EFFECT OF POTASSIUM LEVEL AND SOURCE ON
PHYSIOLOGICAL RESPONSES OF LACTATING COWS
UNDER HEAT STRESS
By
Tareq Abdullah Al-Showeimi
A thesis
Submitted in Partial Fulfillment
of the Requirements for the Master’s Degree
in Animal Production
Department of Animal Production
College of Agriculture
King Saud University
Riyadh, Saudi Arabia
1417 H/ 1996 G
1
CHAPTER I
INTRODUCTION
2
Environmental physiology today emphasizes studies to improve the
animals resistance to environmental stressors, hence, improving their
physiological status and performance. This could be done by different
means such as hormonal and nutritional manipulations, environmental
modifications during critical production phases of the life cycle, and
better define the systematic and cellular basis for stress resistance by
persuit
of
physiological,
neuro-hormonal-immuno-techniques.
Temperature and other stressors are critical to animals well- being and
are useful tools to better understand animal functions, its genetic limits
and potential. Often there may be a negative genetic correlations between
traits for adaptation and production. Nutrational and health constraints
must be resolved to take advantage of the genetic improvement in
production triats, such as milk yields. Outcrossing to highly productive
genotypes (generally imported from temperate regions) will generally be
the most rapid means for improving genetic potential if imported stocks
can be kept alive and fertile under tropical conditions. The ability of
livestock to grow, lactate and reproduce to their maximal genetic potential
is determined by the meteorological and biological environment and their
interactions during growth and developmental stages and at maturity.
An animal’s productivity or even survival depends, largely, on whether
or not it can maintain its body temperature within certain limits, and
tolerate local feedstuffs and resist disease(Bianca,1968; Johnson, 1965;
Johnson et al., 1976). In a thermal environment in which the animal’s
heat production exceeds heat loss, an increasing amount of heat is stored
in the animal’s body, resulting in increased body temperature. When the
latter is significantly elevated, a myriad of homothermic events are
3
initiated. These events include increase in evaporated heat loss by
respiration and skin, in urinary excretion which may aid in conductive
and convective cooling. However, when high temperature and radiation
lessen the ability of the animal to radiate heat, feed intake, metabolism,
body weight and milk yields decrease to help alleviate the heat imbalance
(Johnson, 1980). If the animal does not maintain heat balance by heat loss
mechanisms, thermo-regulation effort will soon result in a decline in
energy intake, heat production, and milk production. The degree of
change in body function and the decline in production depends on
managerial or environmental modifying factors such as the use of shelter,
evaporative cooling (spray), fans, quality (feed additives) and amount of
feed, disease control, and other managerial skills of the farm managers
(Bianca, 1968; Hahn et al., 1976a, b; Johnson, 1965; Morrison, 1983;
Thatcher et al., 1974; Johnson et al., 1976 ; Flamenbaum et al., 1986;
Taylor et al., 1986; Igono et al., 1987; Her et al., 1988; Armstrong et al.,
1988; Ryan et al., 1992). Hormonal manipulations have been lately
approved to asses in altering the severe climatic influences of the
environment and to improve the animal’s performance ( Mohammed and
Johnson, 1985; Burton et al., 1992). Hormones generally regarded as
stimulatory or inhibitory to body or mammary gland growth or milk
synthesis and the possible manner in which environmental stressors may
alter hormones and milk production were summarized in a review by
Johnson (1980).
The negative effects of heat stress upon performance are also
associated with changes in mineral metabolism (Beede and Collier, 1986;
Escobosa et al., 1984; Schneider et al.,1984, 1986, 1988a,b ; West et al.,
4
1987, 1991 ,1992). In theory, dietary inputs of macromineral elements
may be manipulated to support homeostasis and lactational performance
of heat stress cows. Effects of macromineral elements are not
independent, and their interactions may affect the hyperthermic cow
directly from the normothermic cow (Schneider et al., 1988b), which
indicate the need for more research. Due to their metabolic rate, dairy
cows in subtropical desert, and mediterranean areas are above their
thermoneutral zone during the summer (Berman et al., 1985). The
resulting need for enhanced heat dissipation increases electrolyte
requirements due to their losses by sweat and salivia polypnea (Beede et
al., 1984). Providing electrolyte requirements as a percentage of dry
matter (DM) rather than daily intake (NRC, 1989) may oversimplify a
complex sitution, especially at the onset of lactation. Addition of salt
buffers to diets benefited heat-stressed dairy cows in terms of milk yield,
regulation of acid-base balance, and lowering body temperatures
(Coppock et al., 1982a,b; Schneider et al., 1984; Mallonee et al., 1985;
Schneider et al., 1988a, b).
Since the potassium content of most grains is relatively lower than
that in roughages and milk being a major source of its elimination from
the body, feeding high concentrate diets could lead to a possible deficiency
problem (Pradhan & Hemken, 1968). Use of dietary potassium (K) and
sodium (Na) buffer may be warranted to overcome this problem. Both
minerals could be important during heat stress as they are major
regulators of body water balance and acid-base physiology of heatstressed cows. Heat-stressed cows lose more K in sweat than cows that are
not heat stressed (Mallonee et al., 1985). Sodium is required at the kidney
5
for K concentration and to balance bicarbonate excretion electrically.
Potential deficiencies of both elements may be implicated in lower milk
production during hyperthermia. A deficiency of dietary K may result in
reduced feed intake (Pradhan and Hemken, 1968; Beede et al., 1983b). A
reduction in feed intake, especially forages, occurs under heat stress, thus
reducing total K intake (Newton, 1980; NRC, 1989; and Schneider, et al.,
1984). Due to the latter effect and in addition to sweating loss of K
(Johnson, 1967; MacFarlane, 1968; Mallonee et al., 1985) and the greater
demand during lactation for K, plasma (Dennis et al, 1976; Niles, et al
1980) and milk K (Pradhan and Hemken, 1968) may be decreased. Only
small amounts of K are stored in the body, so quantity of K in the diet can
influence the amount of milk produced (IMCC, 1981). Therefore, a higher
deitary K levels than NRC recommendations could be required for dairy
cattle. Significant increase (3-15%) in production of milk by heat-stressed
cows fed higher than recommended (NRC, 1989) levels of dietary Na and
K have been reported (Schneider et al., 1984, 1986; Mallonee et al., 1985;
West et al., 1987).
Cows in heat stress often experience a respiratory alkalosis, resulting
from hyperventilation. Compensation results in urinary bicarbonate loss
in an attempt to balance the ratio of carbonic acid to bicarbonate in the
blood
(Benjamin,
1978).
Also,
reduced
ruminal
pH,
acetate
concentrations, and acetate to propionate ratios, may result in metabolic
acidosis for heat-stressed cows (Bandaranayaka and Holmes, 1976; Niles
et al., 1980; Schneider et al., 1988a). Use of dietary buffers may be
warranted to overcome these problems.
Thermal stress, hence, can represent sizable economic losses to
6
producers of intensively managed livestock due to the reduced
performance and increased energy requirement for maintenance. This is
the case in Saudi Arabia where there are many large milking herds of the
temperate-evolved dairy cows practicing under ambient environmental
temperatures which exceed the upper critical temperature of dairy cows
during a major part of the day and for more than 5 months, when peak
daily temperatures range from 37 to 55 0C. Feeding diets high in
digestable energy and concentrate is a common practice to keep a high
level of milk production. Feeding such diets to heat-stress cows may be
advantageous, because heat increament is lower than that with high
forages diets, but such diets tend to increase acidic condition in the rumen.
The intense solar radiation and high temperatures during the summer
months in Saudi Arabia make environmental alteration necessary if
acceptable milk production and fertility are to be maintained. The
evaporative cooling system was applied with good results (Salah et al.,
1992; El-Nouty et al., 1990b; Ryan et al., 1992). Most of the dairy farm in
Saudi Arabia feed high concentrate diets due to governmental subsidy.
This study was carried out in order to determine the effects of increasing
dietary K content from different salt sources (as buffers) upon the
lactational performance and physiological responses of Holstein cows
raised under the semi-arid conditions of Saudi Arabia. Lactational
performance included feed intake, average daily milk yield, fat
percentage and 4% fat-corrected milk. Physiological responses included
levels of some cellular and non-cellular blood components. Levels of
major minerals in plasma were evaluated. Also, levels of plasma
hormones (T4, T3 and cortisol) related to heat stress were considered. The
7
effect of addition of sodium bicarbonate as a dietary buffer to a diet
containing higher or lower K level were also investigated.
8
CHAPTER II
LITERATURE REVIEW
9
1. EFFECTS OF HEAT STRESS ON LACTATIONAL AND
PHYSIOLOGICAL RESPONSES
1.1. EFFECT ON MILK PRODUCTION, FEED AND WATER INTAKE:
Although there are genetic differences in ability to produce milk at
high environmental temperature, the level of milk production is a critical
factor in assessing the desirability of a specific individual for export to
low tropical environment. Environment directly and indirectly influences
productivity of dairy animals. Degree of environmental impact is modified
by stage of the animal life cycle and adaptation of given breed and species
(Thatcher and Collier, 1981). A high environmental temperature clearly
reduces the milk yield of a cow (DiDios, 1984; Regan and Richardson,
1938; Ragsdale et al., 1949, 1951; Brody et al., 1955; Armstrong et al.,
1988; Salah et al., 1988; El-Nouty et al., 1990a; Ryan et al., 1992), due to
reduced feed intake, but it is not clear whether temperature has any
effect on the percentage composition of milk (Jenness et al., 1974; Stanley
et al., 1975). Regan and Richardson (1938) noted that temperatures above
26.6 0C decresaed milk production, casein content, and SNF levels in high
producing dairy cows. Yields were affected only slightly by increasing
maximum daily temperatures from 8 to 29 0C, but declined rapidly
with higher temperature; fat and protein percentages declined when the
increase in the temperature was from 8 to 37 0C (Rodriquez et al ., 1985).
Folman et al. (1979) found a decline of 8% in milk yield for summer
condition when the mean afternoon temperature was 39.8 0C compared
with winter conditions.
Maintenance energy for dairy cattle was shown to be considerably higher
under thermal stress conditions by Findlay (1968), indicating that most of
10
the additional energy is due to the effect of increased body temperature on
metabolic rate,
and that panting requires relatively little energy.
However, digestability of feed may be increased either through direct
means or due to the fact that the decreased feed intake may result in
longer retention time (Morrison, 1983).
The temperature-humidity index [THI=0.72(Tdb+ Twb )+ 40.6; Maust et
al. (1972)] was used in assessing the climate effects on animal
performance. Milk production starts to decline above 72 THI units, but
marked declines occur around 76-78 mean THI, depending primary on
the period of time above 72. The upper limit of acclimatization for the
average dairy cow has been proposed to be a THI of 75 (Regan and
Richardson, 1938; Johnson et al., 1962; Ingraham, 1974). It was reported
that as THI increased from 70 to 82, rectal temperatures increased in a
linear manner from 38.4 to 40.0 0C (Stott et al., 1961 ; Cavestany et al.,
1985). Ingraham et al. (1979) in Hawaii, found that for each 1 unit
increase in THI, milk production declined 0.32 kg/day. Johnson et al.
(1962) estimated a decline of 0.318 kg/cow/day for each unit increase in
THI. Daily production of the University of Missouri dairy herd, USA, was
related to the THI (Elmasri, 1982). Level of production was compared for
days prior to increase in average THI above 72, and during the 110 day
period in which THI was usually above 72 (Elmasri, 1982). The average
daily losses in milk production for early stage cows was 5.5 kg/day/cow,
mid stage 2.6 kg/day/cow, and late stage 2.9 kg/day/cow for the first 55
days during the summer. The early stage cows tended to recover more
during the last 55 days of the summer. In fact, there is some evidence of
compensation especially in the early stage group. This was not apparent in
11
the mid or late stages. The decline per THI unit for early, mid and late
stage cows was 0.9, 0.43 and 0.48 kg/day /cow. These rather significant
losses are due to the direct climate effect since the total confinement
program did not vary with season.
In general, these overall data
indicated a decline of 0.26 kg/day in milk, and a decrease of 0.23 kg/day
hay consumption for each THI increase. High producers were affected
more than lower producers by the increase in THI. El-Nouty et al. (1990a)
indicated that the rise in THI above the upper critical level for lactating
Holstein cows altered most blood cellular and non-cellular constituents in
high-yielding cows. The mean rectal temperature increased 0.12 0C for
each unit increase in THI. These data reconfirm the close functional
relationship of meteorological environment (THI), rectal temperature
and the influence of rising body temperature on hay consumption and
milk yields (Johnson et al., 1962).
As ambient temperature rises, thermal gradient between animals and
their environment decreases. This reduces quantities of heat animals can
lose to the environment by physical means such as convection and
conduction, thus forcing adoption of survival strategies for dessipation of
body heat (Thatcher and Collier, 1981). Mechanisms of heat losses include
behavioral changes, alteration of body fluid and volume and blood
distribution, and decrease feed intake to decrease heat production (Collier
et al., 1982). The latter act results in lower production or growth and
probably less production per unit of feed
(Morrison, 1983). Heat
generated from metabolic functions associated with lactation, growth, and
maintenance are not exchanged readily in hot environments, and cows
often become hyperthermic when they are exposed to heat stress (Badinga
12
et al., 1985). A high producer will have a heat production level perhaps
1/3 to 1/2 greater than a lower producer but still must dissipate this heat
from the same surface area of the animal (Johnson et al ., 1967). When
the
level
of
milk production was compared with the temperature-
humidity index (THI), there was a greater decline in milk production per
unit increase in THI above 74.5 for high than for low producing animals
(Johnson et al ., 1962). Climatic conditions appeared to have the greatest
influence in the first 60 days of lactation (McDowell et al., 1975). During
this
period, high temperatures restricted feed intake causing a rapid
utilization of body reserves and high losses in body weight. Conversely,
cold temperatures stimulated feed intake resulting in higher yields and
gross efficiency
than for moderate or high temperatures. Armstrong
et al. (1988) reported that, at peak daily ambient temperatures of 42 to 50
0
C, milk production was greater for cows cooled by evaporative cooling
than for contemporary control cows maintained under an open shade.
After
about 60 days of lactation,
environmental
feeding
was
the primary
variable limiting performance irrespective of climate
(McDowell et al., 1975).
Heat stress of lactating cows results in dramitic reductions in
roughage intake and rumination.
Decreases
in roughage intake
contribute to alteration in ratio of acetate/propionate which affects levels
of fat in milk. Rumen pH also declines during thermal stress. Electrolyte
concentrations, in particular sodium and potassium , are also reduced in
rumen fluid of heat stressed cattle (Collier et al ., 1981). The increase in
water intake (Johnson and Yeck, 1964; McDowell, 1972) and respiration
rate (see below) due to increased environmental temperature, lead to the
13
primary concommitant reductions in feed dry matter consumption
(Roman-Ponce et al., 1977; Beede et al., 1981). Increased water intake
assists by its direct cooling effect on the reticulorumen (Bianca, 1964) and
because water serves as the primary vehicle for heat dissipation via
panting and sweating. These effects may be contributed to changes in
acid-base and electrolyte (Joshi et al., 1968; Johnson, 1970; Jenkinson
and Mabon, 1973; Singh and Newton, 1978) balance during heat stress.
Estimated total K loss via the skin was about 11.5% of K intake during a
short period of extreme heat stress (40 0C); this compares with about 1%
or less when air temperature was 25 0C or lower (Jenkinson and Mabon,
1973). The loss of K would be greater with a more natural chronic heatstress.
High plasma levels of epinephrine and norepinephrine were
reported following acute heat-stress exposure in the bovine (Alvarez and
Johnson, 1973). These catecholamines activate sweat glands of cattle
(Allen and Bligh, 1969), and may be involved in regulating sweat gland
activity.
1.2. EFFECT ON RECTAL TEMPERATURE AND RESPIRATION RATE:
Ambient temperature
within the animal lower and upper critical
temperature range is regarded as the zone of thermoneutrality. Within
this zone, minimal
physiologic
cost
and
maximum productivity
normally are achieved for dairy cows, this rangs is between 5 and
24 0C (Johnson, 1976).
Above the upper
critical temperature, the
increased respiratory rate and rectal temperature concomitant with
decline of milk yields and reproductive performance have been observed
in dairy cows. These measures traditionally are used to indicate heat
14
stress (Maust et al., 1972; Collier et al ., 1982; Bitman et al., 1984).
Monty and Wolf (1974) reported that the body temperature of heatstressed lactating cows began to increase when the enviromental
temperature exceeded 30 0C, but similar increase did not occur in heatstressed non-lactating cows. As THI increased from 70 to 82, rectal
temperatures increased in a linear manner from 38.4 to 40.0 0C (Stott et
al ., 1961 ; Cavestany et al ., 1985). El-Nouty et al. (1990a) reported that
mean rectal temperature of Holstein cows increased 0.12 0C for each unit
increase in THI .
DeDios (1984) showed rather clearly a positive relationship (r=+0.37;
P<0.01 ) of increase in rectal temperature on a hot day and level of milk
production. Thus the combination of the greater endogenous heat load of
a higher producer and the external heat load tend to increase the rectal
temperature more in higher lactating animals. Of course there are
individual differences in rectal temperature rise and level of production
which suggest a genetic factore and a production level factor which
influence the animals ability to maintain thermal stability. It is reasonable
to assume that 30 kg of milk per day producers whose body temperatures
only increase by 0.5 0C or less would be most thermally adapted for more
adverse low tropical climates.
Lee et al. (1976) indicated that the respiration rate of dairy cattle was
significantly affected by the season of the year. Chikamune et al. (1986)
reported that the changes in respiration rate in cattle and buffaloes
associated with seasonal changes in the air temperature indicate the
existence of highly significant correlation of this physiological response
with air temperature.
15
Cows under heat stress express higher rectal temperatures and
respiration rates than those maintaind under cooling conditions (Bianca,
1968; Wise et al ., 1988). Panting tends to alter alveolar ventilation which
subsequently alter blood pH, O2, and CO2 and hence, acid-base balance.
1.3. EFFECT ON BLOOD CELLULAR CONSTITUENTS:
Lactation and environmental conditions, mainly seasonal variations in
air temperature, are considered to be physiological stressor which affect
the animal’s biological system. Both stressors are known to exert
significant influence on blood haematological characteristics of dairy
cattle(El-Nouty et al., 1986, 1990a; Lee et al., 1976; Shaffer et al., 1981).
El-Nouty et al. (1986), working on Holstein cows in Egypt, found
significant
seasonal
differences
in
blood
cellular
constituents.
Haemoglobin (Hb) concentrations were slightly higher in spring than in
summer and they attributed the decrease in Hb concentration during
summer to the reduction in cellular oxygen
consumption
and
the
consequent decline in tissue heat production during hot season. They also
found that the values of PCV and
RBC were significantly lower in
summer than in winter which could be related to the heat-induced
reduction in animal`s metabolic rate. Furthermore, Roussel et al. (1970)
suggested that the depression in blood Hb and PCV of cattle subjected to
elevated ambient temperature to be due to the haemodilution effect where
more water is transported into the circulatory system for evaporative
cooling. In Saudi Arabia, the rise in ambient temperature during summer
was associated with lower haematological values of Holstein cows than
those values during the moderate winter season (El-Nouty et al., 1990a).
16
These seasonal variations were significant for Hb, PCV (P<0.01), MCV
and MCH (P<0.05), but not for RBC or MCHC values, suggesting the
association of these depressions with the reduction in cellular oxygen
requirements in order to reduce metabolic heat load and, consequently, to
compensate for elevated environmental heat load (Lee et al., 1976). The
decline in Hb and PCV with elevated ambient temperature during
summer agrees with other reports (Lane and Campbell, 1969; Roussel et
al., 1972; Lee et al., 1976; Shaffer et al., 1981; Singh, 1983; El-Nouty et
al., 1986, 1990a; Tripathi et al., 1987). Rowlands et al. (1974 ) and Hassan
et al. (1981), however, reported an elevation in the concentration of blood
Hb of dairy cattle during the hot season. The increase in blood Hb during
hot season was considered as a defence mecanism to improve the blood
capacity to carry oxygen to overcome the low blood oxyhaemglobin level
(Hassan et al ., 1981).
1.4. EFFECT ON BLOOD NON-CELLULAR CONSTITUENS:
Roussel et al. (1972), using Holstein cows in Louisiana, USA where air
temperature ranges from -4 0C in cool season to 38 0C in hot season,
reported that serum total protein and globulin increased significantly
from cool to hot season. Serum albumin and albumin to globulin ratio
exhibited a significant linear decrease with the increase in ambient
temperature. Shaffer et al. (1981) working on four breeds of dairy cattle,
found that total protein was highest in hot season, medium in intermediate
and lowest during cool season . Also, they found a decline in blood
glucose level of dairy cattle with the rise in enviromental temperature. On
the other hand, plasma osmotic pressure and
glucose level were
17
significantly higher for no shade treatment cows than cows under shade
(Niles et al., 1980).
1.5. EFFECT ON BLOOD HORMONE CONCENTRATIONS :
A number of experimental conditions have been used to evaluate
hormonal secretion during heat stress including short-term temperature
modification using environmental chamber, seasonal comparisons of
hormonal-profiles and the use of micro climatic modification during
period of heat stress. Differences in experimental conditions have
presumably contributed to the disparity of results that are found in the
literature concering hormonal secretions during heat stress.
Progesterone concentrations have been reported to be elevated in cows
under heat stress (Monty and Wolff, 1974; Roman-Ponce et al., 1981;
Johnson, 1984). Others have reported a decrease in progesterone
concentrations during heat stress (Stott and Wiersma, 1973; Rosensberg
et al., 1977; Folman et al., 1983). Similar paradoxes exist in the data for
estradiol (Christison and Johnson, 1972; Gwazdauskas et al., 1981;
Roman-Ponce et al. 1981; Folman et al., 1983) and cortisol concentrations
during heat stress (Stott and Williams, 1962; Christison and Johnson,
1972; Stott and Wiersma, 1973; Abilay et al., 1975; Roman-Ponce et al.
1981). Wise et al. (1988) found no changes in serum concentrations of both
progesterone and estradiol throughout the estrous cycle of lactating
Holstein cows maintained under heat stress or under cooling condition.
Serum cortisol concentrations were higher in the heat stressed cows (Wise
et al., 1988).
18
During hot weather, plasma concentration of thyroid hormones (T3,T4)
are depressed (El-Nouty and Hassan , 1983; Bell et al., 1985). This
appears to be related to the reduction in basal metabolism. Similar results
were obtained by El-Nouty et al. (1990c). Johnson and Vanjonack (1976)
stated that the thyroid function in the lacating
animals
showed
a
general depression in the summer months and was normal or elevated
during winter months .
Reduction in plasma aldosterone obseved during heat stress indicate
the cows may by in marginal K status (Beede et al 1982; El-Nouty et al .,
1980). El-Nouty et al, 1980 reported that Holstein cows subjected to 35 0C
had reduced plasma aldosterone, which was associated with a significant
reduction in serum Na, increased urinary Na excretion and decreased K
in serum and urine. Reduced plasma aldosterone aids in conservation of K
at the expense of Na.
2. POTENTIAL ROLES OF MACROMINERAL ELEMENTS DURING HEAT
STRESS :
Potassium and sodium are important, especially during heat stress, as
they are major regulators of body water balance (Vander, 1980). Sodium
is required at the kidney for K conservation and to balance bicarbonate
excretion electrically (Masero and sieget, 1977; Vander, 1980)
Heat
stress reduced the rate of blood flow to the uterus (Roman-Ponce et al.,
1978), the mammary glands (Lough et al., 1990), and the portal drained
viscera (McGuire et al., 1989). Further more, blood flow to the mucosal
tissues of the ovine stomach was reduced by heat stress (Alexander et al.,
1987). Evidence suggested that the amount of feed consumed also
19
influenced the rate of blood flow from the digestive tract (Bensadoun and
Reid, 1962; Lomax and Baird, 1983; Huntington, 1984). Homeostatic
mechanisms attempt to maintain body temperature during heat stress by
reducing feed consumption (McDowell, 1972); consequently, intake of
essential nutrients is reduced. A key consideration in the provision of
mineral elements for productive functions is heat stress, amount of feed
intake, or both, which affect quantities of mineral elements taken up from
the digestive tract into blood (Sanchez et al., 1994).
During heat stress and the associated perturbations of acid-base
physiology (Sanchez et al., 1994), the demand for cations by the kidney is
increased. In order to correct the acid-base imbalance, excretion of the
anion HCO3 in urine increases dramatically. Because of the alkalemia
resulting from respiratory alkalosis, the rate of renal excretion of
hydrogen ion is depressed. However, the excretion of HCO3 must be
accompanied by the excretion of a cation. Sodium or K is a possibility;
however, Na is more likely. During heat stress, urine concentrations of K
decline (El-Nouty et al., 1980). Plasma K concentration decreases, which
may have reflected a need for K in other processes, such as sweating
(Johnson, 1970; Jenkinson and Mabon, 1973; Mallonee, 1984; Mallonee et
al., 1985). El-Nouty et al. (1980) noted that, during prolonged heat stress,
plasma aldosterone concentrations of Holstein cows declined. This decline
was associated with reduced concentrations of Na and K in blood serum
and of K in urine. However, urinary Na excretion increased, perhaps to
aid in the conservation of K. Urinary excretion of Na increased 80% in a
hot environment compared with excretion in a cooler environment, but
urinary excretion of K increased only 18% ( West et al., 1992). The
20
increased demand of Na for renal excretion and of K for sweating are
consistent with the idea that dietary requirements of each increase during
heat stress (Joshi et al., 1968; Singh and Newton, 1978; Sanchez et al.,
1994). During heat stress, in addition to increased water intake, urinary
excretion was also increased (Van Kampen, 1981). Both responses may be
potentiated by high K intake, as K is known to stimulate water
consumption (Austic, 1979) and urine production (Kondo and Ross, 1962)
in a thermoneutral environment.
Potential deficiencies of Na and K my by implicated in lower milk
production during hyperthermia. Under conditions of heat stress, Beede
et al . (1983a) observed that increasing the K level of the diet from 0.8
to 1.2 % improved DM intake and milk production of cows in early
lactation. Also, Schneider et al. (1984) again using cows subjected to heat
stress found an increase in DM intake and milk yield when dietary K
increased from 1.3 to 1.8%. It
is
apparent that the lactating cow
requirement for K varies depending on the degree of heat stress.
Potassium content of milk is about 0.15 % ( Miller, 1979), which may
represent 15 to 40% of total daily intake of lactating cows, depending on
production and feed intake. Only small amounts of K are stored in the
body, so quantity of K in the diet can influence amounts of milk produced
(International Mineral and Chemical Corp., 1981). Milk yield increased
when cows subjected to heat stress received supplemental K (Collier et al.,
1982). Erdman et al (1980a) found that increasing dietary potassium
increased dry matter intake while dietary sodium had no effect on the
intake.
21
Also, in chicken, heat stress depresses plasma K concentration (Huston,
1978; AitBoulahsen et al., 1989), enhances urinary K excretion, and
reduces body K retention (Deetz and Ringrose, 1976; Smith and Teeter,
1987).
3. POTASSIUM FUNCTIONS AND REQUIRED LEVELS
Potassium is the principal cation of the intracelluar fluid; but it is also a
very important constituent of the extracellular fluid because it influences
muscle activity, notably cardiac muscle. It is the muscle tissue which
serves as the principal store of potassium in the body.Within the cells it
functions, like sodium in the extracellular
fluid, by influencing acid-
base balance and osmotic pressure, including water retention, and also in
the metabolic processes taking place in the cells (especially in the
metabolism of carbohydrates through activation of ATPase). High
intracellular potassium concentrations are also essential for protein
biosynthesis by ribosomes. If added to protien deficient diets, the gain in
weight of piglets and their feed utilization improves (Leibgol’ts, 1967). A
number of enzymes, including the glycolytic enzyme pyruvate kinase,
require K for maximal activity. Potassium ions, in conjunction with
sodium ions, participate in producing the “resting potential” and “active
potential” in nerve and muscle function. Potassium ions in the
erythrocytes effect the transport of oxygen and CO2 by haemoglobin.
The role of potassium in dairy cattle nutrition was reviewed by Ward
(1966) who indicated that K represents an important fraction of cation
contents of the rumen fluid, so maintaining
a desirable medium for
bacterial fermentation. It is believed that potassium is indispensable for
22
normal microflora activity, in particular for cellulolytic microorganism.
The maximum concentration of K ion in the rumen fluid is noted 3-4
hours after feeding; it is connected with the content of common salt in the
diet (Poutiainen, 1970). Hubbert et al. (1958) have shown that K is
essential for cellulose digestion in an in vitro system. Maintenance of
osmolarity within plasma is important to maintain a desirable moisture
content of the rumen fluid (Balch and Johnson, 1950; Nicholson et al.,
1960). Balch and Johnson (1950) have shown that a higher moisture
content favours cellulose digestion by the cow. Potassium might be
somewhat more effective than Na in this case due to its slow absorption
from the rumen. Its apparent absorption (digestability) varies with its
intake with the feed (Paquay et al., 1969). In ruminants, the potassium
concentration is 25-75 meq/l of rumen fluid, the principal source of the
element being the food. Its intake with the saliva is small (Hennison and
Lewis, 1961). When the concentration of potassium in the diet of cows was
increased from 174 to 425 g/day, the digestive processes became
intensified (Sineshchekov, 1965). The amount of chyme formed per kg of
dry substance increased (owing to the increased secretion of digestive
juices), the contents of potassium and water in the body increased, and the
sodium concentration decreased. The overall quantity of chyme passing
through the intestine in 24 hours increased by a factor of 1.5
(Sineshchekov, 1965).
Ward (1966) suggested a minimum K requirement for lactating cows of
0.6% of the dietary DM intake. Subsequently the minimum requirement
of K for lactating cows was established at 0.8% of dietary DM intake
(Dennis et al., 1976; Erdman et al ., 1980b). This is slightly higher than
23
the dietary concentrations of 0.5% and 0.6% deemed adequate for sheep
and beef cattle, respectively (Telle et al., 1964; Roberts and St. Omer,
1965). A potassium content of 0.7% appear to be adequate for cows in
mid to late lactation; however, the feed intake and potassium in serum
indicate that 0.7% dietary potassium may not be optimal for high
producing cows in early lactation (Dennis & Hemken , 1978). When
0.66% level of potassium was fed to groups previously fed 0.45 and 0.55%
potassium, feed intake increased by 3.0kg and 1.3kg, respectively, but
increased by 0.2kg in the group previously fed the 0.66% level (Dennis,
1975). Feed conversions indicate that the 0.55% K ration would be the
most efficient; however, these calculations ignore changes of body weight.
Since feed intake is frequently the limiting factor for high producing dairy
cows, differences in intake and body weight gains should be considered in
interpreting the results for use over the entire lactation. Milk production
was not affected by feeding the diets containing 0.045, 0.55 or 0.66% K
(Dennis et al., 1976). Du Toit et al. (1934) reported that a ration providing
K as 0.32% of the dry matter was adequate to maintain milk production
of 7.57 liters per day over a period of two lactations. This ration had no
effects on appetite. Another study (Pradhan and Hemken, 1968) observed
lower, but statistically nonsignificant, daily milk production which about
paralled a significantly lower feed intake in cows fed a K deficient ration
(0.26% K). Fat content of milk in all these studies was not affected
significantly by the level of dietary K.
The decrease in dietary sodium and potassium level are related to
increases in loss of urinary sodium and loss of skin potassium
(Collier et al ., 1981). It was calculated (Ward, 1966) that urinary K as a
24
percentage of total K output for non-lactating cows was 86% and for
lactating cows K output in urine represented 75%, feces 13%, and in
milk 12% of the total. Fecal K seems to be arise principally from
endogenous sources (Ward, 1966). Brounwer (1961) suggests that K
excretion is higher in cattle due to the highest fecal water excretion. There
is an obligatory excretion by cattle of K, both in feces and urine
(Campbell and Roberts, 1965). Some K excretion is probably necessary to
prevent sever alkalosis (Pickering, 1965). The extracellular concentration
of hydrogen ions can affect the entry of potassium into the cell (Bishop et
al., 1985). A high hydrogen ion concentration (acidosis) depresses the
secretion of potassium by the kidney because it is trying to rid the body of
the excess hydrogen ions. This leads to retention of potassium in the body.
The increased amount of the hydrogen ions competing with potassium ions
to enter the cell impedes, also, the entry of potassium into the cell, which
increases the plasma potassium concentration. The concentration of K in
the urine of ruminants varies strongly with its intake in the feed
(Kochanov, 1974).The relative proportion of K in the overall ionogram of
the urine strongly increases with age (Kochanov, 1974).
Potassium interacts with several other minerals including sodium,
magnesium, calcium and phosphorus in ruminants and non-ruminants
(Gruner et al., 1950; Jacobson et al., 1972). Addition of Na to ruminant
rations has had a negative effect on K absorption (Fontenot et al, 1960;
Renkema et al., 1962; Campbell and Roberts, 1965; Scott, 1970; Grace,
1988). Extremly high deitary K appears to have a positive effect on Ca
(Newton et al., 1972; Pradhan et al., 1974), and a negative effect on Mg
balance (O’Dell et al., 1960; Commonwealth Bureau of Nutrition, 1971),
25
while low levels (<1.5% K) appear to have a negative effect (Paquay et al.
1968; Pradahan et al., 1974). Normally , K has no specific effect on Mg
requirement. Evidence existed, also, for a K by Cl interaction on milk
yield (Tucker et al., 1988). With a relatively high Cl diet (1.25%), a
positive linear increase in milk yield occurred with increasing dietary K
(from 0.73 to 1.91% K). Paquay et al. (1969) observed a positive
correlation between K and Cl in the urine of cows, to maintain electrical
neutrality of the urine. Physiological needs for K and Cl may have
accounted for dietary K by Cl interaction on lactational performance
(Sanchez et al., 1994).
Increasing K levels in the diet increased water consumption and urine
output (Fisher et al., 1994), which has major implications for water
management. Fisher et al. (1994) found that water intake of cows fed
midium (3.1%) and high (4.6%) K total mixed ration (TMR) increased
(P<0.05) compared with the cows fed the low (1.6%) K TMR. This
observation supports the general view of Russell and Chow (1993) that
supplementation with either Na or K bicarbonate results in an increase in
water intake. Murphy (1992) did not mention dietary intake of K as a
factor which stimulates water intake. Association with the feeding of
medium and high K TMR (Fisher et al., 1994) was a marked increase in
urine output (P<0.05) presumably required by the need to excrete K (St.
Omer and Roberts, 1967). The concentration of Na in the urine tended to
be lower when the high K TMR was fed but total excretion of Na was
greater (P<0.05), when the medium and high K TMR were fed (Fisher et
al., 1994), with the Na loss being less (P<0.05) for cows on the high K diet
compared with those on the medium K diet. Decreases in the urinary
26
excretion rate of Ca and Mg with dietary additions of potassium chloride
had been observed by Tucker and Hougue (1990) and also by Fisher et al.
(1994), but physiological cause of this decreased excretion has not been
postulated. The retention of Ca and Mg was not influenced by the level of
K in the diet (Fisher et al., 1994). The higher losses of Mg in the feces was
made up for by less Mg being lost in the urine. The retention of Na was
lower (P<0.05) when cows fed the two diets supplemented with potassium
carbonate. However, St. Omer and Roberts (1967) found that potassium
carbonate supplementation did not influence Na balance. There was a net
loss (P<0.05) of K from the cows fed the medium K TMR compared with
the feeding of either the low or high K TMR in Fisher et al. (1994) study.
The amount of the four elements excreted in feces, urine and milk in the
latter work were similar to those observed for K (Paquay et al., 1969), Na
(Lomba et al., 1969), Mg (Lomba et al., 1968), and Ca (Paquay et al.,
1968). It was concluded that high K levels in the diet interfere with the
absorption of Mg but not Ca. However, K did appear to interfere with the
utilization of Ca as indicated by the lower amount of Ca in the milk and
urine.
The total cationic concentrations (K+Na+Mg) in milk, blood plasma,
and whole blood remained essentially the same during the low and
adequate potassium diet (Pradhan and Hemken, 1968). The decrease in
potassium concentrations in the milk and blood plasma when fed the
deficient diet (high concentrates) was
increases
compensated
primarily
by
in sodium concentration. There was negative correlation
between potassium and the two other elements (Na, Mg) in milk during
27
periods of low as well as adequate potassium diets
(Pradhan and
Hemken, 1968).
The toxicity of K has not been a major concern (Ward, 1966) because of
the ability of ruminants to excrete excess amounts of K in the urine
(Paquay et al., 1969). However, a number of studies (Tomas and Potter,
1976; Wylie et al., 1985; Rahnema and Fontenot, 1990) have
demonstrated an impairment of Mg absorption when excess K was
infused into the rumen. Newton et al. (1972) found that the apparent
absorption of magnesium Mg was decreased markedly by increasing the
K content of diet from 0.6 to 4.9%. The absorption of Ca did not appear
to be as sensitive to increasing levels of dietary K (Greene et al., 1983b;
Rahnema and Fontenot, 1990). Concern has been expressed by the
National Research Council (1989) that dietary K levels greater than 3%
may interfere with the utilization of both Ca and Mg.
Serum cation concentrations were not affected by feeding diets
containing 0.45, 0.55 or 0.66% K (Dennis et al., 1976). There was a linear
response in plasma K levels (P<0.05) with increasing level of K in the diet
of med-lactation Holstein cows. This effect of excess dietary K on plasma
Mg levels was similar to that observed by Greene et al. (1983a), but the
effect on plasma Ca was not as great as the decrease observed by Greene
et al. (1983b) or Newton et al. (1972).
Covariance analysis, based on pre-experimental values, of blood serum
cations did not reveal any significant differences due to the K content of
the rations (Dennis, 1975). The serum K, Na, Mg and Ca concentrations
were within the ranges normally given for these cations. Significantly
28
lower plasma K was in K deficient cows (Pradhan and Hemken, 1968),
but no consistent trends were in Dennis (1975) serum K data.
Furthermore, dietary K level did not influence blood hematocrit and
plasma glucose levels; however, there was a drop (P<0.05) in plasma urea
nitrogen when a high K diet (4.6%) was fed compared with the low (1.6%)
and medium K (3.1%) diets (Fisher et al., 1994). This could be due to
decreased intake and the selection against the concentrate portion of the
high K diet.
The principal mechanism determining the homeostasis of K in the
body is at the level of the kidneys. Its regulation involves the mineral
corticoids- aldosterone and deoxycorticosterone. The effect of these
hormones on the excretion of K ions is probably secondary, being derived
from their effect on the re-absorption of Na ions in the renal tubules.
Nevertheless the two processes are interconnected because the secretion of
aldosterone is stimulated only if Na level in the plasma decreases and the
K level increases simultaneously. By eliminating excess K ions through the
kidneys, the regulating mechanisms maintain a constant Na:K ratio in the
extracellular fluids. Mineral corticods probably influence the regulation
of membrane permeability and the mechanism of the sodium pump
(Jaquot et al., 1960).
4. EFFECTS OF MINERAL ELEMENTS IN DRINKING WATER DURING
HEAT STRESS
Water serves as the major vehicle for heat dissipation by evaporation
and is the principle component of milk (Beede, 1992). High water salinity
(total dissloved solids, TDS) affected osmotic balance in rats and high Cl
29
in drinking water was less deleterious than SO4 (Heller, 1933). Heller also
reported that dairy cows could adapt physiologically to survive when
given water with 15,000 ppm of NaCl. Later, Frens (1946) found that
10,000 ppm of NaCl was the maximum amount tolerated before milk yield
declined. Water intake was 7% greater, but feed intake and milk yield
tend to be less (Jaster et al. 1978), for cows offered high saline water (2500
ppm NaCl) compared with tap water (196 ppm of TDS).
In Saudi Arabia during hot weather, Challis et al. (1987) found that
desalination (reverse osmosis) of drinking water originally containing
4387 of ppm TDS markedly improved cow performance. Desalination
reduced the concentrations of SO4 and Cl anions about 94 to 95%, and
TDS to 434 ppm. Desalination increased water intake of cows by 37%,
green intake by 38%, and milk yield during the first 8 weeks of lactation
by 27% compared with intakes of cows offered control drinking water.
When control water was reintroduced to cows previously receiving the
desalinated water, milk yield dropped about 4 kg/d by the 2nd wk after
the change. This study showed that a combination of high TDS and
particularly high concentrations of SO4 and possibly Cl in drinking water
during hot weather can be deleterious for lactating cows. Based on
measured water intake and concentrations of SO4 and Cl in control water,
cows in the above study consumed 2.1 times more S and 1.1 times more Cl
from drinking water than that recommended for the diet (NRC, 1989).
The desalinated water contributed only about 10% of that recommended
in the diet for each element. High SO4 and Cl could affect acid-base
balance and health status and may have been the cause of reduced milk
yield and grain intake (Challis et al., 1987). Research to determine the
30
maximal amount of SO4 and Cl in drinking water that will not affect
lactational performance and health during heat stress is not available.
Water hardness (contains more than 121 ppm of Mg plus Ca) appears not
to influence animal health or productivity (Graf and Holdaway, 1952;
Blosser and Soni, 1957).
5. EFFECTS OF DIETARY MINERAL BUFFERS DURING HEAT STRESS
Excellent reviews on the use of dietary buffers have been written by
Erdman (1988) and Staples and Lough (1989). The main intent here is to
focus on the potential need and use of some dietary buffers (NaCl, KCl,
NaHCO3, K2CO3, and KHCO3) in diets of cows under heat stress.
Feeding sodium bicarbonate (NaHCO3) improve feed intake and
lactational performance of cows in the hot weather. Escobosa et al., (1984)
showed higher feed intake, water intake, actual and 4% fat-corrected milk
yields and percent milk fat, concentrations of K in blood plasma of cows
fed 1.7% NaHCO3 than did cows fed CaCl2 (2.28%) or NaCl (0.23%)
during natural heat stress. Schneider et al. (1986) reported that
supplementation of sodium bicarbonate (1% of DM resulting in 0.55%
total dietary Na, dry basis) in the diet during hot weather increased feed
intake, actual and 4% fat-corrected milk yields and percent milk fat, and
caused a slight depression in plasma Na concentrations compared with no
NaHCO3 (Schneider et al., 1986), but concentration of K were not
affected. Also, higher 4% FCM yield was detected with NaCl
supplementation than with no added NaCl. When Na was fed, whether as
NaHCO3 or another Na source, bicarbonates and NaHCO3 in saliva
increased, and salivary flow also increased (Emery, 1976). It was
31
suggested that a major effect of feeding NaHCO3 was not its direct effect,
but rather supplying more Na to stimulate salivary buffering capacity,
which may help explain why lactational performance was improved by
either NaHCO3 or NaCl addition (Schneider et al., 1986). Sodium
bicarbonate is a compound known to cause potassium to move back into
the cell and administered in case of hyperkalemia (Henry, 1984). This
effect may restore the level of K inside the cell for better function,
regardless of its extracellular concentration.
West et al. (1986, 1987) compared concentrates containing no buffer,
1.8% KHCO3 , 1.2% K2CO3, or 1.5% NaHCO3 when offered with a
maximum amount (10 kg/d of DM) of a forage blend (50% corn silage,
30% whole cottonseed, and 20% chopped bermudagrass hay, DM basis).
Intake of concentrate was not different among treatments. However,
intake of forage blend was highest with 1.2% K2CO3, higher with 1.8%
KHCO3 than with no buffer, but not different with 1.5% NaHCO3
compared with no buffer or 1.8% KHCO3. Previously, 1.0% KHCO3 in a
total mixed ration (TMR) composed of 25% cottonseed hulls and 75%
concentrate reduced feed intake and milk yield (P<0.01) during hot
weather (Schneider et al., 1986). Milk fat percentage was higher (3.2%)
with 1.2% K2CO3 than with 1.5% NaHCO3 (2.4%; P<0.1). Yield of 3.5%
FCM was higher with 1.2% K2CO3 than with 1.5% NaHCO3 (West et al.,
1986, 1987).
In a second comparison (West et al., 1987), the forage blend (40%)
and concentrate (60%, DM basis) were fed as TMR. The dry matter
intake was higher for cows fed the diet containing K2CO3 than for those
fed the other diets containing KHCO3, or NaHCO3. Yield of 3.5% FCM
32
was higher for cows fed K2CO3 than those fed KHCO3, or NaHCO3. Milk
fat percentage was higher for K2CO3 than for cows fed no buffer; and was
intermediate for cows fed KHCO3 or NaHCO3. Feed intake and
lactational performance in both comparisons (West et al., 1986; 1987)
clearly were superior with K2CO3, followed by KHCO3. Sodium
bicarbonate did not improve lactational performance over that with no
buffer (West et al., 1986).
Bicarbonate is the second (after chloride) most abundant anion in the
ECF. Carbon dioxide is carried in different forms of bicarbonate ion
(HCO3-), carbonic acid (H2CO3), and dissolved CO2, with bicarbonate
accounting for over 90% of the total CO2 at physiologic pH. It is the
major component of the buffering system in the blood. Carbonic
anhydrase in red blood cells catalyzes the reaction of CO2 and H2O to
generate H2CO3-, which diffuses out of the cell in exchange for chloride,
to maintain ionic charge neutrality within the cell. In this way, potentially
toxic CO2 in the plasma can be converted to the effective buffer,
bicarbonate. The latter can be used to help buffer hydrogen ion
imbalances. This buffering by electrolytes is effective only for fine
adjustments. The kidneys are responsible for a large part of the hydrogen
ion homeostasis by constantly replenishing buffer and either neutralizing
or eliminating H+ in the tubules (Bishop et al., 1985). In the kidneys, most
(85%) of the bicarbonate ion is reabsorbed by the proximal tubules, with
15% being reabsorbed by the distal tubules (Guyton, 1986).
6. EFFECT OF DIETARY ELECTROLYTE BALANCE
33
The acid-base balance of animals is dependent upon the balance
between anions and cations in the blood and can affect animal
performance (Relman, 1972; Chan, 1974; Fredeen et al., 1988; Tucker et
al, 1988a; Jackson et al., 1992; Ross et al., 1994; Pauchon et al., 1995).
Although several ions can affect the acid-base balance, sodium, potassium
and chlorine ions are the main elements implecated in such control.
Therefore, the dietary cation-anion (electrolyte) balance (DEB), defiend
quantitively as milliequivalents of [(Na+ + K+) - Cl-]/100 g of diet DM
(Mongin, 1980), is a more important determinant of dietary impact on
systemic acid-base status and performance in lactating dairy cows than
actual dietary concentrations of Na, K, and Cl (Stewart, 1978; Tucker et
al., 1988a; Tucker and Hogue, 1990). An excess of fixed cations cause
alkolosis, while an excess of fixed anions can cause acidosis (Mongin,
1981; Patience et al., 1987; Fauchon et al., 1995). It affected Ca
metabolism in peripartum dairy cows (Lomba et al., 1978; Block, 1984;
Gaynor et al., 1989), and P metabolism in young calves (Beighle et al.,
1988).
In growing steers, intake, ADG, feed efficiency and blood
bicarbonate increased as DEB increased from 0 to 450 meq kg-1 (Ross et
al., 1994). Similarly, intake and daily gains of dairy calves increased as
DEB was increased from 0 to 370 meq kg-1 (Jackson et al., 1992).
Lactating
cows were found capable of maintaing a normal acid-base
status with a DEB of 400-500 meq kg-1 (Fredeen et al., 1988), but lactation
parameters were not presented. In growing lambs, Fauchon et al. (1995)
indicated that diets containing between 500-700 meq (Na+ K- Cl) kg-1
stimulated growth by allowing greater feed intake and greater daily gain
with little effect on nutrient digestibility. West et al. (1992) found also a
34
linear increase in intake of heat-stressed dairy cows as DEB increased
from 100 to 464 meq kg-1.
Block (1984) reported that the incidence of milk fever during lactation
was reduced from 47.4% in cows fed a cationic diet to 0% when cows fed
an anionic diet during the dry period. Conversely, work by Tucker et al.
(1988) showed that a cationic diet (20 meq Na+ K- Cl/100g of feed)
resulted in greater milk yield than when diets containing -10, 0, or 10
meq/100 g were fed. The lower intake observed with the diets containing 100 meq kg-1 (Tucker et al., 1988a) seems to result from inclusion of
CaCl2 in these diets and the same effect of CaCl2 on feed intake in dairy
cattle has been shown by Coppock et al. (1982a) and by Escobosa et al.
(1984), probably by affecting the palatability of the diets, or by the
metabolic acidosis caused by CaCl2 (Mongin, 1981;Yen et al., 1981 ). West
et al. (1991) altered DEB by changing K and Cl content in the diet using
potassium bicarbonate or calcium chloride, reported that milk yield and
4% FCM of primiparous cows improved linearly with increasing DEB
with insignificant treatment by phase (cool or hot environment)
interaction, whereas dry matter intake (DMI) of cows improved
quadrtically with increasing DEB. A treatment by phase interaction for
DMI was detected, although the intake reached a plateau at a similar
DEB during the cool and hot phases.
Fauchon et al. (1995) found an increase in water intake with increasing
dietary cation-anion balance; this seems to be a response to increased
osmolality of rumen contents. However, Forbs (1986) reported that
increased osmolality is associated with a decrease in intake. Higher intake
seems most likely to be associated with acid-base parameters in blood.
35
Kellaway et al. (1977) observed that the addition of bicarbonate to the
diets of young calves increased intake, but also produced changes in blood
acid-base parameters comparable to those observed in Fauchon et al.
(1995) study. It is plausible that changes in blood acid-base parameters
excert a direct effect on hypothalamic areas controlling feed intake. It has
been demonstrated that intake can be manipulated by changes in ionic
composition of cerebrospinal fluid and hypothalamic tissue (Seoane and
Baile, 1973; Seoane et al., 1975). An acidotic acid-base profile would be
associated with a decrease in intake while a mildy alkalotic one would be
associated with an increase.
The response to DEB appeared to be mediated through blood buffering
and impact on physiologic systems of the cow accompanying the
absorption of monovalent ions from the gastrointestunal tract. Absorption
of Na and K is linked to the generation of systemic bicarbonate (HCO3-),
whereas Cl absorption increases systemic free proton (H+) generation
(Guyton, 1986). As a result, as DEB increases, blood pH tends to rise,
whereas a reduction in DEB increases systemic H+ generation and lower
blood pH (Tucker et al., 1988). Consequently, diets that contain the same
DEB, whether achieved by altering concentration of dietary Na, K, or Cl,
should yield similar proportions of HCO3- and H+ and should have similar
effects on systemic acid-base status. This was confirmed (Tucker et al.,
1988) with DEB ranging from -10 to +20 but has not been investigated at
balances above +20. However, investigation of even higher balances has
shown that blood pH continues to increase linearly as DEB increases from
+12 to +64 (Pradhan and Hemken, 1980), but the impact of dietary
mineral concentrations, independent of DEB, has not been evaluated in
36
this range. Addition of sodium bicarbonate to the Cl-containing diet
restored blood pH, blood bicarbonate, and feed intake from decrease to
normal (Yen et al., 1981; West et al., 1991).
37
CHAPTER III
MATERIALS AND METHODS
38
This study was conducted at Al-Aziziah farm, about 120 Km south of
Riyadh, during the summer of 1994. A total of
fourty eight healthy
lactating Holstein cows, past the peak (118-134 day) of the 3rd or 4th
lactation, were alotted to eight groups of six cows in each, assigned
randomly to one of eight experimental diets, according to calving date,
lactation number, and daily milk yield.
The animals were housed in groups under an open shaded barn, but
had no additional mechanical cooling. Before start of experiment, animals
were under cooling system and milked thrice daily. During experimental
time (2 weeks adaptation + 8 wks experimental) cows were milked twice
daily at approximately 0700 and 1700 h. They were offered (adlibitium) a
total mixed ration (TMR, 79% DM), consists of 60% concentrate mixture
and 40% forages (dry basis, Table 1) formulated to meet the current
recommendations (NRC, 1989).
To the basal TMR, potassium chloride (KCl), KCl plus sodium
bicarbonate (NaHCO3), potassium bicarbonate (KHCO3) alone or plus
KCl, or potassium carbonate (K2CO3) alone or pluse KCl buffers were
added as shown in Table 2 forming eight different diets. Experimental
cows were assigned randomely upon these eight diets (6 cows/ diet). Half
of the mentioned diets (A, C, E and G) contained
a lower level of
potassium (1.25% of DM) than (1.75% of DM) the other half (diets B, D,
F and H). Source (buffer) of K supplementation in the diet was in
replacement of an equal proportion of the ground corn.
39
Table 1. Composition of basal diet as dry matter (DM) basis.
Ingredient
% DM
Corn silage
20
Beet pulp
15
Rhodes hay
5
Ground corn
37.2
Soy bean meal
5
W.cotton seed
15
Urea
1.0
Dicalcium
0.5
phosphate
Limestone
1.0
Magnesium 0.2
oxide
Trace minerals
0.1
& vitamins
Total (% DM) 100
CP
1.60
1.20
0.45
3.77
2.70
3.08
2.8
---
Fat
0.38
0.21
0.07
1.58
0.08
3.41
-----
ADF
6.20
5.10
1.55
1.13
0.50
5.18
-----
K
0.248
0.023
0.086
0.208
0.103
0.189
-----
Na
0.040
0.029
0.014
0.054
0.002
0.008
-----
-----
-----
-----
-----
-----
---
---
---
---
---
15.6
5.73
19.66
0.86
0.147
40
Table 2. Description of dietary potassium treatments (level x source)
as a percentage of dietary dry matter.
Diet
KHCO3
K2CO3
NaHCO3
KCl
Total
NaCl
Total
Na
A
B
C
D
E
F
G
H
0
0
1
1
0
0
0
0
0
0
0
0
0.69
0.69
0
0
0
0
0
0
0
0
0.85
0.85
0.75
1.70
0.00
0.95
0.00
0.95
0.75
1.70
K
1.25
1.75
1.25
1.75
1.25
1.75
1.25
1.75
1.00
1.00
1.00
1.00
1.00
1.00
0.40
0.40
0.54
0.54
0.54
0.54
0.54
0.54
0.54
0.54
Total *EB meq.
Cl
1.28
1.72
0.92
1.32
0.94
1.36
1.00
1.40
19.5
19.9
29.7
31.2
29.1
30.1
27.4
29.0
*EB= electrolyte balance (Na+K-Cl).
41
Potassium and sodium bicarbonate supplementation contributed equal
quantities of bicarbonate (0.61% of total dietary DM). Also, Potassium
bicarbonate and K2CO3 contributed equal quantities of potassium (0.39%
of total dietary DM). Total calculated electrolyte balance, EB=(Na+ KCl), was varied between 195 and 312 meq/kg of feed DM (Table 2), due to
the varied levels of K and Cl in the diets. Source of Cl variation among
diets was KCl and/or NaCl. All diets were equal in sodium (Na+ ) by
addition of NaCl to contain 0.54 % Na (recommended level by Schneider
et al., (1986) was 0.55 % Na). Diets A and B had a normal DEB value
(Schnieder et al., 1986), and the other diets had higher than normal DEB
values.
During the eight-week experimental period, daily milk yield (twicedaily machine milking) for each cow were recorded. Milk samples were
collected once a week from each milking and fat content was shortly
analyzed, in order to calculate fat-corrected milk yield (4%FCM). The
quantity of feed refusals were monitored every 24 hours, for each group,
and feed intake was calculated. Excess water
was provided for each
group and intake was measured daily through a water meter connected to
a drinking basin.
Blood
samples were collected at 14.00 h twice a week from the
coxygial vien using 10 ml (100 x 16 mm) vacutainer
tubes [Becton
Dickinson]. Lithium heparin was used as anticoagulant. The tubes were
placed immediately in ice. Whole blood was analyzed soon after collection
for haemoglobin content (Hb), packed cell volume (PCV) , red blood cell
counts (RBC). Blood Hb concentration and RBC counts were determined
using Miniphotometer (MP plus Nr : 25). Packed cell volune was
42
determined
using Hematocret. Mean cell volume (MCV), mean cell
haemoglobin (MCH) and mean cell haemoglobin concentration (MCHC)
were calculated using the formulas proposed by Schalm (1975).
Plasma was obtained by centrifugation of blood at 860 xg for 20 min,
and were stored at -20 C until analyzed. Total protein (TP), albumin (A),
cholesterol (CHOL) and glucose (GLU) in stored plasma were determind
spectrophotometrically
using
commercial
reagent
kits
(Randox
Laboratories Ltd., Ardmore, UK). Globulin (G) concentration was
calculated as the difference between TP and A, and A/G ratio was
calculated.
Inorganic (K, Na, Ca, Mg) plasma constituents were determind by
atomic absorption spectrophotometer (Perkin-Elmer Ltd., UK). For Mg,
Ca and K determinations, the plasma was diluted 50 folds, and for Na 500
folds, in 0.25% strontium chloride. Chlorine and phosphorus were
determined in plasma using commercial kits furnished by Diagnostica
Merck; E. Merck, P.B. 4119; D-6100 Darmstadt 1, W. Germany). Urine
was collected for two consecutive days within the last two weeks of the
experiment, only from 3 cows of each group. Concentrations of Na, K, and
Cl in the urine were determined also by the same atomic absorption
spectrophotometer.
Plasma hormone levels, thyroxine (T4), triiodothyronine (T3) and
cortisol were measured by a direct solid phase I125 based radio
immunoassay method using commercial kits (Diagonistic Products Co.,
Los Angeles, CA, USA). Pooled plasma were included routinely in the
assays and the coefficients of variation for intra- and inter-assay were
43
respectively 8.2 and 8.0% for T4, 7.6 and 8.2 for T3, and 8.5 and 8.9% for
cortisol.
Maximum and minimum ambient temperatures, relative humidities
and solar radiation (black globe temperature) were
through
out the experimental period. Dry
recorded
daily
(Tdb)and wet (Twb) bulb
temperatures were measured at 9.00 and 16.00 h of two days a week to
calculate the temperature-humidity index (THI) using the following
equation by Maust et al. (1972):
THI = 0.72 (Tdb + Twb) + 40.6 .
THI is used as an indicator for environmental heat load on animals with
valus over 72 for dairy cows (Johnson, 1987). Also rectal temperature and
respiration rate were measured before blood sample collection, using a
clinical thermometer.
Data were subjected to statistical analyses at King Saud University
Computer Center using SAS program (SAS, 1986). The least-Squares
means (LSMEANS) and the simple correlation (CORR) procedures were
applied to the data. The following model was used:
yijkl = u+ Ki + Bj + DEBk+ Ki*DEBk+ Cm + eijklm,
where, yijklm is the mth observation of the lth cow fed the jth diet (buffer)
that had the ith potassium level and the kth electrolyte balance (DEB); u is
the overall mean; Ki is the effect of the ith level of potassium in the diet
(I=1 and 2); Bj is the effect of the jth buffer (source of K, j=1, 2, ..... and
8); DEBk is the kth DEB level (k=1 and 2); Ki*DEBk is the interaction
between dietary potassium and elctrolyte balance; Cm is the effect of the
mth cow (m=1, 2,.......and 48); and eijklm is the residual term.
44
The G/A and T3/T4 ratios have a binomial, rather than a normal
distribution, therefore the square root of each ratio was transformed to its
arcsine to achieve normality assumption.
45
CHAPTER IV
RESULTS AND DISCUSSION
46
Table 3 shows the meteorological data recorded during the 8-wk
experimental period (4th of August until 28th of September, 1994). The
average maximum air temperature during the whole experimental period
was 45.37 0C with a range of 41.57 to 48.31 0C. The corresponding values
for the minimum air temperature were 22.36, and 21.21 to 24.5 0C.
Average maximum and minimum relative humidities were 27.23 and
14.15 %, respectively. The sunrise average hours was 9.36 hrs. The
average black globe temperature (solar or radiation temperature) was
43.28 0C with a maximum value of 47.75 and a minimum value of 39.14
0
C. The calculated temperature-humidity index (THI) averaged 91.35
units and ranged between 90.3 and 97.8 units. These values indicated a
heat stress environment on animals as it exceeded the upper critical limit
(72 units) for dairy cattle (Johnson, 1987). Under almost similar THI
values, El-Nouty et al. (1990c) working with lactating cows in a herd very
close to vacinity of the herd of the present study, indicated a significant
rise in rectal temperature of cows during the summer hot season
compared with that during the winter season, which in turn caused a
respiratory enhancement. The average rectal temperature (Table 3) of the
cows of the present study was 39.86+ 0.07 0C, which is significantly higher
than the average rectal temperatures (39.12 0C) of a selected group of
lactating cows from the same herd during the less heat stress spring time,
in which THI value was 71.2 units, and than those reported by El-Nouty et
al. (1990c). Cows apparently were unable to dissipate sufficient heat to
prevent
a rise in body temperature during the experimental period. Cows were
experiencing a degree of hyperthermia as respiration rates (Table3) were
47
well above those reported for cows in the thermoneutral zone (Brody,
1945). The increase in rectal temperature could probably be due to the
higher metabolic heat load induced by both lactation and environmental
conditions (Berman et al., 1985).
48
Table 3. Means and standard errors for daily ambient temperatures (oC), the
temperature-humidity index (THI), rectal temperature (oC) and respiratory rate
(no/min) during the eight-week experimental period.
WEEK
Tmin*
Tmax
Tr
Tdb
Twb
THI
RT
RR
1 24.50+0.49 48.31+0.55 47.75+0.65 41.00+0.66 38.50+1.88
97.84+1.62 40.22+0.09 85.64+2.11
2 23.00+0.49 47.00+0.55 44.93+0.65 40.00+0.66 33.00+1.88
93.16+1.62 40.34+0.09 91.39+2.11
3 22.37+0.49 46.06+0.55 43.50+0.65 37.00+0.66 37.00+1.88
93.88+1.62 39.83+0.09 84.90+2.11
4 23.75+0.49 46.62+0.55 43.37+0.65 37.00+0.66 25.50+1.88
85.60+1.62 39.78+0.09 86.56+2.11
5 21.50+0.49 46.00+0.55 42.62+0.65 35.00+0.66 31.00+1.88
88.12+1.62 39.67+0.09 90.63+2.11
6 21.25+0.49 43.18+0.55 42.37+0.65 39.00+0.66 29.00+1.88
89.56+1.62 39.70+0.09 92.52+2.11
7 21.31+0.49 44.25+0.55 42.50+0.65 37.00+0.66 35.00+1.88
92.44+1.62 39.56+0.09 85.05+2.11
8 21.21+0.49 41.57+0.55 39.14+0.65 35.50+0.66 33.50+1.88
90.28+1.62 39.78+0.09 88.94+2.11
_______ _________ _________ _________ _________ _________
Overall 22.38+0.41 45.44+0.52 43.34+0.60 37.75+0.33 32.81+0.94
_________ _________ _________
91.41+0.81 39.86+0.07 88.20+1.49
*Tmin = minimum temperature; Tmax = maximum temperature; Tr = Solar radiation temperature;
Tdb= dry bulb temperature; Twb = wet bulb temperature; THI = temperature-humidity index;
RT = rectal temperature; RR = respiratory rate.
49
1. EFFECTS OF DIETARY POTASSIUM LEVEL
1.1. Production Responses:
Least squares means of daily
feed intake, milk yields and fat
percentage, and feed efficiency are presented in Table 4.
1.1.1. Feed intake
Daily feed intake was not affected by 1.25% compared with 1.75% K in the
diet (Table 4). Mallonee et al. (1985) reported that total daily feed consumption
of Holstein cows increased 8.5% as dietary K increased from 0.66 to 1.08%
but then decreased 4.8% as K content increased to 1.64%. This later results
indicated that 0.66% K was too low to maximize dry matter intake, but 1.64%
was not more effective than 1.08% K. The normal current requirement is 0.8%
K (NRC, 1989). In another experiment (Schneider et al., 1984) with heat stress
lactating cows, 1.8% K resulted in a significant 5.1% increase of feed intake
compared with 1.3% K, indicating a need for a higher K level in diets during
heat stress time. These later dietary K levels were close to those levels of the
present study. However, no significant change in feed intake was observed in
the present study due to level of dietary K as more stressful heat condition
was imposed on cows compared to that
of Schnieder et al. (1984). Mallonee et al. (1985) found that responses of cows
to increasing dietary potassium in shade were small relative to that under no
shade.
50
Table 4. Least-squares means and standard errors for daily feed intake (DFI),
milk yield, fat percentage, fat-corrected milk (FCM), and feed effeciency (FE), as
affected by dietary potassium (K) level in heat stressed Holstein cows.
1.25% K
1.75% K
DFI (kg DM)
17.44±0.09A
17.29±0.09A
Milk (kg):
Morning
10.89+0.16A
10.07±0.16B
Evining
10.87+0.27A
9.83±0.22B
Total
21.76+0.40A
19.89±0.40B
Fat %
Morning
3.64+0.06A
3.50±0.06A
Evening
3.48+0.06A
3.30±0.06B
3.56+0.05A
3.40±0.05B
4% FCM
20.31+0.34A
18.07±0.34B
FE
(kg DM/kg milk)
0.82+0.02A
0.88+0.02B
Average
* Different superscript letters indicate significant differences among means
(P<0.05), otherwise indicate similarity.
51
Cows of the present study were kept under shade. It seems also that cows
ate more during night time to compensate for any reduced feed intake
during the hottest time of the day, and this overcame any effect of dietary
K. This could be a reason for not finding significant differences in feed
intake among groups.
1.1.2. Milk Yield and Fat Percentage
Milk yield for evening milking (1700 h), most closely corresponding to
the hottest portion of the day, was reduced 9.6% in the 1.75% K cows
(P<0.01) compared with 1.25% K cows (Table 4). For morning milking
(0630 h), corresponding to the cooler portion of the day, this reduction
was 7.6% (P<0.01). Total average reduction in daily milk yield due to
increased dietary K level from 1.25 to 1.75% (r=-0.37, p<0.01) was about
8.6% (P<0.01, Table 4). Mallonee et al. (1985) reported a curvilinear
trend (P<0.001) for milk yield with increasing dietary K in Holstein cows,
increased 7.4% under no-shade and only 1.7% under shade environment
as dietary K content increased from 0.66 to 1.08%, whereas decreased
slightly in both environments as dietary K increased to 1.64%. Other
experiments with heat stressed cows have shown significant positive milk
production responses to 1.5 compared with 1.0% K (Schnieder et al.,
1984) and 1.8 compared with 1.3% K (Schnieder et al., 1982). Also, under
conditions of heat stress, Beede et al. (1983) observed that increasing the
K level of the diet from 0.8 to 1.2% improved DM intake and milk
production of cows in early lactation. It is apparent that the lactating cow
requirement for K varies depending on the degree of heat stress.
Effect of dietary K on milk fat was insignificant (P>0.1) for morning
milking and significant (P<0.05) for evening milking and average daily
52
milk fat content (Table 4). However, although the correlation between
dietary K content and milk fat content was not significant (r=-0.22,
P>0.05), fat-corrected milk (FCM) was significantly (P<0.0001) reduced
by about 11%, by increasing dietary K level (r=-0.46, P<0.0001) from 1.25
to 1.75% (Table 4). As a collective results, increasing dietary K from 1.25
to 1.75% decreased feed efficiency (P<0.01) from 0.81 to 0.89 kg DM/kg
milk produced (Table 4). Cows under 1.25% K level in the diet were
expressing lower (P<0.05) rectal temperature (39.67 C) than those under
the 1.75% K dietary level (40.05 C), with a slight increase in respiration
rate on the first group (Fig-1). This significant decrease in RT of the lower
K group may explain partly the above result. This suggested that heatstressed cows under the present study may require a dietary K level less
than the 1.75 to maximize milk yield.
1.2. Physiological Responses
1.2.1. Blood Cellular and Non-Cellular Constituents
Levels of K in the diet had no significant effects on any of the cellular
components of blood (Table 5). Also, plasma concentrations of glucose and
total protein did not alter by the variation in dietary K level (Table 5).
The insignificant variations in feed intake may explain this result. No
variations were found among the two groups of cows in the main
components of plasma protein (albumin and globulin).
53
RT (oC)
39.67+0.07A
40.05+0.07B
RR (no./min)
89.88+1.23A
87.09+1.23A
Figure 2. Rectal temperature (RT) and respiratory rate (RR) as affected by dietary potassium (K)
level in heat stressed Holstein cows.
54
However, cholesterol concentrations in plasma were reduced (P<0.01)
with increasing dietary K (Table 5). Fisher et al. (1994) found no
significant effect of dietary K level on blood hematocrit and plasma
glucose levels. Lactation affects kidney function in goats (Maltz et al.,
1981; Olsson et al., 1982) and cows Fettman et al., 1984). The duriation
from normal in the amount of water consumed (Table 6) and urine output
and electrolyte concentrations (Table 7), would activated reninangiotensin system (Olsson et al., 1982), and hence, increases urine
capacity, enabling the cow to improve its osmoregularity capacity in both
groups as indicated by similar plasma volume (Table 5) and osmalality
(Table 6).
1.2.2. Plasma electrolyte concentrations, osmolality and water intake
Plasma Na and K concentrations (Table 6) were not affected by dietary
K level. Dietary Na level was constant for all animals. Plasma values of
both minerals were lower than, but not far from normal levels (McSherry
and Grinyer, 1954), implicating the need for additional dietary potassium
or/and sodium during heat stress in other processes, such as sweating
(Johnson, 1970; Mallonee, 1984; Jenkinson and Mabon, 1973 Mallonee et
al., 1985); otherwise, cows being adapted well to have a lower K or Na
blood content with their excess being secreted in urine or lost through
sweating. During heat stress, urine concentrations of K decline (Huston,
1978; El-Nouty et al., 1980; AitBoulahsen et al., 1989). Also, urinary Na
excretion may be increased, perhaps to aid in the conservation of K, more
with low than high dietary K (see Table 7). El-Nouty et al. (1980) noted
that, during prolonged heat stress, plasma aldosterone concentrations of
Holstein cows declined. This decline was associated with reduced
55
concentrations of Na and K in blood serum and of K in urine. Urinary
excretion of Na increased 80% in a hot environment compared with
excretion in a cooler environment, but urinary excretion of K increased
only 18% (West et al., 1992). Deetz and Ringrose (1976) and Smith and
Teeter (1987) found that heat stress reduces body K retention as well as K
plasma concentration. The increased demand of Na for renal excretion
and of K for sweating are consistent with the idea that dietary
requirements of each minerals (Na and K) increase during heat stress
(Schnieder et al., 1986; Sanchez et al., 1994). There is an obligatory
excretion by cattle of K, both in feces and urine (Campbell and Roberts,
1965). Some K excretion is probably necessary to prevent sever alkalosis,
because K ions are exchanged in the kidney for hydrogen ions and vice
versa (Pickering, 1965). The extracellular concentration of hydrogen
ions can affect the entry of potassium into the cell (Bishop et al., 1985). A
high hydrogen ion concentration (acidosis) depresses the secretion of
potassium by the kidney because it is trying to rid the body of the excess
hydrogen ions. This leads to retention of potassium in the body. The
increased amount of the hydrogen ions competing with potassium ions to
enter the cell impedes, also, the entry of potassium into the cell, which
increases the plasma potassium concentration. It appears that cows in the
present study were in a stat of respiratory alkolosis due to sever heat
stress expressed by some subnormal K levels in plasma.
In an other study (Tucker and Hogue, 1990) an increase (P<0.04) in
plasma K for cows receiving high dietary K (2.17%) compared with a
basal diet (1.32% K) in the form of added KCl was observed. This was in
agreement with results of Deetz et al. (1982) but contrasts with those of
56
O’Connor et al. (1988) and Schneider et al. (1984, 1986), who reported
that plasma K was not altered in response to increased dietary K. The
maximum dietary level of K in the work of Tucker and Hogue, (1990)
was higher than those of Schneider et al. (1984, 1986) and of that of the
present study. The effect of adding different source of K in the diet could
play a significant role in this discrepancy (section 3). Covariance analysis,
based on pre-experimental values, of blood serum cations did not reveal
any significant differences due to the K content of the rations (Dennis
1975). The serum K, Na, Mg and Ca concentrations were within the
ranges normally given for these cations. Significantly lower plasma K was
in K deficient cows (Pradhan and Hemken, 1968), but no consistent trends
were in Dennis (1975) serum K data.
Magnesium and phosphorus concentrations in plasma were not
affected by K levels in diet, but Ca concentration did increase by
increasing dietary K from 1.25% to 1.75% (Table 6, P<0.05). Normally,
K has no specific effect on Mg requirement. Extremly high deitary K
appears to have a positive effect on Ca (Newton et al., 1972; Pradhan et
al., 1974; Greene et al., 1983b), and a negative effect on Mg balance
(Commonwealth Bureau of Nutrition, 1971; O’Dell et al., 1960; Greene et
al., 1983a), while low levels (<1.5% K) appeare to have a negative effect
(Paquay et al., 1968; Pradahan et al., 1974). Fisher et al. (1994) reported
that the retention of Ca and Mg was not influenced by the level of K in the
diet. The higher losses of Mg in the feces was made up for by less Mg
being lost in the urine.
57
Table 5 . Least-squares means and standard errors for blood cellular and noncellular constituents as affected by dietary potassium (K) level in heat-stressed
Holstein cows.
Parameter
1.25% K
1.75% K
Hb
8.18+0.39A
8.84+0.39A
RBC
4.11+0.07A
4.26+0.07A
PCV
33.15+0.56
A
34.71+0.56
A
MCV
82.54+2.03A
MCH
MCHC
Parameter
GLUCOSE
(mg/dl)
1.25% K
1.75% K
52.40+2.04A
53.28+2.04A
8.95+0.12A
8.87+0.13A
CHLOSTERO
L (mg/dl)
191.19+2.79A
175.67+2.78B
84.43+2.03A
ALBUMIN
(AL) %
3.18+0.04A
3.31+0.04A
20.27+0.92A
21.23+0.92A
GLOBULIN
(GL) %
5.77+0.14A
5.56+0.14A
25.29+1.03A
26.26+1.03A
AL/GL.
0.56+0.02A
0.60+0.02A
TOTAL
PROTEIN
%
* Hb= Haemoglobin (%); RBC= Red blood cells (x106/mm3); PCV= Packed cell volume (%);
MCV= Mean cell volume(fl); MCH= Mean cell haemoglobin (pg); MCHC= Mean cell haemoglobin concentrations (%).
Different superscript letters indicate significant differnces (p< 0.05) among means of the same
parameter ; otherwise indicate similarity.
58
Table 6. Least-squares means and standard errors for daily water
intake, plasma macromineral levels and osmolality as affected by
dietary potassium (K) level in heat-stressed Holstein cows.
Variable
1.25% K
1.75% K
Na (mEq/l)
92.24+0.72A
92.68+0.72A
K (mEq/l)
3.28+0.09A
3.22+0.09A
Cl (mEq/l)
89.88+0.66A
91.72+0.66B
PEB (Na+K-Cl mEq/l)
5.70+1.28A
4.53+1.28A
Mg (mg%)
0.39+0.03A
0.41+0.03A
Ca (mg%)
0.35+0.01A
0.38+0.01B
P (mg%)
4.78+0.11A
4.83+0.11A
WI (l/day/cow)
140.88+7.43A
115.62+7.43B
OSM (mOsm/l)
317.99+1.84A
317.56+1.84A
* Different superscript letters indicate significant differnces (p< 0.05)
among means of the same parameter ; otherwise indicate similarity.
59
Table 7. Least-squares means and standard errors for daily
urine output and concentrations of sodium, potassium and
chlorine as
affected
by dietary potassium (K) level in heatstressed Holstein cows.
Variable
1.25% K
1.75% K
Urine output (l/day)
12.12+0.71A
15.34+0.97B
Na (mEq/l)
14 9.49+0.72A
77.75+0.72B
K (mEq/l)
141.15+0.09A
92.75+0.09B
Cl (mEq/l)
238.00+0.66A
137.50+0.66B
UEB(Na+K-Cl
meq/l)
52.69+1.28A
33.00+1.28B
* Different superscript letters indicate significant differnces (p< 0.05)
among means of the same parameter ; otherwise indicate similarity.
60
It was concluded that high K levels in the diet interfere with the
absorption of Mg but not Ca. However, K did appear to interfere with the
utilization of Ca as indicated by the lower amount of Ca in the milk and
urine (Fisher et al., 1994). Concern has been expressed (NRC, 1989) that
dietary K levels greater than 3% may interfere with the utilization of both
Ca and Mg. The dietary K levels in the present study were less than this
level.
The isotonic situation (normal osmolality) seen with the present study is
most often the result of excess salt as well as water excretions. Animals in
the present study drank a large amount of water (Table 6) to compensate
with the water lost in urine (Table 7), through skin or respiration due to
severe heat stress, although feed intake was not affected. Murphy (1992)
did not mention dietary intake of K as a factor which stimulates water
intake. However, Fisher et al. (1994) reported a significant increase in
water intake with increasing K level in the diet. The later levels of K were
much higher than those reported in Murphy (1992) and in the present
study. Electrolyte balance (Na+K-Cl) was higher in the groups of cows fed
the 1.25% K diets compared with the 1.75% K diets, although
nonsignificantly (Table 6). This was a result of the significant reduction in
Cl level of plasma of the former groups (Table 6) as related to dietary Cl
levels (see Table 2). The lower K diets contained an average of 1.04% Cl
compared with 1.45% Cl in the higher K diets. All groups had plasma Cl
values close to the lower edge of the normal values (McSherry and
Grinyer, 1954). We believe that all values of Na, K and Cl in plasma are
normal. In the present study, all animals under heat stress were able to
maintain a normal electrolyte and water balances (osmolality did not
61
change, Table 6). The practical implications of the increase in water
consumption as it influences dilution of rumen digesta and rate of passage
from the rumen and major increase in urine volume may be more
important than the possible disruption of the utilization of minerals.
Because sodium and its associated anions account for 90% of the
osmotic activity of the plasma, any change in osmolality is usually due to a
change in soduim concentration. No change in Na was observed in plasma
due to dietary K level. All cows recieved the same amount of Na in thier
diets, although it seems to be some how high (0.55%). The natural
response to the thirst sensation is to consume more fluids, thus increasing
the water content of the ECF, diluting out the elevated Na levels, and
decreasing the osmolality of the plasma. Thirst is, therefore, important in
mediating fluid intake (Zilva and Pannall, 1979). The other means of
controlling osmotic pressure and water content is to control its excretion
or loss from the body through the kidneys, lungs, sweat, and feces. This is
accomplished in two ways. The hypothalamus, by responding to changes
in osmotic pressure in the bloodstream, plays a part in maintaining water
balance by controlling loss of water through the kidneys. Increased
plasma osmolality, occurring when the water concentration is decreased
relative to the concentration of the sodium and other electrolytes, causes
the stimulation of the antidiuretic hormone (ADH, vasopressin) secretion.
This hormone is secreted from the posterior pituitary gland and acts on
the cells of thedistal tubules and collecting ducts in the kidneys to increase
water reabsorption. As water is conserved, osmotic pressure decreases,
turning off ADH secretion (Natelson and Natelson, 1975; Zilva and
Pannall, 1979). Aldosteron has an opposing effect. When water content is
62
high, the decreased renal sodium in the blood flow causes aldosteron
secretion from the adrenal cortex (Bishop et al., 1985). A reduced blood
volume, an increased extracellular fluid potassium level, physical stress
and release of the renin enzyme from the kidneys (Weldy, 1988) are also
factors that stimulate secretion of aldosteron. This hormone promotes
renal reabsorption of sodium in exchange for secretion of potassium by
the renal tubules, which promotes retention of water, expansion of
extracellular fluid volume, and increases blood (plasma osmolality)
pressure (Henry, 1984). With a normal dietary sodium consumption,
almost all the sodium is reabsorped. Excessive sweating during heat stress
stimulates aldosterone secretion, which acts on the sweat glands to
conserve sodium and chloride.
1.2.3. Plasma Hormone Concentrations
Level of K in the diet showed no significant effects on the
concentration of both cortisol and thyroxine in plasma (Table 8).
Concentration of triiodothyronine (T3) was affected significantly (P<0.05)
by the level of K in the diet. It was higher with the lower K diets. The
T4/T3 ratios were similar when the lower as well as the higher K diets
were fed (Table 8). The nonsignificant changes in T4 and cortisol
concentrations in plasma of cows of present study could be due to the
insignificant changes observed in their dietary intake. The higher T3
level in plasma of cows fed on the 1.25% K diets compared with
those on the 1.75% K those on the 1.75% K diets could be an
indication of increased avaliability of the most potent thyroid hormone
(T3) to the cells for more feed utilization (see Table 4), by more
conversion of the secreted T4.
63
Table 8. Least-squares means and standard errors
for plasma hormone concentrations as affected by
dietary potassium
(K) level in heat-stressed
Holstein cows.
Hormone
1.25% K
1.75% K
T3
139.15+2.71A
131.27+2.71B
T4
4.54+0.14A
4.33+0.14A
T4/T3
0.33+0.01A
0.33+0.01A
Cort.
0.76+0.04A
0.79+0.04A
T3=Triiodothyronine( ng/ml) , T4=Thyroxine(ug/dl) , Cort=Cortisol (ug/dl).
Different superscript letters indicate significant differnces (p< 0.05) among
means of the same raw; otherwise indicate similarity.
64
However, the higher K group may utilized more T3 in order to regulate
thier body temperature, although it was not enough to do so.
2. EFFECT OF DIETARY ELECTROLYTE BALANCE
2.1. Production Responses
The
dietary
cation-anion
(electrolyte)
balance
(DEB),
defiend
quantitively as milliequivalents of [(Na+ + K+) - Cl-]/100 g of diet DM
(Mongin, 1980), is a more important determinant of dietary impact on
systemic acid-base status and performance in lactating dairy cows than
actual
dietary
concentrations
of
Na, K, and Cl (Stewart,
197************************************************************
***************************************************************
***************************************************************
***************************************************************
***************************************************************
***************************************************************
***************************************************************
***************************************************************
***********c diet (20 mequiv Na + K - Cl/100g of feed) resulted in
greater milk yield than when diets containing -10, 0, or 10 meq/100g
were fed. The lower intake observed with the diets containing -100 mequiv
kg-1 (Tucker et al., 1988) seems to result from inclusion of CaCl2 in these
diets and the same effect of CaCl2 on feed intake in dairy cattle has been
shown by Coppock et al. (1982b) and by Escobosa et al. (1984), probably
by affecting the platability of the diets, or by the metabolic acidosis
caused by CaCl2 (Mongin, 1981; Yen et al., 1981). In the present study,
similar or different dietary mineral (K and/or Cl) concentrations from
65
different buffer sources were utilized to achieve different DEB (diets A
and B, 19.7; diet G, 27.4; diets E and H, 29.1; diets C and F, 29.9, and diet
D, 31.2 mEq/100 g of diet DM; Table 3). No significant changes in daily
feed intake or milk yield due to variation in dietary EB were detected
(Table 9). However, increasing DEB from 20 to 30 meq/100g feed DM
decreased fat content of milk (P<0.05) and, hence, 4% FCM. West et al.
(1991) altered DEB by changing K and Cl content in the diet using
potassium bicarbonate or calcium chloride, reported that milk yield and
4% FCM of primiparous cows improved linearly with increasing DEB
with nonsignificant treatment by phase interaction, (cool or hot
environment) whereas dry matter intake (DMI) of cows improved
quadrtically with increasing DEB. A treatment by phase interaction for
DMI was detected, although the intake reached a plateau at a similar
DEB during the cool and hot phases.
2.2. Physiological Responses
The response to DEB appeared to be mediated through blood buffering
and impact on physiologic systems of the cow accompanying the
absorption of monovalent ions from the gastrointestunal tract. Absorption
of Na and K is linked to the generation of systemic bicarbonate (HCO3-),
whereas Cl absorption increases systemic free proton (H+) generation
(Guyton, 1986). As a result, as DEB increases, blood pH tends to rise,
whereas a reduction in DEB increases systemic H+ generation and lower
blood pH (Tucker et al., 1988). Consequently, diets that contain the same
DEB, whether achieved by altering concentration of dietary Na, K, or Cl,
should yield similar proportions of HCO3- and H+ and should have similar
effects on systemic acid-base status. This was confirmed (Tucker et al.,
66
1988) with DEB ranging from -10 to +20 meq/100g DM. However,
investigation of even higher balances has shown that blood pH continues
to increase linearly as DEB increases from +12 to +64 (Hemken, 1980),
but the impact of dietary mineral concentrations, independent of DEB,
has not been evaluated in this range.
In the present study no apparent changes in rectal temperature among
cows fed diets of different EB (Table 9). Also, we did not observe any
significant changes in plasma EB values due to DEB (Table 10). Body
(milk) temperature of cows appeared to be related to the level of feed
consumed (West et al., 1991). Also, the latter authors found that blood and
urinary EB increased linearly with increasing DEB. Cows with greater
feed intake exhibited higher body temperatures, especially in hot weather.
Digestion and metabolism of feed nutrients create high energy, so the
greater body temperature in the work of West et al. (1991) associated
with increasing DEB probably was related to the amount of feed
consumed. However, Schneider et al. (1988a) reported increased evening
respiration rate and elevated rectal temperatures in cows fed high
mineral diets (high in Na, K, and Cl) compared with those offered the
basal diet, even though feed intake was not different among treatments.
They theorized that the elevated respiratory rate was necessary
to
counteract the acidogenic effects of the diet.
Table 9. Least-squares means and standard errors for daily feed
intake, milk yield , fat %, fat-corrected milk (FCM), rectal
temperature (RT) and respiratory rate (RR) as affected by dietary
electrolyte balance in heat-stressed Holstein cows.
Paramater
200 meq/kgDM
300 meq/kgDM
67
DFI
17.21+0.15A
17.41+0.09A
MILK YEILD
20.99+0.58A
20.77+0.33A
FAT%
3.65+0.08A
3.42+0.05B
FCM
19.89+0.53A
18.95+0.30A
RR
89.05+1.53A
87.92+0.88A
RT
39.86+0.08A
39.85+0.04A
Different superscript letters indicate significant differnce (p< 0.05) *
among means of the same raw; otherwise indicate non- signif icant.
68
This was the case for cows of the present study that offered the most
acidogenic diet (diet B; KCl with high K content and lower DEB); They
had the higher respiratory rate (see Table 12). Cows under heat stress
hyperventilate in an effort to maintain body temperature. The resulted
elevated respiratory rate due to heat stress may lowered blood partial
pressure of CO2 (pCO2) and creating a blood carbonic acid dificit, thus
increasing blood pH (Benjamin, 1978). Blood pH increased with
increasing DEB in both cool and hot environment (West et al., 1991). The
increases in respiratory rate were similar to blood pH changes; however,
respiratory rate was much higher in the hot phase of the study (West et
al., 1991).
Table 10 showed that plasma concentrations of K and Na were not
affected by the variations in DEB. The variation in plasma K
concentrations was small and remained within the normal physiological
range for ruminants (Fredeen et al., 1988; Ross et al., 1994). The stability
of plasma Na concentration under varying DEB levels has been
previousely documented in cattle (Tucker et al., 1988; West et al., 1992;
Ross et al., 1994). Sodium stability in extracellular fluids is well controlled
by endocrine system, mainly through renal excretion, in order to maintain
normal vital functions that require Na.
Other studies (Coppock et al., 1979; Fettman et al., 1984) involving
lactating dairy cows have demonstrated that plasma Cl responds to
changes in the Cl concentration of the diet. In our study, plasma cationanion balance was related negatively (P<0.02) to dietary Cl, but not
related to changes in DEB.
69
Table 10. Least-squares
means
and
standard
errors
for plasma
macromineral levels, water intake and osmolality as affected by dietary
electrolyte balance in heat-stressed Holstein cows.
200
300
Paramater
mequiv/kgDM
mequiv/kgDM
Na
92.75+0.86A
92.16+0.49A
K
3.25+0.10A
3.24+0.06A
CL
90.50+0.84A
91.09+0.48A
5.43+1.59A
3.82+0.93A
Mg
0.15+0.01A
0.16+0.00A
Ca
0.28+0.00A
0.29+0.00A
P
4.86+0.08A
4.89+0.04A
WI
133+10.6A
127 +6.10A
OSM
317+1.61A
320 +0.91A
*Different superscript letters indicate significant differnce (p< 0.05) among
means of the same raw; otherwise indicate non- signif icant.
70
This in agreement with the finding of Tucker et al. (1988), but disagrees
with that of Tucker and Hogue (1990). The similar plasma EB observed
in the latter study might be attributed to the fairly constant DEB among
diets. In the present study, plasma Cl decreased from 93.54 to 89.75 meq/l
as DEB increased from 197 to 312 meq/kg. Tucker et al. (1988) reported
that plasma Cl levels in dairy cows decreased from 105.4 to 100.0 meq/l
when the DEB level was increased from 0 to 200 meq/kg. Similarly, Ross
et al. (1994) reported a decreae in Cl concentrations from 104.2 to 99.0
meq/l when DEB level was increased from 0 to 450 meq/kg in steers. In
growing lambs, Fauchon et al. (1995) reported that Cl concentrations
decreased from 104.6 to 101.5 meq/l as DEB level increased from 100 to
500 meq/kg. Decreased plasma concentrations would be expected since
renal excreetion of excess Na requires a concomitant excretion of an ion
(Cl-) to maintain electroneutrality.
Concerning minerals found in hard tissues (Ca, P and Mg), plasma
Ca, P and Mg levels were not affected by DEB levels (Table 10). Since P
and Mg metabolism is closely associated with that of Ca, the same
tendencies should be observed (McDonald et al., 1988). In growing lambs,
P concentrations in serum were not affected by the DEB differences
(Fauchon et al.,1995), but serum Mg concentrations did declined linearly
as DEB increased (P<0.01). Increasing dietary Na was found to decrease
absorption and retention of Mg in sheep (Poe et al., 1985). This could
explain the insignificant variations in Mg concentrations of cows fed on
diets with different EB but with the same Na. However, even though Mg
absorption and retention were slightly decreased in Poe’s experiments,
(1985) serum Mg concentrations remained unchanged. A decrease in
71
plasma Ca has been noticed when sheep are changed from a highroughage
(elevated DEB) to a high-concentrate (low DEB) diets
(Hun**********************************************************
***************************************************************
***************************************************************
***************************************************************
***************************************************************
72
***************************************************************
73
***************************************************************
***************************************************************
*************y cation-anion balance; this seems to be a response to
increased osmolality of rumen contents. However, Forbs (1986) reported
that increased osmolality is associated with a decrease in intake. Higher
intake seems most likely to be associated with acid-base parameters in
blood. No significant differences in water intake were found among group
of cow fed on diets of different EB (Table 10).
3. EFFECT OF SOURCE (BUFFER) OF DIETARY K SUPLEMENTATION
Significant variations in daily feed intake among dietary groups were
only detected within the 1.25% K level (Table 11). The DMI was higher
(P<0.05) for cows fed the diet containing K2CO3 than for those fed on the
other diets containing KCl or KHCO3, respectively. Sodium bicarbonate
supplementation to diet containing KCl increased feed intake to level
comparable to that of K2CO3 diet. This latter supplementation was also
effective within the 1.25% K diets only. Again supplementation of the
lower K diets with either K2CO3 alone or KCl+NaHCO3 increased milk
yield, although not significant, compared with other suplementations.
Also, addition of NaHCO3 to KCl-containing diet at 1.25% K level
increases milk fat content (P<0.01), and hence, fat-corrected milk
(P<0.05).
Buffers may have greater value in hot than in moderate or cold
weather, because alkali element (Na, K) suplementation and buffering are
needed during heat stress, and the most common buffers are acceptable
74
sources of both needs of dairy cows (Escobosa et al., 1984; Schneider et
al., 1984; West et al., 1987). Higher intake has been observed by Fauchon
et al. (1995) in growing lambs when sodium bicarbonate is added to the
diet; this effect has been attributed to the buffering capacity of
bicarbonate on rumen pH (Kellaway et al., 1977). However, in Fauchon et
al. (1995) experiment, the 700 meq diet contained three times as much
bicarbonate as the 500 meq diet and in spite of this, intake was similar
with both diets.
Escobosa et al., (1984) showed higher feed intake, water intake, actual
and 4% fat-corrected milk yields and percent milk fat, concentrations of
K in blood plasma of cows fed 1.7% NaHCO3 than did cows fed CaCl2
(2.28%) or NaCl (0.23%) during natural heat stress. Schneider et al.
(1986) reported that supplementation of sodium bicarbonate (1% of DM
resulting in 0.55% total dietary Na, dry basis) in the diet during hot
weather increased feed intake, and 4% fat-corrected milk yields and
percent milk fat, and caused a slight depression in plasma Na
concentrations compared with no NaHCO3, but concentration of K were
not affected. Also, higher 4% FCM yield was detected with NaCl
supplementation than with no added NaCl. When Na was fed, whether as
NaHCO3 or another Na source, bicarbonates and NaHCO3 in saliva
increased, and salivary flow also increased (Emery, 1976). It was
suggested that a major effect of feeding NaHCO3 was not its direct effect,
but rather supplying more Na to stimulate salivary buffering capacity,
which may help explain why lactational performance was improved by
either NaHCO3 or NaCl addition (Schneider et al., 1986). Addition of
sodium bicarbonate to the Cl-containing diet restored blood pH, blood
75
bicarbonate, and feed intake from decrease to normal (Yen et al., 1981;
West et al., 1991).
West et al. (1986, 1987) compared concentrates containing no buffer,
1.8% KHCO3 , 1.2% K2CO3, or 1.5% NaHCO3 when offered with a
maximum amount (10 kg/d of DM) of a forage blend (50% corn silage,
30% whole cottonseed, and 20% chopped bermudagrass hay, DM basis).
Intake of concentrate was not different among treatments. However,
intake of forage blend was highest with 1.2% K2CO3, higher with 1.8%
KHCO3 than with no buffer, but not different with 1.5% NaHCO3
compared with no buffer or 1.8% KHCO3. At level of 1.0% KHCO3 in a
total mixed ration (TMR) composed of 25% cottonseed hulls and 75%
concentrate reduced feed intake and milk yield (P<0.01) during hot
weather (Schneider et al., 1986). Milk fat percentage was higher (3.2%)
with 1.2% K2CO3 than with 1.5% NaHCO3 (2.4%; P<0.1). Yield of 3.5%
FCM was higher with 1.2% K2CO3 than with 1.5% NaHCO3 (West et al.,
1986, 1987). Belibasakis and Triantos (1991) reported a nonsignificant
increase in mean daily actual milk yield when the cows were fed on diet
containing
Na2CO3
compared
with control cows both under hot
condition. In contrast, the difference of 4% fat-corrected milk yield was
significantly (P<0.05) higher for supplemented than for control cows.
Also, Coppock et al. (1982b) found no change in milk yield when NaHCO3
was added to ration of sorghum sudan hay and concentrate for lactating
cows. Our results suggest that the K2CO3 treatment increased
significantly the 4% fat-corrected milk yield of the cows during hot
weather by about 14% over the KHCO3 treatment (Table 11).
76
The higher milk fat content, and hence, fat-corrected milk observed in
cows received K2CO3, or NaHCO3 +KCl (Table 11) may be due to the
buffering capacity of these salts. Because buffers sometimes increase milk
fat content during heat stress when cows are fed on a high concentrate
diet (Erdman, 1988), it was difficult to determine whether the effect of the
K2CO3 or NaHCO3 on milk fat content in our experiment was due more
to the hot weather or to the high concentrate diet. Schneider et al. (1986)
found an increase in milk fat content from the addition of NaHCO3 to
maize silage and concentrate diet for lactating cows during hot weather.
Similar results have been reported by West et al. (1987) with addition of
K2CO3 to maize silage, hay and concentrate diets. However, Coppock et
al. (1982a) found no change in milk fat content when NaHCO3 was added
to a ryegrass hay and concentrate diet. Belibasakis and Triantos (1991)
found that addition of K2CO3 to the diet of lactating cows during hot
weather increased the blood concentration of TG significantly, which may
explain the significant increase in fat content found in the present study.
In a second comparison (West et al., 1987), the forage blend (40%)
and concentrate (60%, DM basis) were fed as TMR. The dry matter
intake was higher for cows fed the diet containing K2CO3 than for those
fed the other diets containing KHCO3, or NaHCO3. Yield of 3.5% FCM
was higher for cows fed K2CO3 than those fed KHCO3, or NaHCO3. Milk
fat percentage was higher for K2CO3 than for cows fed no buffer; and was
intermediate for cows fed KHCO3 or NaHCO3. Feed intake and
lactational performance in both comparisons (West et al., 1986; 1987)
clearly were superior with K2CO3, followed by KHCO3. Sodium
bicarbonate did not improve lactational performance over that with no
buffer (West et al., 1986).
77
Source of potassium suplementation did not affect Na, K, Mg and P in
blood. However, plasma Cl and Ca levels were affected by source of K in
the diet (Table 12). Ruminal infusions of potassium chloride (Anderson
and Pickering, 1962) and dietary additions of potassium carbonate (St.
Omer and Roberts, 1967) have resulted in increases in plasma K levels.
The KCl+NaHCO3 diet had the highest Ca concentration in plasma.
Escobosa et al. (1984) found that the addition of NaHCO3 to diets for
lactating cows increased the blood concentrations of K, but no difference
was found in blood Na. Schneider et al. (1984) suggested that Na and K
concentrations in blood were unaffected by the addition of NaHCO3 or
KHCO3 to diets for dairy cows during hot weather. Decreases in the
urinary excretion rate of Ca and Mg with dietary additions of potassium
cholride had been observed by Tucker and Hougue (1990) and also by
Fisher et al. (1994), but physiological cause of this decreased excretion has
not been postulated.
Animals fed dietary buffers consumed comparatively more water
may be attributed to high osmolality of rumen contents in these groups,
because both rumen water flux and water rumen permeability are
affected by rumen osmotic presure (Warner and Stacky, 1972).
Intraruminal infusions of NaHCO3 increased the salivary secretion
(Rogers and Davis, 1982). Thus comaratively higher water intake (see
Table 12) along with probably more salivary secretion due to the
addition of NaHCO3 in diets could account for higher rumen fluid
outflow rate (Puri and Kapoor, 1996) which help in increasing the fiber
digestion in the rumen. Kellaway et al. (1977) observed that the addition
of bicarbonate to the diets of young calves increased intake, but also
78
produced changes in blood acid-base parameters comparable to those
observed in Fauchon et al. (1995) study. It is plausible that changes in
blood acid-base parameters excert a direct effect on hypothalamic areas
controlling feed intake. It has been demonstrated that intake can be
manipulated by changes in ionic composition of cerebrospinal fluid and
hypothalamic tissue (Seoane and Baile, 1973; Seoane et al., 1975). An
acidotic acid-base profile would be associated with a decrease in intake
while a mildy alkalotic one would be associated with an increase.
Addition of NaHCO3 and KCl to diets benefited heat-stressed dairy cows
in terms of milk yield, regulation of acid-base balance, and lowering
body temperatures (Coppock et al., 1982; Schneider et al., 1984;
Mallonee et al., 1985; Schneider et al., 1988a, b). Sodium bicarbonate is
a compound known to cause potassium to move back into the cell and is
administered in case of hyperkalemia (Henry, 1984). This effect may
restore the level of K inside the cell for better function, regardless of its
extracellular concentration. Our results indicated that both K2CO3 alone
and NaHCO3
+
KCl increased significantly (P<0.05) body temperature
(Table 13).
Most of blood components were not affected due to different source
K in the diet (Tables 14, 15 and 16). Cows fed on diets containing
NaHCO3 showed lower T3 than the diets containing only KCl (P<0.05;
Table 16). Escobosa et al. (1984) found that the addition of NaHCO3 to
diets for lactating cows increased the blood concentrations of glucose.
Thyroid hormone appear to enhance the actions of epinepherine in
stimulating glycogenolysis and gluconeogenesis and to potentiate the
actions of insulin on glycogen synthesis and glucose utilization.
79