HEAT STRESS
Getting Back to Basics
By Mel DeJarnette, reproductive specialist
It’s no secret that many areas of the United States (especially the
tropical, subtropical, and arid zones) experience significant economic
losses each year due to stresses suffered by cattle during warm weather.
These losses are manifested directly as losses in milk production, weight
gain, or indirectly as a result of decreased reproductive performance.
This decrease in reproductive performance is usually expressed as an
increase in services/conception, decreased conception rates, increased
average days open and decreased pregnancy rates.
Basics Practices
Obviously, the most basic management practice for handling cows in heat
stress environments is to provide protection from direct sunlight. Cows
should have free access to shaded areas during how weather. Be it from a
grove of trees, shade screens, or a pole barn. Shade, whether natural or
artificial, can reduce the heat stress on production. Cows which are allowed
access to shaded areas have been observed to produce 10.7% more milk and
had 19.1% higher conception rates than the controls without shade.
Shade is especially important at the feedbunk and watering troughs. Since
heat stress reduces feed intake, cows should not be baking in the sun while
they try to eat. Also, placing feed and water close to shade structures
will reduce the amount of physical activity required to move between these
areas and thus, decrease the potential heat load on the animal. While any
shade would be beneficial, cows which travel some distance from shaded
areas to feeding areas may easily become hot enough to lose their appetite
before they get there. Other practices which reduce daily physical activity
will also help reduce the potential for heat stress. Designing walkways and
loafing areas for proper drainage so cows will not have to trudge through
mud and providing effective fly control are a couple of examples.
Crowding should also be avoided. Cows cannot effectively dissipate their
own body heat when they are cramped and exposed to the additional body heat
of surrounding cows. Decrease the number of cows per milking group to reduce
the amount of time spent in hot, crowded allies and holding pens. This may
require temporary fencing of additional space or moving a group of dry cows
to a new location.
Nutrition
Since cows will often decrease feed intake by as much as 25% in heat stress
situations, they need to get more energy from the feed they do consume.
Therefore, rations fed during periods of heat stress should contain a higher
percentage of protein and energy. Also, much of the internal heat produced
by animals is a result of digestion of roughage and fiber. Thus, reducing
fiber intake will help to relieve the heat load on animals. Both of these
strategies may be met by increasing the percentage of concentrates in the
diet. Lactating dairy cows may also require additional sodium and potassium
in their diet during periods of heat stress due to the considerable amounts
lost by perspiration. Any alterations or modifications to the diet, however,
should be done under the guidance and approval of a qualified nutritional
consultant and/or veterinarian.
Estrus Synchronization
The inter-estrus period has not been shown to be significantly affected
by periods of high environmental temperature. However, estrus periods of
cows experiencing heat stress in the southeastern U.S. have been observed
to be shorter in duration and lower in intensity than those of cows in more
temperate environments. Heat stressed cows will be reluctant to spend their
energy on mounting and other physical activities. Thus, cows exposed to high
environmental temperatures will cycle fairly normally (every 18-24 days);
but, due to the shorter duration of the estrus period and the decrease in
physical activity (i.e. mounting, walking fences, etc.) it will be much
harder to catch these animals in heat.
Heat-stressed cattle appear to respond extremely well to prostaglandin
injections. Therefore, estrus synchronization programs could be effectively
implemented with other environmental management systems.
A.I. Best During Heat Stress
Bulls are also very susceptible to heat stress. Some bulls exposed to
even short periods of thermal stress have significant reductions in semen
quality that may last for more than a month. Extended periods of high
environmental temperatures may also reduce the quantity of sperm in each
ejaculate as well as the bulls sex drive or libido. Bulls at Select Sires
are housed in air conditioning or well ventilated barns to minimize effects
of heat stress. A.I. during heat stress periods will expose cattle to better
quality semen than a bull running with the herd could provide.
Summary
Minimizing stress is essential to achieving and maintaining maximum
productivity and profitability in any cattle operation (beef or dairy).
Every summer, the stress associated with increased environmental temperatures
results in decreased performance in herds throughout the United States.
Although we cannot change the weather, we can alter our management practices
in order to minimize the effects of hot weather on our animals.
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