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Breeding cattle

Heat stress can put quash reproductive performance

There are a lot of things that can keep your cows from getting bred. Summer heat is one.

Hot weather is not the best time to breed cows because it can affect reproductive performance. But there’s more to it than that.

Large animals and those carrying a lot of condition have more trouble dissipating heat than smaller, leaner animals, so heat stress is often a bigger problem in feedlots than in pastured cattle, but excessive heat can also adversely affect reproduction rates in cow-calf operations. Dark-colored animals and those with the most insulation (body fat) have the most problems. Cattle that originated in tropical regions tend to be more heat-tolerant than British and European breeds.

Some cattle are genetically better equipped to handle stress of heat and humidity and less likely to suffer negative consequences. Also, cattle that are not adapted to a certain environment may suffer more stress.

If you are raising cattle in fescue country, for instance, it’s not only hot in summer but fescue pastures may contribute to heat stress. Alkaloids in endophyte-infected fescue interfere with blood flow from the body core to peripheral tissues and the animal’s ability to dissipate heat is drastically impaired. This results in significant heat stress, which means decreased feed intake, decreased weight gain/milk production, and poor reproduction.

If cattle get too hot, conception rates drop, and cows that are already pregnant may suffer early pregnancy loss. “If a cow’s body temperature gets too high, this may kill the embryo,” says Don Spiers, professor emeritus, Division of Animal Science, University of Missouri, who has worked with beef and dairy cattle for many years looking at heat stress.

Heat stress can affect early embryonic development. When you observe conception and pregnancy rate in a herd, you may notice problems occurring not during the hot months, but a few months later, since it takes time to show these effects. The frustrating problem is trying to determine the correlation between heat stress and reproduction because it is delayed. You don’t always know if heat compromised the pregnancy until later. 

Some producers are now calving in late spring or early summer rather than during the cold weather of February-March, to be more in synch with nature. But this means breeding during heat of summer, which can be a problem. Also, if you are using AI, you are working/handling cows during the heat, which is an additional stress and conception rates may drop.

If you have to move cattle or work cattle, it pays to watch weather forecasts and try to choose a day that won’t be during a heat wave. Sometimes, however, breeders need to get cattle in for various steps in a heat synchronization and AI program and timing is crucial, regardless of weather, says Julie Walker, beef specialist, South Dakota State University. 

“More people are going to April-May calving to match forage with nutritional needs of the lactating cow (and reduce labor at calving, not having to worry about cold weather in February-March) so now they have to deal with heat instead. They are breeding cows in July and August—which are often the hottest months.”

“This means that if they are putting in CIDRs and synchronizing, and have to pull the CIDRs at a given time, they are under the clock and may have to get those cows in when it’s very hot,” says Walker.

Stress may be a factor with epigenetics as well. “What the parents are exposed to—what the cow is experiencing during pregnancy—can affect the embryo or fetus, or even affect eggs in the female,” Spiers says. “The female is born with all the eggs for her lifetime. If she suffers heat stress that reduces blood flow to those eggs (because she is routing more of the blood to the skin to try to dissipate body heat), the eggs that are developing could be malnourished and not the prime eggs you’d want.”

Stress can take many forms, including hot weather, rough handling, vaccinating/processing/deworming at the same time you are running cows through the chute for applying heat-synchronizing drugs or insemination. Heat stress increases cortisol levels, and this in turn alters the hormones important for reproduction. Suppression of secretion of gonadotropin hinders ovulation.

The gametes in both the cow and the bull produced during hot months will be affected. It takes about 90 to 130 days for development of gametes. This is why we see the impact later, such as in late fall, rather than during the hot months.

Bull fertility can also be affected by heat. Some people put valuable bulls in an air-conditioned barn for part of the time, so they are not as adversely affected by heat stress and more able to breed cows, with better fertility.

“We are not sure what to tell producers when they ask about the best time of year to breed cattle,” says Spiers. There are plusses and minuses to every calving/breeding season. Each producer has to figure out what works best for his/her own situation.

Smith Thomas is a rancher and freelance writer based in Salmon, Idaho. The opinions of the author are not necessarily those of beefmagazine.com or Farm Progress.

TAGS: Breeding
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