population has apparently not been evaluated in buffaloes and yaks. In addition, buffalo ovaries
contain only about 20% of of the antral follicles found in cattle ovaries [15].
Erickson [16] reported that the number of growing follicles per ovary pair in 22 to 50 day-old
Herefords, 12 months old, 19 to 24 months old and 7 to 9 years old were 93+18, 248+33, 233+38
and 154+15 respectively. The study in yaks showed that the average total number of growing
follicles per ovary pair in 1 month-old calves, 1 year-old heifers, 2 year-old heifers and 7 to 10
year old yak cows were 210+76, 815+95, 895+142 and 445+88, respectively [10]. There are
significant differences between age groups.
Rajakoski [8] carried out quantitative estimation of the number of Graafian follicles >1 mm in
cattle with reference to seasonal, cyclical and left-right variations.
Normal and atretic follicles >1 mm were found in equal numbers, on an average 46.3 and 46.0
respectively in each pair of ovaries. He found that cyclical differences in the number of follicles >5
mm indicated two growth waves during a sexual cycle. One hundred and thirty ovaries contained
a similar number of follicles >1 mm, however, there was a greater number of normal follicles >5
mm in the right than in the left ovary. Several subsequent studies using similar techniques further
supported the theory of follicular waves [1,17,18], although these studies relied on single timepoint data. Using ink marking at laparotomy, it was demonstrated that ovulatory follicles only
became the largest follicle on the ovary within 2 to 3 days of oestrus, and confirmed the dynamic
nature of ovarian turn-over [19-21]. More recent studies using transrectal real-time ultrasound
techniques have confirmed and extended the previous histological and gross morphological data
showing two and three major phases of growth of large follicles during the bovine estrous cycle.
The ovulatory follicle is selected around 3 days prior to ovulation [22-26].
In cattle each wave of follicular development is characterized by simultaneous emergence of
medium-sized (>4 mm in diameter) growing follicles from a pool of smaller follicles. One of these
follicles rapidly emerges as the dominant follicle (>7 mm in diameter) and continues to develop,
while the others undergo atresia and regress. In cattle it usually takes 5 to 7 days for the
dominant follicle to reach ovulatory size [27,28]. The dominant follicle normally reaches a
maximum size of about 15 mm in diameter and remains dominant for a few days, until it becomes
atresic and regresses, and is replaced within approximately 5 days by a new dominant follicle
developing from the next wave of follicular development. If luteal regression occurs during the
growth phase or early period of dominance, then the dominant follicle will continue to develop to
preovulatory size (up to 20 mm in cattle) and will eventually trigger the hormonal cascade leading
to ovulation [29].
In cattle follicular waves appear to be a constitutive characteristic since they are present prior to
puberty [30], throughout most of pregnancy [31] and the post-partum period [32,33], as well as
during estrous cycles. The number of waves per estrous cycle is usually two [34] or three [24,25]
and reflects a genetic and environmental influence [35]. Singh et al. [36] delineated the pattern of
development and atresia of large follicles (>8 mm) on the surface of ovaries of buffalo heifers.
The authors concluded that their findings agreed with Rajakoski’s theory that the follicles at
midcycle become atretic and that a new growth wave of follicles begins around midcycle and
gives rise to the follicle(s) which would ovulate after estrus [8]. Danell [4] reported that the
average number of Graafian follicles in the ovaries of cycling buffalo heifers was 46.3 and in the
non-cycling animals 57.89. The average number of follicles in the left and right ovaries was 23.8
and 22.5 in cycling heifers and 35 and 32.8 in non-cycling ones, respectively. Danell also
indicated that a significant correlation existed between the number of primordial follicles and the
number of Graafian follicles >1 mm (p = 0.048). Evaluation of the number of follicles of 2 mm on
the ovaries was carried out at three different time periods of the estrous cycle and revealed a
greater number of follicles between Days 1 - 8 and 9 - 11 than during Days 12 - 21 [4].
Compared with cattle and buffalo, there is little available data on Graafian follicles in the yak. One
study on yaks [10] showed that the average total number of Graafian follicles in 1 month-old
calves, 1 year-old heifers, 2 year-old heifers and 7 year-old cow were 36.5+14.2, 41.7+12.3,
37.8+9.8 and 42.5+14.5, respectively. The difference was not significant among age groups or
between the right and left ovaries within each age group. The numbers of atretic Graafian follicles
in 1 month-old calves, 1 year-old heifers, 2 year-old heifers and 7 to 10 year-old yak cows were
22.1+55.6, 21.2+7.6, 21.5+4.7 and 25.3+6.7, respectively. No significant differences exist
between age groups. The time of sampling was during the reproductive season, although the
animals showed no signs of estrus. Therefore, comparatively, in the non-estrus phase of the
cycle, the numbers of Graafian follicles in yaks were lower than in cattle, but basically similar to
those in buffaloes.
Conclusion
From current available data on the development of follicles in yaks, it is clear that the follicular
number at each developmental phase in yaks is lower than in cattle, but it is similar to in
buffaloes. Buffalo and yak are both seasonal breeders in which ovarian function is greatly
suppressed during extreme climatic conditions [15]. Conditions are typically harsher for yaks. It is
well known that nutritional deficiency is one of the important conditions that limits production.
Possible effects of nutrition on ovarian follicle function merits consideration.
There is a strong need to identify the factors responsible for the low reproductive efficiency in
yaks. Available information on the patterns of follicle development in yaks is inadequate and
further study is needed. Further studies are needed to understand the processes of follicle
recruitment, development and atresia and the temporal patterns of follicle selection, dominance,
follicle numbers and preovulatory changes and follicular dynamics using techniques which permit
serial assessment of changes occurring overtime. Emphasis may be directed towards
investigating follicular waves as a functional unit. The knowledge obtained from such basic
research may then be used to develop and test models for enhancing yak reproductive efficiency.