Nutritional Strategies for Managing the Heat-Stressed Dairy Cowl
Joe W. West
Animal and Dairy Science Department, University of Georgia,
Coastal Plain Station, Tifton 31793-0748
ABSTRACT: Heat stress results from the animal's
nutrients are used less efficiently. An excess of
degradable dietary protein is undesirable because of
energy costs to metabolize and excrete excess N as
urea. Optimizing ruminally undegraded protein improves milk yield in hot climates. Mineral losses via
sweating (primarily K ) and changes in blood acidbase chemistry resulting from hyperventilation reduce
blood bicarbonate and blood buffering capacity and
increase urinary excretion of electrolytes. Theoretical
heat production favors feed ingredients with a lower
heat increment, such as concentrates and fats,
whereas forages have a greater heat increment.
Improved dietary energy density and the lower heat
increment associated with the inclusion of dietary fat
must be coupled with limitations t o fat feeding to
avoid ruminal and metabolic disorders. Numerous
nutritional modifications are used for hot weather
feeding; however, many need further investigation to
achieve specific recommendations.
inability to dissipate sufficient heat to maintain
homeothermy. Environmental factors, including ambient temperature, radiant energy, relative humidity,
and metabolic heat associated with maintenance and
productive processes, contribute to heat stress. The
focus of this article is to identify environmental and
metabolic factors that contribute to excessive heat
load, describe how disruption of homeothermy alters
physiologic systems of the cow, and discuss nutritional
modifications that help to maintain homeostasis or
prevent nutrient deficiencies that result from heat
stress. Changes in diet are needed during hot weather
to maintain nutrient intake, increase dietary nutrient
density, or to reestablish homeostasis. Formulation for
adequate nutrient intake is challenging because of the
competition between nutrient density and other needs
for the cow, including energy density and adequate
dietary fiber. Lower DMI during hot weather reduces
nutrients available for absorption, and absorbed
Key Words: Dairy, Heat Stress, Intake, Metabolism, Milk
01999 American Society of Animal Science and American Dairy Science Association. All rights reserved.
Introduction
adaptations and may be favorable or unfavorable to
the economic interests of humans, and that, nevertheless, they are essential for survival of the animal.
The impact of heat stress on livestock is broad in
geographic terms. Dairy cattle across the southern
United States and in many subtropical and tropical
regions are subject to high ambient temperatures
and(or) high relative humidity for extended periods.
In the southeastern United States, high ambient
temperature and relative humidity exceeding the
temperature-humidity index ( THI) associated with
heat stress, persist for 4 to 6 mo of each year.
In an excellent review, Beede and Collier (1986)
identify three management strategies that minimize
the effects of thermal stress: 1) physical modification
of the environment, 2 ) genetic development of heattolerant breeds, and 3 ) improved nutritional manage-
High ambient temperature, relative humidity, and
radiant energy compromise the ability of the lactating
dairy cow to dissipate heat, resulting in heat stress.
These environmental factors, coupled with metabolic
heat, create difficulties in maintaining thermal
balance. This results in elevated body temperature,
which in turn initiates compensatory and adaptive
mechanisms to reestablish homeothermy and
homeostasis. Stott ( 1981) stated that these readjustments to maintain homeostasis are referred t o as
lThe author gratefully acknowledges the assistance of Brett
Norman for assistance in literature review and June Womack for
assistance in manuscript preparation.
27
22
WEST
ment practices. Nutritional modifications to compensate for the effects of heat stress have been developed,
but the response to dietary changes may be improved
by environmental modifications. Feeding excessive
quantities of nutrients, such as crude protein, can
contribute to reduced efficiency of energy utilization,
potentially adding to stress levels. Therefore, a
thorough understanding of dietary modifications to
minimize heat stress is necessary. The objectives for
this paper are to summarize environmental factors
that cause heat stress, describe the physiologc or
metabolic effects of the stressors, and discuss the
nutritional practices developed to compensate for the
effects of the stress.
Effects of Environmental Stress on Physiologic
Responses of the Cow
Reactions of cattle and other homeotherms to
moderate climatic changes generally are compensatory
and are directed at maintaining or restoring thermal
balance (Kibler and Brody, 1953). The numerous
physiologic mechanisms for coping with heat stress
have been reviewed (Blackshaw and Blackshaw,
1994). These include sweating, a more rapid respiratory rate, and greater vasodilation with increased
blood flow to the skin surface. A reduced rate of
metabolism, decreased DM and nutrient intake, and
altered water metabolism all occur in response to heat
stress. Unfortunately, responses to heat stress often
have negative effects on the physiology of the cow and
on milk yield.
Environmental Effects on Nufrient
Irttake and M i l k Yield
Feed DMI starts to decline and maintenance
expenditures increase when environmental temperatures exceed 25°C (NRC, 1981). However, THI may
describe more precisely the effects of the environment
on the cow's ability to dissipate heat. Milk yield and
TDN intake declined slightly when the THI exceeded
72 and declined sharply when 76 was exceeded
(Johnson et al., 1963). Milk yield declined when body
temperature exceeded 38.9"C, and, for each 0.55"C
increase in rectal temperature, milk yield and intake
of TDN declined 1.8 and 1.4 kg, respectively (Johnson
et al., 1963). Consequently, minimizing the increase
in rectal temperature can improve nutrient intake and
subsequent milk yield.
Genetic differences exist for heat tolerance of cattle.
Bos indicus breeds are more heat-tolerant than Bos
taurus breeds because of greater sweating capacity
and lower metabolic rate (Blackshaw and Blackshaw,
1994). Milk yield decline for dairy cows exposed to
temperatures of 24 or 34°C and to low ( 3 8 t o 46%) or
high ( 7 6 to 80%) relative humidities, was greatest for
Holstein, intermediate for Jersey, and least for Brown
Swiss cows (Johnson and Vanjonack, 1976). Holstein
cows are larger and have less surface area relative to
internal body mass and would be expected to dissipate
heat less effectively than would Jerseys. Brown Swiss
are generally acknowledged as being more heattolerant than Holsteins, but generally produce less
milk than Holsteins. Milk temperatures for Holstein
and Guernsey cows increased as production increased
from low, medium, and high levels during hot weather
(Igono et al., 19851, an indication that high yielding
cows are more sensitive to hot conditions. Heat
production was about 14% greater for cows subjected
to hot ( 3 5 ° C ) vs cool (20°C) conditions and was
reflected in elevated rectal temperatures (Robinson et
al., 1986).
Environmental conditions such as temperature and
humidity are interrelated, and the combined effects
must be considered when determining effects on
intake and milk yield. The mean daily temperature
had the greatest effect on milk yield and rectal
temperature when compared with minimum and
maximum temperatures (Kabuga and Sarpong,
1991). Holter et al. ( 1 9 9 6 ) reported that the minimum daily THI was more closely correlated with DMI
than maximum THI in Jersey cows. Reductions in
DMI commenced when minimum THI exceeded 56 to
57 and continued until THI reached 72. Corresponding
maximum THI ranged from 71 to 85; DMI declined 4.4
kg/d, or 22%. Similarly, correlation of DMI with
minimum and maximum THI for Holsteins was -.63
and -.62, and maximum THI had a .95 correlation
with minimum THI (Holter et al., 1997). During heat
stress, DMI was reduced 22% for multiparous cows
and 6% for primiparous cows, probably reflecting
smaller body size for the primiparous cows as well as
lower DMI and less metabolic heat production.
Effects of weather conditions on intake and milk
yield are most likely mediated through changes in
body temperature. Across early, middle, and late
stages of lactation, 9% of the variation for milk yield,
13% for milk fat, 5% for feed intake, and 65% for
rectal temperature were attributable to weather
conditions (Maust et al., 1972). However, weather
conditions during the 2 to 3 d preceding the day of
temperature measurement were most closely associated with milk yield, and composition and comparisons using a single day of temperature measurement
J. h i m . Sci. Vol. 77, Suppl. 2/J. Dairy Sci. Vol. 82, Suppl. 2/1999
MANAGING HEAT-STRESSED DAIRY COWS
could be misleading. In addition, DMI and milk yield
for cows in midlactation were reduced most by hot
weather conditions (Maust et al., 19721, possibly
because early-lactation cows rely on body stores for a
portion of the nutrients for production whereas latelactation cows consume fewer total nutrients. Araki et
al. (1984) reported that lactating cows were more
sensitive to the effects of heat than were nonlactating
cows, consistent with the greater metabolic heat
production for the former.
Metabolic Effects of Heat Stress
That Affect Nutrient Intake
Johnson et al. ( 1967) termed cows that experienced
sharp increases in body temperature upon exposure to
high ambient temperatures as heat-intolerant, and
those with lesser increases were termed heat-tolerant.
When ambient temperatures were increased from
18°C to 29"C, the heat-intolerant cows exhibited a
1.4"C increase in body temperature and a 4-kg decline
in daily milk yield, compared with a .7"C increase and
2-kg decline for heat-tolerant cows. Such responses
suggest metabolic or physiologic differences among
cattle that could be exploited if the mechanisms were
identified.
Hormonal changes that occur in response t o heat
stress may play an integral role in the decline in
productivity. Plasma growth hormone concentration
and growth hormone secretion rate declined with hot
temperatures (35°C) (Mitra et al., 1972). Growth
hormone content in milk of low, medium, and high
production groups declined when THI exceeded 70,
possibly reflecting suppressed production of growth
hormone so that metabolic heat production is reduced
(Igono et al., 1988). Plasma growth hormone reductions that occurred with heat-stressed cows did not
occur in thermoneutral conditions for cows fed restricted intakes that were similar to those consumed
during heat stress (McGuire et al., 1991). The change
in plasma growth hormone was probably in response
to effects of high temperature rather than to reduced
nutrient intake. The thyroid hormones triiodothyronine and thyroxine declined in response t o heat stress
(Johnson et al., 1988; Magdub et al., 19821, which is
probably an attempt to reduce metabolic heat production in the cow. However, reduced metabolic rate
compromised productive capability as well. Greater
plasma content of epinephrine and norepinephrine
with high ambient temperatures is an indicator of
stress response (Alvarez and Johnson, 1973) and
reduces rate of passage in the digestive tract. Slower
passage rate can lead to greater gut fill and limited
23
intake but improved digestibility because of greater
residence time in the gut. Mean total-tract retention
time for cattle was 36.6 and 43.2 h when the ambient
temperatures were 18 and 32°C (Warren et al., 1974).
Digestibility of DM, ADF, and NDF was 6.7, 11, and
8.1% greater for cattle in the hot environment,
partially offsetting the reduced nutrient intake that
occurred with reduced DM consumption. Cortisol
response to heat stress challenge is often variable, but
workers evaluating the effects of increasing fiber
content of the diet on the response to hot, humid
weather reported a linear increase in plasma cortisol
with increasing fiber content of the diet (West et al.,
19981, suggesting that greater stress associated with
greater dietary fiber content occurred.
Reduced blood flow during heat stress may reduce
nutrient uptake and mammary blood flow. Cows given
ad libitum access to feed or with intake restricted to
75% of the ad libitum amount in a thermoneutral
environment or with ad libitum access in heat stress
had intakes of 15.1, 11.5, and 11.1kg/d, respectively.
Portal vein plasma flow was reduced by 14% in the
thermoneutral environment for cows fed the restricted
diet compared with ad libitum consumption (McGuire
et al., 1989). In addition, net flux of alpha amino-N
declined by 20 and 35% for cows fed restricted diets
and for cows in the heat stress environment-ad
libitum feeding treatment. The authors indicated that
a portion of the negative effects of heat stress could be
explained by reduced nutrient intake and reduced
nutrient uptake by the portal drained viscera of the
cow. Similarly, cows with restricted intake in thermoneutral conditions or ad libitum intake in heat
stress conditions had reduced blood flow to the
mammary gland when compared with cows on ad
libitum consumption in thermoneutral conditions
(Lough et al., 1990). Changes in blood flow that alter
the supply of nutrients for milk production are at least
partially related to reduced nutrient intake during hot
weather; however, a direct effect of heat stress on
blood flow to the viscera may be involved. Interestingly, exogenous compounds can alter circulatory flow
and contribute to greater stress. Beef heifers fed
endophyte-infected tall fescue (Al-Haidary et al.,
1994) or administered ergovaline (Al-Haidary et al.,
1995) and exposed to hot environmental temperatures
had greater core temperatures and respiratory rates.
Heifers consuming ergovaline had lower temperatures
for the back and hips; the authors suggested that
decreased peripheral blood flow reduced heat loss via
the skin. Endophyte-infected tall fescue has been
associated with greater heat stress, which is consistent with these findings. Each avenue of heat dissipation is necessary to the maintenance of homeothermy.
J. h i m . Sci. Vol. 77, Suppl. 2/J. Dairy Sci. Vol. 82, Suppl. 211999
24
WEST
Effects of Cooling on Intake and Milk Yield
Environmental modification may be necessary if
productivity is to be maintained by the dairy cow with
high genetic potential. Maintenance of DMI is critical
to production, and minimizing the increase in body
temperature during hot conditions translates directly
into greater feed consumption. The most obvious
environmental modification is the use of shade.
Shading in a hot, humid climate reduced rectal
temperature by 2 to 4.1%, respiratory rate by 29 to
60%, improved DMI by 6.8 to 23.2%, and improved
milk yield by 9.4 to 22.7% compared with unshaded
cows (Mallonee et al., 1985; Schneider et al., 1984,
1986). Cows shaded during the dry period yielded 4.5
and 13.6% more milk at 100 and 305 d postpartum
and delivered calves weighing 3.1 kg more compared
with cows receiving no shade while nonlactating, even
though cows were handled similarly postpartum
(Collier et al., 1982b). This suggests that cows may
have had greater energy reserves entering the lactation. Energy use by the cow is affected by the degree of
heat stress. Grazing cows that were unshaded had
higher respiratory rates and rectal temperatures than
unshaded cows, and shaded cows with no concentrate
supplementation had milk yield similar to that of
unshaded cows fed 3.5 kg/d of concentrate (Valtorta et
al., 1996). Shading was found to be profitable,
whereas concentrate supplementation without shade
was not.
Additional benefits from cooling cattle with fans
and sprinklers can be derived in hot or hot and humid
climates. Reports as early as the 1940s (Seath and
Miller, 1948) demonstrated that, used independently,
fans and sprinklers reduced rectal temperature and
respiratory rate but when used together the effects
were synergistic. Sprinklers used in combination with
fans also reduced diurnal variation of body temperature. Much of the response t o cooling is a direct result
of increased DMI. Bucklin et al. (1991) summarized
studies from Florida, Kentucky, and Missouri and
reported improvements in DMI of 7.1, 9.2, and 7.1%
and milk yields of 11.6, 15.8, and 8.6% for the three
states, respectively. Rectal temperatures for the Kentucky and Missouri cows were reduced by 0.5 and
0.4"C. High producing cows in early lactation were
most sensitive to heat stress, and milk yield declined
significantly when rectal temperatures exceeded 39°C
for more than 16 h (Igono and Johnson, 1990).
Because genetic potential for milk yield has improved
and the predominant dairy breed has become Holstein,
the impact of heat stress on production has increased.
Physiologic improvements with cooling included
greater circulating growth hormone concentrations
and lower prolactin concentrations (Igono et al.,
1987). Accompanying these physiologic changes were
a 2.3 and 2.0 kg/d increase in DMI and milk yield.
Similar to shade work with nonlactating cows (Collier
et al., 1982b), cooling with fans and sprinklers during
the dry period increased mean 150-d milk yield at 150
d by 3.5 kg/d, and the effect was greater with
increasing age (Wolfenson et al., 1988). Milk yield
was 0.9, 2.5, and 7.3 kg per day greater in lactation 2,
3, and 4+ for cows that were cooled during the dry
period. It is apparent that with increasing production,
body size, and maturity, cows become more sensitive
to the effects of environmental stress. Effective
nutritional programs need to be implemented with
some environmental modification if high production is
to be maintained.
Heat Production and Heat Increment
The processes associated with maintenance, digestion, activity, metabolism, and production create a
large amount of heat. Coppock (1985) reported that
heat production for a 600-kg cow yielding 40 kg of 4%
fat milk accounted for 31.1% of consumed energy,
second only t o fecal energy losses of 35.3%. Heat
energy is further partitioned into heat associated with
waste formation and excretion ( 3 % 1, product formation (52.9%), fermentation (8.3%), digestion
(12.2%), and maintenance (23.5%) (Coppock, 1985).
Heat production, though advantageous during cool
temperatures, is a liability during hot weather.
Obvious advantages to reducing metabolic heat
production in the cow during hot weather are improved metabolic efficiency and reduced heat to be
dissipated. High milk yield requires the intake of
large quantities of nutrients and greater heat production. Cows at high (31.6 kg/d) and medium (18.5 kg/
d ) milk yield had 48.5 and 27.3% greater heat
production when compared with dry cows (Purwanto
et al., 1990). However, cows in mid and late lactation
were more adversely affected by hot weather compared with early lactation cows, despite greater yield
for the early lactation cows (Maust et al., 1972). The
higher producing early-lactation cows were probably
less sensitive to the effects of high ambient temperatures because of less total DMI and the more eficient
use of mobilized body tissue reserves. The heat
production associated with the use of body tissue
stores is approximately one-half of that produced by
direct conversion of dietary ME to milk (Coppock,
1985).
Identifying dietary modifications to reduce heat
production and improve efficiency of energy use has
J. h i m . Sci. Vol. 7 7 , Suppl. 215. Dairy Sci. Vol. 82, Suppl. 2/1999
MANAGING HEAT-STRESSED DAIRY COWS
merit during hot weather. Digestion and metabolism
of nutrients creates heat, and heat increment is
defined as energy expenditures associated with the
digestion and assimilation of food (Baldwin et al.,
1980). The conversion efficiencies of intermediate
products, such as acetate and glucose, t o end products,
such as fatty acids, is 68 to 72 and 82 to 85%,
respectively, and partial efficiencies for the conversion
of acetate and dietary fat to milk fat are in the ranges
of 70 to 75 and 94 to 97%, respectively (Baldwin et al.,
1985). Infused W A resulted in varying heat increment, depending on the specific W A infused (Armstrong and Blaxter, 1957; Moe, 1981). Differing
efficiencies for nutrient utilization offers the potential
t o formulate diets with a lower heat increment.
Theoretical diets were formulated for beef cattle with
high, low, and intermediate heat increments. Rations
with high heat increment contained more forage fiber;
those with low heat increment contained more grains.
Using the NE,, NE,, and ME energy equations, the
author determined that expected performance improvements with low heat increments more than offset
greater ration costs during high temperatures (Brokken, 1971).
Can these differences in energy efficiency be
exploited in practical diets for animals in hot weather?
Low efficiency for use of acetate may account for the
low net energy of feeds high in fiber (Moe, 1981) and
provides the rationale for feeding low fiber diets
during hot weather. The high efficiency of fat utilization and its low heat increment led Coppock (1985) to
comment that fats are undervalued by current feed
evaluation systems when they are fed within or above
the zone of thermoneutrality.
Nutritional Modifications for Feeding
During Hot Weather
Fiber Feeding
Because there is greater heat production associated
with metabolism of acetate compared with propionate,
there is a logical rationale for the practice of feeding
low fiber rations during hot weather. Feeding more
concentrate at the expense of fibrous ingredients
increases ration energy density and should reduce
heat increment. Kurihara (1996) fed diets of Italian
ryegrass or corn silage plus soybean meal to cows
exposed t o 18, 26, or 32°C. Ryegrass resulted in a
greater heat increment at higher temperatures
without changes in ME intake (because of force
feeding via ruminal cannulae), probably resulting
25
from greater heat production associated with acetate
production. The author reported that DMI decreased
in the order of ryegrass hay, corn silage, beet pulp,
and concentrates with high environmental temperature. The decline in ME intake over the range of 18 to
32°C was 22% for cows fed the ryegrass hay diet but
was only 12.9% for the corn silage plus soybean meal
diet, and energy retention was far less for the ryegrass
hay diet. Cows fed diets containing 100, 75, or 50%
alfalfa, with the remainder being a mix of corn and
soybean meal, had a mean efficiency of conversion of
ME to milk of 54, 61, and 65% (Coppock et al., 1964).
Heat production for the three diets was 688, 647, and
620 kcal per megacalorie ME. The greater metabolic
heat production for high fiber diets was confirmed by
Reynolds et al. (19911, who fed pelleted diets of 75%
alfalfa or 25% alfalfa, with the remainder as concentrate. Diets containing 75% alfalfa resulted in greater
heat production and reduced retained energy, and the
greater 0 2 intake by portal drained viscera and liver
accounted for 44 and 72% of heat increment for the
concentrate and alfalfa diets, respectively.
Altered proportions of ruminal VFA may explain a
part of the differences in heat increment with fiber
feeding during heat stress. Volatile fatty acids constitute a large proportion of the energy available to the
cow (Van Soest, 19821, and declining intake during
heat stress reduces the quantity of VFA in the rumen
because fermentable carbohydrate is reduced.
However, heat-stressed cattle force-fed via ruminal
cannulae to achieve constant intake had reduced total
VFA production with altered molar proportions of VFA
compared with cattle in thermoneutral conditions
(Kelly et al., 1967). The mechanism for this decline is
not understood but could relate to combined effects of
heat stress on ruminal buffering and efficiency of
microbial VFA production. Cows subjected to high
environmental temperatures selectively decrease the
quantity of forage consumed relative to concentrates
(McDowell, 1972). Increased feeding of concentrates
is a common practice during conditions conducive to
heat stress, but maximal benefit from concentrates
appears to be approximately 60 to 65% of the diet
(Coppock, 1985). Excessive concentrate feeding leads
to acidosis and the associated production, health, and
metabolic difficulties.
Does the greater heat production associated with
high fiber diets actually translate into measurable
performance effects? In early work, cows fed low fiber
diets during hot weather had greater daily milk yield,
lower body temperature, and slower respiratory rates
compared with those fed high fiber diets (Leighton
and Rupel, 1956; Stott and Moody, 1960; Tsai et al.,
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26
WEST
1967). The addition of 30 or 10% molasses to diets
containing 35 or 45% alfalfa, respectively, had little
effect on milk energy produced per unit of feed energy
(20.7 and 20.0%) for cows in an 18.3"C environment.
However, in a 31.1"C ambient environment, the
efficiencies were 19.2 and 17.3% for the 30 and 10%
molasses diets (Wayman et al., 1962). In addition,
total feed intake declined by 18.8 and 31.8% for the 30
and 10% molasses rations. Extra carbohydrate furnished by the molasses probably contributed to
greater efficiency of energy use. It has been proposed
that diets that contain sufficient concentrates for high
propionate production supply adequate NADPH to
enable acetate to be converted to fat, whereas, in high
roughage diets, the larger amount of acetate and
smaller quantity of propionate produced results in a
metabolic excess of acetate, which must be utilized as
heat as a result of a futile cycle (MacRae and Lobley,
1982; Parker, 1984). They suggested that extra N as
crude protein provides for more efficient use of
available VFA by providing amino acids for glucose
synthesis; thus, ruminants might be able to efficiently
utilize larger quantities of acetate from roughage
feeds if an additional source of reduced coenzyme were
available. This may be particularly true for subtropical and tropical regions where adapted forages often
are very high in fiber and low in readily fermentable
carbohydrate.
Rarely are total forage diets fed to lactating dairy
cows in the United States and the carbohydrate
furnished in mixed diets, such as those in practical
total mixed rations, may improve the efficiency of
energy use. Nonlactating cows fed diets containing
100% alfalfa hay or 30% alfalfa hay plus concentrates
used ME from infused acetic acid with 27% efficiency
for 100% hay diets, which improved t o 69% efficiency
for the 30% hay diets (Tyrrell et al., 1979). The large
amount of highly fermentable carbohydrate fed in
typical high-concentrate diets should minimize the
heat production observed in the very high fiber diets,
which were used in research settings.
Although high fiber diets contribute to heat stress,
the level of intake is far more critical to the total
amount of metabolic heat produced. Growing heifers
fed pelleted rations containing 75% alfalfa or 25%
alfalfa produced 48.8 and 45.5 MJId of heat (Reynolds
et al., 1991). However, when the low and high intake
(4.2 and 7.1 kgld DMI) heifers were compared, heat
production was 38.2 and 56.1 MJId. Therefore, intake
effects have a substantial effect on heat production
and must be considered in designing an effective
nutritional and environmental management program.
In sheep, heat production in the gut was not greatly
affected by diet composition but increased exponentially with increasing ME intake (Webster et al.,
1975). The authors concluded that 25 to 30% of total
heat increment resulted from intake and digestion.
Work in Alabama supports the concept of the interaction of high intake with greater heat stress. Cows
were fed diets containing 20, 17, and 14% ADF across
varying environmental conditions. Dry matter intake
was greatest with the lower fiber diets in cool weather,
yet there was a more rapid decline in intake with the
low fiber, higher intake diets as the environmental
temperature increased (Cummins, 1992). Cows fed
the diet containing 14% ADF were more sensitive to
changes in minimum ambient temperature than for
those fed higher ADF diets, but total DMI was also
greater for these cows. Intake declined for cows fed
four diets with a range for NDF from 27 to 35% with
increasing NDF content during cool weather, but the
decline was less severe during hot weather, indicating
that the level of intake was more critical than fiber
content during hot weather (West et al., 1998).
Research suggests that lower fiber, high grain diets
may indeed reduce metabolic heat production and
contribute to lower heat load in the animal. Further,
the low fiber, high grain diets provide more efficiently
used end products, which contribute to lower dietary
heat increment. However, low fiber, high grain diets
must be balanced with the need for adequate fiber to
promote chewing and rumination to maintain ruminal
pH and cow health. Research to compare high and low
quality forages for effects on energy and heat increment should address the question of adequate fiber vs
high concentrate diets for hot weather.
Feeding Fats and Concentrates
The reduction in DMI and related efficiency of
nutrient utilization that occurs with heat stress
requires greater dietary nutrient density if milk yield
is to be maintained at a level comparable to that at
thermoneutral conditions. The addition of fat to the
diet of lactating dairy cows is a common practice, and
the greater energy density and the potential to reduce
heat increment of high-fat diets may be particularly
beneficial during hot weather. The addition of 3 to 5%
fat to the ration can be achieved without toxic effects
t o ruminal microflora (Palmquist and Jenkins, 19801,
and, in thermoneutral conditions, cows fed diets with
25% of ME from a protected tallow had 8 to 13.6%
higher efficiency for use of ME for lactation than those
not receiving supplemental tallow (Kronfeld et al.,
1980). The conversion of dietary fat to body fat is
highly efficient when compared with the conversion of
J. h i m . Sci. Vol. 7 7 , Suppl. WJ. Dairy Sci. Vol. 82, Suppl. 211999
MANAGING HEAT-STRESSED DAIRY COWS
acetate to fatty acids (Baldwin et al., 1980). Improved
efficiency and lower heat increment should make fat
especially beneficial during hot weather. However,
results from the addition of dietary fat during hot
weather have been mixed. Both Brahman cross and
British crossbred steers fed diets containing 9.2% fat
during hot weather had lower body temperatures (.3
to .4"C lower) than those fed diets containing 2.5% fat
(O'Kelly, 19871, suggesting less heat production in
those steers. Cattle fed diets containing 10% of
concentrate as soybean oil or hydrogenated vegetable
oil during hot weather had similar body temperatures
and efficiency of energy utilization for lactation
compared with controls (McDowell et al. 1969; Moody
et al. 1967). Feeding 15% whole cottonseed or 3% of a
ruminally protected fat to cows during hot weather
resulted in a small increase in fat-corrected milk yield
with protected fat, and no improvements in respiratory rate or rectal temperature (Saunders et al.,
1990).
Ruminally-protected fats allow the inclusion of a
substantial quantity of fat in the diet, which could
lower heat increment significantly. Holter et al.
(1992) added 15% whole cottonseed, or 15% whole
cottonseed plus 5 4 kg of a calcium salts of fatty acids
and reported that heat production in excess of
maintenance declined by 6.7 and 9.7%, and total heat
loss declined by 4.9 and 7.0%, respectively. The high
fat levels produced a measurable decline in heat
production in thermoneutral conditions. Knapp and
Grummer ( 199 1) offered diets containing 5% added
fat (60% prilled fatty acids, 40% tallow) to cows held
at thermoneutral (2O.S0C, 38% relative humidity) or
hot environmental conditions ( 3 1.8"C, 56% relative
humidity) and reported a nonsignificant increase in
milk yield of 1.1 and .3 kg in the cool and hot
environments, although fat-corrected milk increased
by 2.7 and 1.8 kgld. No diet x environment interaction
occurred. Feeding diets containing 4.6% or 7.4% fat
(from 3% added prilled fatty acids) and housing in a
shade or shade plus evaporatively cooled environment
did not improve milk or fat-corrected milk yields
(Chan et al., 1997). Cooling improved milk yield ( P <
.08), but rectal temperatures were unchanged by
either cooling or dietary fat content, and no interactions occurred, again indicating no apparent benefits
from lower heat increment associated with fat feeding.
The authors speculated that high rectal temperatures
across treatments could have resulted from cow
movement associated with the collection of rectal
temperatures, but performance variables did not
indicate added benefits from fats during hot weather.
High levels of prilled fatty acids (5% of diet DM)
fed to cows that were blocked according to calving
27
season (cool or warm seasons) improved milk yield
only for the warm season (Skaar et al., 1989).
Maximum ambient temperatures during the warm
season ranged from 29.4 to 35.0°C, and fat levels were
high relative t o other studies. Conversely, in Georgia
cows that calved in fall or summer and were fed diets
with or without added tallow to supply 1 kg/d had
improved NE intake with added fat. Milk yield was
unchanged and efficiency of NE1 utilization was
reduced by added fat (Niango et al., 1991). Fatsupplemented cows lost more BW than did controls
and had lower fiber digestion, which suggests that fat
content impaired digestion. The literature for fat
supplementation is inconclusive relative to the
benefits of added fat during hot weather. Additional
research should include a comparison of several levels
of added fat imposed over an extended period of time,
implemented before the exposure t o temperatures
above the upper critical level for the cow.
Protein Nutrition in Hof Climates
As DMI declines, the quantity of consumed
nutrients, including CP, also declines and a negative
protein balance may occur (Hassan and Roussel,
1975; Ibrahim et al., 1970; Kamal and Johnson,
1970). During hot weather, dietary protein density is
often increased to compensate for lower intake, and
cows offered diets containing 20.8% CP during hot
conditions had greater DMI and milk yield than those
offered diets containing 14.3% CP (Hassan and
Roussel, 1975). The low CP diet was calculated to be
adequate for the level of production achieved, and
improved milk yield was associated with greater DMI
for the high protein diet.
It should be noted that there is an energetic cost
associated with feeding excess protein, and dietary N
above requirement reduces ME by 7.2 kcaWg of N
(Tyrrell et al., 1970). Milk yield declined from 24.5 to
23.1 kg when dietary CP increased from 19 to 23%
(Danfaer et al., 1980); the energy cost of synthesizing
and excreting urea accounted for the reduced milk
output (Oldham, 1984). Cows offered diets of two
protein solubilities (40 and 20.9%) during thermoneutral and heat-stress conditions consumed more
feed and yielded more milk in both environments
when fed the less soluble protein diet (Zook, 1982).
Although there were no overt measures of increased
heat stress for the highly soluble protein diets during
heat stress, N balance was improved for the low
solubility diet. Hassan and Roussel ( 1975) reported
that blood nonprotein-nitrogen content was positively
correlated with rectal temperature, which is consistent
with reduced efficiency of energy use for excessive N
J. Anim. Sci. Vol. 77, Suppl. 215. Dairy Sci. Vol. 82, Suppl. 2/1999
28
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(Tyrrell et al., 1970). Thus, the energy necessary to
form urea from excess protein appears as heat
production and decreases the proportion of NE1 in ME,
and the energy lost in excreted urinary N from excess
protein decreases the proportion of ME in DE (NRC,
1989).
Until recently, research into the effects of excessive
dietary protein content, solubility, or degradability
has been lacking, especially under heat stress conditions; recent studies investigating dietary protein
content as well as composition suggest an interaction
between protein availability and environment. Diets
with low (31.2% of CP) and high (39.2% of CP)
ruminally undegradable protein fed during hot
weather had no effect on DMI; however, milk yield
increased by 2.4 kgld and blood urea protein declined
from 17.5 to 13.3 mgl100 mL for the diet containing
higher undegradable protein (Belibasakis et al.,
1995). Arizona workers compared diets with high
(18.4%) and low (16.1%) CP and high ( 5 7 to 60%)
and medium (40%) protein degradability during hot
weather (Higginbotham et al., 1989b). Milk yield and
persistency was lowest for cows offered the high
protein, high solubility diet, whereas blood urea N was
highest. However, similar diets fed under moderate
temperature conditions had little effect on performance (Higginbotham et al., 1989a1, suggesting that
excessively high dietary protein content may have
deleterious effects only in a heat stress environment.
In two trials, cows were fed diets with high ( 6 1 to
64%) and low (47 to 5 5 % ) protein degradabilities to
cows under shade or shade plus evaporative cooling
environments (Taylor et al., 1991). They reported
reduced milk yield for low degradability diets in the
first trial but significant improvements in both DMI
and milk yield for low degradability diets during the
second trial. In the first trial, temperatures were
cooler and a lower quality of escape protein source
(corn gluten meal) was used, whereas, in the second
trial, temperatures were hotter and a higher quality of
ruminal escape protein (corn gluten meal and blood
meal) was used, suggesting that the degree of heat
stress as well as protein quality are factors during hot
weather. Cows fed diets with a similar ruminally
undegradable protein content but from high quality
(blood, fish, and soybean meals) or lower quality
(corn gluten meal) sources were kept in shade or
shade plus evaporatively cooled environments (C hen
et al., 1993). Lysine was -98 and 3 9 % of DM for high
and low quality protein diets. Although protein quality
x environment interaction was not significant, cows
fed high quality ruminal escape protein gave 3.8 and
2.4 kg more milk in the evaporatively cooled and
shaded environments, respectively, than those fed low
quality proteins. The authors theorized that the
greater response to high quality protein for cows in
the cooled environment was because the amount of
protein metabolized for energy was reduced and less
energy was used in converting NH3 to urea. In
addition, cows in the cooled environment had higher
milk yield and thus greater protein demand.
Huber et al. (1994) summarized protein research
by concluding that, when cows are subject to hot
environmental conditions, ruminally degradable protein should not exceed 61% of dietary CP and should
not exceed NRC recommendations (NRC, 1989) by
greater than 100 g N/d. One hundred grams of N is
equivalent to about 3.1% CP in the diet, assuming 20
kg DMI/d. In addition, the authors reported that cows
fed diets containing 241 g/d lysine (1%of DM) yielded
3 kg more milk than for diets containing 137 g/d lysine
(.6% of DM).
Electrolytes and Acid-Base Chemistry
During hot weather, declining DMI and high
lactation demand requires increased dietary mineral
concentration. However, alterations in mineral
metabolism also affect the electrolyte status of the cow
during hot weather. The primary cation in bovine
sweat is K (Jenkinson and Mabon, 1973), and sharp
increases in the secretion of K through sweat occur
during hot climatic conditions (Jenkinson and Mabon,
1973; Johnson, 1967; Mallonee et al., 1985). The
absorption of macrominerals, including Ca, P, and K,
declined during hot temperatures (Kume et al., 1987;
Kume et al., 1989). Kume et al. (1986) also reported
that trace element requirements may increase with
elevated environmental temperature. Lactating cows
subjected to hot climatic conditions and supplemented
with K well above minimum NRC recommendations
(NRC, 1989) responded with greater milk yield
(Mallonee et al., 1985; Schneider et al., 1984; West et
al., 1987). Cows supplemented with 5 5 % Na during
hot climatic conditions also demonstrated greater feed
DMI and milk yield compared with those receiving
.18% Na (Schneider et al., 1986).
Potassium and Na are the primary cations involved
in the maintenance of acid-base chemistry. The cow
subjected to hot climatic conditions can have acid-base
disturbances resulting from respiratory alkalosis,
subsequent renal compensation by increasing urinary
excretion of bicarbonate and Na, and renal conservation of K (Collier, 1982a). Electrolytes are a key
element of acid-base chemistry and their supplementation during heat stress may be critical to
J. h i m . Sci. Vol. 77, Suppl. WJ. Dairy Sci. Vol. 82, Suppl. 211999
MANAGING HEAT-STRESSED DAIRY COWS
homeostatic mechanisms. Block ( 1984) pioneered the
use of dietary anions and cations to alter the acid-base
chemistry of the periparturient cow, creating a metabolic acidosis and altering Ca metabolism. However,
during extreme heat stress, cows exhibited a respiratory alkalosis with elevated blood pH and reduced
concentrations of blood C02 and bicarbonate
(Schneider et al., 1984). Although the respiratory
alkalosis, which was indicated by elevated blood pH,
suggested an excess of bicarbonate, the elevated pH
actually was a result of a carbonic acid deficit created
by C02 expiration via hyperventilation (Benjamin,
198lj. Escobosa et al. (1984) found that providing
diets that were high in Na and K and normal for C1
resulted in greater DMI and milk yield, compared
with diets high in C1 and normal for Na and K.
Calculated cation-anion difference (mEq Na + K - C1)
was 35.0 mEq/100 g of feed DM for the high cation
diet and -14.4 meq/100 g of feed for the high C1 diet. A
high C1 diet depressed DMI and was associated with
low blood pH and reduced blood buffering. Others
have reported similar results, with greater DMI and
milk yield from higher cation-anion balance (alkaline) diets (Tucker et al., 1988; West et al., 1991). In
work designed to compare efficacy of increasing cationanion difference using either K or Na to increase
cation content of the diet, DMI increased linearly with
increasing cation content, and Na or K was equally
effective as a cation source (West et al., 1992).
The improved intake or milk yield observed when
more alkaline diets are fed to lactating cows may be a
result of improved blood buffering or correction of
mineral deficiencies; however, separating the two
effects is very difficult. Perhaps acid-base chemistry
for the cow is optimized by providing the correct
balance or ratio of electrolytes. Baldwin et al. (1980)
stated that futile cycles contribute substantially to
maintenance energy expenditures and that Na and K
transport may account for 20 to 30% of those costs.
Cows subjected to heat stress in climatic chambers
and fed a high mineral diet (1.93, .68, and 1.85% of K,
Na, and C1, respectively) or basal diet (.97, .19, and
.2% of K, Na, and C1) responded with more rapid
respiratory rates at night for the high mineral diet, as
well as higher rectal temperatures. Dry matter intake
and milk yields were similar among treatments
(Schneider et al., 1988a). This was in the presence of
lower blood pH, probably resulting from the high C1
content of the high mineral diet. Greater urinary NH3
was observed, and NH3 was probably serving as a
companion ion for hydrogen ion excretion.
One of the difficult issues associated with the
electrolytes researched revolves around the com-
29
pounds used to alter mineral content of diets. In some
studies, salts such as NaCl or KC1 were used, whereas
in others Na or K bicarbonates or carbonates were
used. The cation-anion difference approach to formulation considers all elements that are critical to the
equation. In studies in which the equation was not
considered in choosing mineral supplements, results
may be clouded by unknown interactions. Florida
researchers studied the cation-anion difference approach to electrolyte formulation through empirical
modeling of numerous macromineral research studies
from several locations (Sanchez et al., 1994). Data
were presented for individual macrominerals and for
cation-anion difference ( N a + K - C1, mEq/100 g DM)
during summer and winter seasons. Dietary K and Na
elicited increased DMI response at levels well above
NRC ( 1989) recommendations, whereas increasing
dietary C1 depressed DMI, especially during hot
weather. The combination of respiratory alkalosis and
increased renal bicarbonate filtering probably reduced
blood buffering capacity sufficiently to make the dairy
cow more susceptible to the effects of an acidifying
agent such as C1. The DMI response to increasing
cation-anion difference was similar during cool and
hot weather, which suggests that the effect is consistent across these environments (Sanchez et al., 1994).
This is consistent with Georgia work in which Na and
K were equally effective for increasing the cationanion difference (West et al., 1992). Intake was
improved by 5.5 and 5.2 kgld in cool and hot weather
when the cation-anion difference was increased in the
range of -79 to 324 mEq/kg of feed DM (West et al.,
1991).
Heat stress is cyclic in nature, with cows generally
being at the peak of their stress by midafternoon and
cooling somewhat in the evening and early morning
hours. Cows may exhibit a respiratory alkalosis in the
afternoon but physiologic responses may actually
overcorrect so that when cows become cool a metabolic
acidosis occurs. During the day, cows in an unshaded
environment had higher rectal temperatures and
respiratory rates than shaded cows, but at night both
measures were lower for cows with no shade
(Schneider et al., 1988b). Blood bicarbonate was
consistently lower for the no-shade cows, and urine pH
was elevated during the day and low at night,
following trends for blood pH. Ammonium concentration was greater during the cool evening period for
cows housed in climatic chambers, reflecting greater
H+ ion excretion in response to acidosis (Schneider et
al., 1988b). Cows in early lactation during hot
weather exhibited a marked decline in plasma Na, K,
and C1 concentrations during the day, with concentra-
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30
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tions returning to normal at night (Maltz, 1994). The
authors speculated that active absorption of Na from
the rumen, which is dependent on VFA concentration
in the rumen, was limited by reduced VFA production
associated with lower DMI, and that C1 absorption is
coupled with Na absorption. In addition, K and C1
losses in sweat are significant. The greater losses of K
and C1 ions, coupled with potential for reduced
absorption and intake of Na, may account for the
deficit that occurred during the day. During the cooler
hours of the day, DMI was greater, and at night
concentrations of elements returned to normal (Maltz
et al., 1994). Wide swings in the acid-base chemistry
and blood electrolyte content of the cow occur within a
short time span. Sanchez et al. (1994) suggested that
feeding programs targeted at physiologic needs of the
cow at specific times might address the differing
physiologic conditions of the cow that occur throughout the day. They also theorized that, because the
heat-stressed cow eats more feed during the cooler
evening hours and less during the day, ruminal acid
production would be greater at the time when the cow
is experiencing a compensated metabolic acidosis.
Feeding strategies designed around the current physiological status of the cow might help alleviate the
metabolic extremes caused by heat stress and improve
performance. More work is needed to determine the
effectiveness of altering electrolytes and cation-anion
difference in the lactating cow, especially during hot
climatic conditions, and to determine if targeted
feeding is a viable option.
Water Utilization and Metabolism
Water is the most critical nutrient for cattle, and
the need is intensified for those subjected to hot
climatic conditions. Classical work demonstrated that
water losses through the lungs increased gradually
and losses from the skin increased sharply as ambient
temperature increased from about 16°C to 35°C
(Kibler and Brody, 1950). Cows acclimated to 21.1"C
and then exposed to 32.2"C for 2 wk increased water
consumption by 110%, and water losses from the
respiratory tract and from the skin surface increased
by 55% and 177% at the higher temperature
(McDowell and Weldy, 1960). Such changes increase
the volume of water needed.
Water requirements have not been extensively
investigated because water is usually supplied in
abundance. Water consumption is closely tied to feed
DMI; Murphy et al. (1983) reported a highly significant ( P < .0001) correlation of .626 between DMI and
water intake. Water consumption increased from 35.6
L/d at 24 d prepartum to 65.2 L/d at 42 d postpartum
as DMI increased from 9.6 to 16.2 kgld and milk
production increased to 25.7 kgld (Woodford et al.,
1984). Quality of water may affect consumption.
Water with 4,100 mgL of total dissolved salts from an
added complex salt mixture increased water consumption compared with fresh water with less than 450 mgl
L total dissolved salts, but persistency of milk yield
was reduced by 25 and 27% in winter and summer by
the high salt water (Wegner and Schuh, 1988). In
Kuwait, brackish well water, which contained 3,574
mg/L of total dissolved solids had little effect on water
Consumption when fed under hot, arid conditions and
resulted in a slower rate of decline in milk yield
during the course of the experiment when compared
with fresh water (Bahman et al., 1993). Intake of DM
increases with Na and K supplementation during hot
weather and with more alkaline diets. A positive
correlation between sodium intake and water consumption has been demonstrated (Murphy et al.,
19831, and supplementation of diets with high Na and
K content will probably increase water intake during
hot conditions. Maltz et al. (1994) indicated that
water turnover is related to DMI, and milk yield and
respiratory-cutaneous water loss by the cow are major
factors increasing ion outflow under heat stress.
Greater water content in the rumen tends to
accelerate ruminal turnover (Silanikove, 19921,
which may be beneficial to cows during hot weather
because the rate of passage of digesta is slower and
may contribute to gut fill. Consumed water may have
a direct cooling effect via the reticulorumen (Beede
and Collier, 1986) in addition to the cooling effects of
panting and sweating. Cows offered drinking water at
10°C or 28 to 30.2% consumed more DM (Baker et
al., 1988; Milam et al., 1986) and yielded more milk
for the cool water treatment (Milam et al., 1986).
Cows offered chilled drinking water generally consumed less of the chilled water, but the amount of
heat absorbed by the chilled water was still significantly greater than for the warm water treatment.
Chilled water cooled the cow, as evidenced by reduced
respiratory rate (Lanham et al., 1986). Initial water
temperature is critical and the control water temperature for the Texas studies just discussed was relatively
high. Economic benefit for the practice must be
considered, but the benefits of the consumption of
large quantities of cool water on the comfort of the cow
and ultimately DMI and milk yield is apparent and
should be considered when devising a water supply
system for the dairy herd.
J. h i m . Sci. Vol. 77, Suppl. 215. Dairy Sci. Vol. 82, Suppl. 211999
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MANAGING HEAT-STRESSED DAIRY COWS
Vifamins
The topic of heat stress and vitamin requirements
is addressed briefly in regards to niacin use (NRC,
1989), and regarding vitamin A for poultry (NRC,
1981). It was noted that the requirement and the
absorption of vitamin A for poultry was unchanged by
heat-stress conditions. It becomes apparent that there
is a dearth of information in the scientific literature
regarding the effects of heat stress on requirements or
metabolism of vitamins, although an early study
showed that heat stress caused a decline in vitamin A
stores in liver of steers (Page et al., 1959). The effect
on gain or nutrient utilization has not been elucidated.
In practical terms, it appears that adjustments in
vitamin content to account for reduced DMI are
justified. Further investigation into the effects of high
ambient temperature and subsequent heat stress on
the requirements and metabolism of specific vitamins
is needed. Only then can the impact of heat stress on
individual vitamins be ascertained and recommendations established for use in hot climatic conditions.
Level of production may determine how effective some
nutritional practices are during heat stress conditions.
Cattle supplemented with niacin ( 6 g/d) during
summer months increased milk yield by .9 kg/d
compared with controls (Muller et al., 1986).
However, with cows yielding greater than 34 kg of
milk per day, yield improved 2.4 kg/d for niacin.
Niacin can prevent ketosis and is involved with lipid
metabolism. Energy intake for early lactation cows is
often inadequate and fat mobilization is excessive.
Muller et al. (1986) speculated that niacin improved
milk yield by affecting lipid and energy metabolism,
by stimulating protein synthesis by ruminal microorganisms or by causing other effects on ruminal
microorganisms. The greater response for high producing cows indicates that these cows are more subject to
stress, have greater nutritional and metabolic demands, and may derive greater benefit from nutritional modifications than lower producers. It also
indicates the need to conduct research with high
producing cows so that those demands can be identified. Di Costanzo et al. (1997) reported that supplementation with niacin at three levels ( 12, 24, 36 g/
c0w.d-l) resulted in skin temperatures at the rump
and tail of about 0.3% lower with niacin supplementation, an indication that vasodilation and greater blood
to flow to the peripheral tissues associated with niacin
use improved heat loss by the cow. Rectal temperature
and milk yield were unchanged.
Cattle fed endophyte-infected tall fescue seed to
induce fescue toxicosis exhibited reduced herbage
intake per grazing session as ambient temperature
increased (Dougherty et al., 1991). Fescue toxicosis
increases susceptibility t o heat stress, and cattle
ingesting the endophyte often exhibit greater rectal
temperature and seek out shade and mud holes during
the day. Supplemental thiamine improved grazing
intake by extending grazing time during hot weather.
The authors theorized that the expression of tall
fescue toxicosis in beef cattle subject to high ambient
temperatures may involve a thiamin deficiency, which
was partially alleviated by the supplemental thiamine.
Implications
The modern dairy cow with high genetic potential
consumes and metabolizes a large quantity of
nutrients, and it is apparent that if the cow is to
produce to her potential that some heat abatement
procedures are necessary to dissipate the large
amount of metabolic heat produced. By minimizing
the increase in body temperature that occurs with hot
environmental conditions, greater DMI is encouraged
and the gross efficiency with which dietary nutrients
are used by the cow is improved. A reduction in body
temperature reduces sweating and panting. The loss
of electrolytes via skin secretions is minimized and
disturbance of acid-base chemistry from hyperventilation is moderated. Modification of the environment by
shading and cooling enhances the quantity of
nutrients consumed by the cow but can also affect
nutrient requirements.
With the onset of hot weather, cows will adapt to
the environmental conditions and DMI and milk yield
will usually stabilize at a level below that during cool
weather. Reformulation of diets is necessary to
achieve sufficient nutrient density to maintain
nutrient intake. However, some nutrients are required
at still higher levels during heat stress ( K and Na, for
example), and supplementation above NRC minimums (NRC, 1989) results in improved production
during hot weather. There is evidence that a ratio or
proportion of mineral elements benefits the heatstressed cow, and the concept of dietary cation-anion
difference may have merit. If so, not only is dietary
electrolyte content important, but so is the source of
electrolytes as well; KC1 or NaCl are neutral in the
cation-anion difference equation, whereas Na or K
carbonates or bicarbonates contribute to alkalinity of
the diet. Apparently, an excess of acidogenic ions such
as C1 reduces DMI in the cow, especially during hot
weather; this could result from the reduced blood
J. h i m . Sci. Vol. 77, Suppl. 215. Dairy Sci. Vol. 82, Suppl. 2/1999
32
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buffering capacity that occurs in the presence of a
respiratory alkalosis. Although inadequate nutrient
intake is of particular importance during hot weather,
excesses of nutrients such as protein can contribute t o
lowered efficiency in the cow. The practice of boosting
nutrients such as CP to high levels to minimize
potential deficiencies can have negative consequences
because nitrogen in excess of requirement must be
metabolized and excreted, requiring energy. Recent
research demonstrates that protein content, ruminally
undegradable protein value and quality of amino acids
delivered to the lower digestive tract play a complex
role in the nutrition of the heat-stressed cow. Additional research is needed in this area to establish
minimum and maximum protein content of the diet
and to determine the effects of protein content on the
efficiency of nutrient use by the cow.
Heat increment of the dietary ingredients plays a
role in both the efficiency with which nutrients are
utilized and heat generation potential of the diet.
There is a potential for formulation of diets that are
particularly favorable for use during hot weather.
High fiber diets result in production of large quantities of ruminal acetate, which is used with less
efficiency than propionate. The efficiency of use for
ME from high fiber diets is lower than for diets with
high levels of rapidly fermentable carbohydrate,
providing a scientific basis for the practice of feeding
low fiber diets during hot weather. Low fiber diets
must be implemented cautiously because of the
necessity for sufficient fiber to promote good rumen
function and t o maintain animal health and wellbeing. Use of very high quality forages for hot weather
feeding may ensure both adequate content of digestible fiber and improved content of fermentable sugars
to enhance production of rumen VFA. Dietary fats
have a relatively low heat increment because of a high
efficiency of use by the cow. Fats may be of particular
value during hot weather and may be undervalued by
current energy equations because of the lower heat
increment. Results from research with fat feeding
during hot conditions have been variable. Rumen inert
fat allows inclusion of relatively high levels of fat in
the diet, and experiments t o elucidate the effects of
feeding these diets during hot weather should be
conducted. Because high levels of dietary fat reduces
the amount of fermentable energy in the rumen, the
quantity of protein and ruminal escape value for that
protein may be particularly critical when feeding high
fat diets.
Practical management considerations such as frequent feeding and ready water availability are logical.
Water should be available in holding pens, travel
alleys, and near feed bunks. Provision of fresh feeds
through multiple feedings encourages frequent feeding
bouts by cows, and making the feeding area as
comfortable as possible through shading and(or)
cooling should enhance feeding frequency and total
intake. Animal behavior during hot weather indicates
that the animal will consume more feed during cooler
evening hours and feeding quantities and schedules
should be adjusted to accommodate changes in behavior because of season of feeding. Research evaluating animal facilities and environmental conditions
including temperature, humidity, and photoperiod for
their effect on feeding behavior may be beneficial in
designing feeding and management programs for the
lactating cow during heat-stress conditions.
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