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Managing for reproductive success of dairy cattle during the heat

Pregnancy rates in dairy cows can be drastically reduced when temperatures
rise in the summer. Dairy cows are highly susceptible to heat stress due to
the metabolic heat produced during lactation. Producers use a variety of
management strategies to minimize the loss of pregnancy rates.
In the Midwest, lower estrus detection and conception rates occur from June
through October. This results in lower pregnancy rates. During times of heat
stress, reduced pregnancy rates can be caused by a number of factors
including reduced expression of estrus, impaired egg quality and abnormal
follicular function, including cystic and anovular cows.
Early embryonic death is significantly higher when cows are heat stressed
due to an altered uterine environment, reduced blood flow to the placenta,
and sensitivity of the embryo to elevated temperatures during the first
three days after breeding.
One strategy employed by many producers in the southern states is to
concentrate on getting cows bred before the heat, making an extra effort to
ensure that any cow past the voluntary waiting period and not confirmed
pregnant is inseminated. Heat stress reduces the length and intensity of
estrus behavior and results in fewer cows receiving artificial insemination
(A.I.). Utilizing a synchronization program in heat stressed cattle will
maximize service rate, thus enhancing pregnancy rate.
Heat abatement is essential in the weeks preceding breeding, as well as the
first week after breeding. The time of greatest susceptibility is
immediately after the onset of estrus and early post-breeding. Providing
adequate shade and water to cows on pasture can help keep them cool,
resulting in increased embryo survival. In the barn and holding areas, the
use of fans and sprinklers will help to cool the cow and the air around her.
Make sure that the sprinklers are set to the appropriate time interval and
that the droplet size is large enough to soak the cow to the skin – not a
mist, which will sit on top of the hair and insulate the cow. Sprinklers
should be running for between one to three minutes in every 10 to 15
minutes. It is critical that the sprinklers be turned off and fans are
running for evaporation to occur, resulting in the cows feeling cooler
because some of their body heat is used to evaporate the water. Avoid
overcrowding pens and keep up on fly control to prevent bunching.
Although bulls may be used to help compensate for cows not showing estrus,
they cannot improve the altered uterine environment, blood flow to the
placenta or embryo sensitivity. Besides, bulls are also affected by heat
stress. They are less active, breed fewer cows, and have lower quality semen
when they are hot. In addition, just two to three days of exposure to
temperatures over 85°F can reduce semen quality for the following eight
weeks.
Nutrition is another important factor in maintaining reproduction in the
summer.
Research has demonstrated that negative energy balance is correlated with
impaired reproductive performance. When cows reduce intake as a result of
heat stress and fall into a negative energy balance situation, there are
negative effects on plasma concentrations of insulin, IGF-1 and glucose,
which result in poor follicular development, poor quality of oocytes and
reduced expression of heat. Minimizing dry matter intake losses during heat
stress is critical.
Keeping cows cool will result in more frequent meals and reduce slug
feeding. It is a good idea to feed fresh feed more often, place extra
waterers in return alleys and provide shade at the bunk area. Review your
ration before the heat hits to make sure there is adequate fiber, potassium,
sodium and buffers if needed.
Source: Faith Cullens, Michigan State University Extension

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