Heat stress can affect reproductive performance of beef cattle. The real concern for the beef cattle producer is that it doesn't seem to cool down in the evenings. Therefore, it is difficult for the cattle to get rid of the heat load that was built up during the day. Heat stress is a major cause for reproductive failure in cattle that are grazing endophyte-infected fescue. A fungus in fescue (endophyte) produces chemical compounds that reduce the ability of an animal to dissipate heat. The inability to dissipate the heat load results in reproductive failure. In females, heat stress can affect embryo implantation. Heat stress can cause abortion in cattle up to 45 days of gestation. Females that are heat stressed typically experience embryonic loss in the first week of gestation and are repeat breeders. Fetal loss can also occur when females are heat stressed from 25 to 45 days of pregnancy. Heat stress can also affect bull fertility. Heat stress affects spermatogensis (production of sperm). Because the testicles and tubules store sperm, the effect of heat stress doesn't show up until about 45 to 60 days post-heat stress. One of the mechanisms to keep the testicles cool is that the muscles relax, allowing the testicles to hang farther from the body. What can you do? Allow access to plenty of clean water. Most pastures have trees that will provide shade. Avoid working cattle during the hot temperatures. Finally, keep the flies under control. It seems that when cattle have a fly load, they tend to bunch. This causes less air circulation around the cattle, and increased heat is generated.

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