by: Heather Smith Thomas

Many cattle producers are changing their calving season to late spring/early summer instead of late winter, to get away from labor-intensive calving during cold weather. There are advantages to having cows calve on green pasture where conditions are cleaner for baby calves than the melting snow and mud of early spring. The cows are on green pasture at a time they need good nutrition for lactation. But calving in April, May or June means breeding in July, August or September. Hot weather can impact the breeding season because it adversely affects fertility and reproduction rates, according to Dr. Ram Kasimanickam, Department of Veterinary Clinical Sciences, Washington State University.

“Heat stress will increase cortisol levels, and this in turn alters the hormones which are important for reproduction, including suppression of secretion of gonadotropin. This in turn hinders ovulation,” says Kasimanickam.

“Heat stress can also affect early embryonic development if the cow does become pregnant. This may cause some additional infertility issues. When you observe the conception and pregnancy rate on a farm or ranch, you may notice problems occurring not during the hot months, but a few months later, since it takes time to show these effects. The gametes (in both the cow and the bull) produced during the 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,” he says.

“Recent findings are also interesting, looking at the impact of maternal stress on the developing fetus. We now have a better understanding about how maternal heat stress might affect the offspring.” The fetus suffers stress during development that will have a lasting impact after birth—on future reproductive performance of that calf when it grows up. The affects observed in later life are not so much due to how the animal was fed and raised (the environment it grew up in) as due to the impact during fetal development.

“If the mother is suffering from heat stress, this can have a serious impact on the developing fetus. It may have a negative affect on the calf's immune function, and impair metabolic function, and/or the future reproductive function. There can be a multitude of adverse affects in the offspring, due to maternal stress.” Most people don't think about this. They see the cow had a live calf and assume everything will be ok.

“The production and reproduction abilities of both male and female fetuses can be affected when they become adults,” explains Kasimanickam.

Heat Stress Affects A.I. Conception Rates -- Heat stress in cows can also impact AI programs as well as natural breeding with bulls. Conception rates with AI are always somewhat lower than conception rates with bull breeding, but good management and a good AI technician can maximize conception rates with AI. Part of good management is minimizing stress—before, during, and after insemination—since stress can lead to early embryo loss. Stress can take many forms, including hot weather, rough handling of the cattle, vaccinating/processing (deworming, etc.) at the same time you are running the cows through the chute for applying heat-synchronizing techniques or insemination.

Julie Walker, Beef Specialist, South Dakota State University, says working cattle in hot weather can be detrimental when doing heat synchronization and AI programs. Any time you have to move cattle or work cattle, it pays to watch weather forecasts and choose a day that won't be during a heat wave. Sometimes, however, breeders need to get cattle in for the various steps in a heat synchronization and AI program and timing is crucial, regardless of weather.

“More people are going to April-May calving to match forage with nutritional needs of the lactating cow (not having to worry about cold weather in February-March) so now they have to deal with heat instead. They are breeding in July and August—which are often the hottest months. This means that if they are putting CIDRs in and synchronizing, and have to pull the CIDRs at a given time, they are under the clock and may have to get cows in when it's very hot. If they plan to do a fixed-timed AI in the morning, they are probably pulling CIDRs at 7 p.m. the evening before,” says Walker.

“If the cattle are not very close to the corrals, and we have to pull CIDRs at 7, this might mean we have to go get the cattle at 4 or 5 p.m. and that's still during the hottest part of the day. We won't be able to give those cows any time to rest and cool down and rehydrate because we have to sort off the calves before we put the cow through the chute. This often equates to 3 hours of working cattle in the heat of the day,” she says.

“A person needs to think about the heat, and plan ahead. It might be a situation where you'd want to move the cattle closer to the corrals in the morning, while it's still cool, so you don't have to drive them as far to the facility that evening. Then they are refreshed and watered before you start working them, and maybe you'll only be moving them for 15 or 20 minutes to get into the working facility. When moving and working cattle that time of year, make sure there is always ample clean fresh water for them, so they won't have any hesitation about drinking and getting rehydrated,” she says.

“If there's a fountain-type tank make sure there's enough water pressure to keep it full. If there are calves with those cows, we have to make sure the calves can reach the water, if it's in a tank,” says Walker.

“We always need to think about heat stress and try to minimize it while moving or working cattle, because heat stress can impact reproduction. A lot of the risk is during early pregnancy, for cows, and heat also has a negative impact on bull fertility,” says Walker.

“Another thing to take into consideration if you are doing heat detection during hot weather is that cows won't be very active during the hottest part of the day; they will be lying around in the shade if possible and trying to stay cool,” she says. You'll have your best luck checking cows for evidence of cycling activity in the early morning or late evening, and they may be most active during the night.

“When breeding cows that late in the summer when it's hot, it really helps with heat detection to use patches on the cows, to know which ones have been ridden. We may not be out there during the coolest part of the day/night to see the riding activity, so we can use that tool to help us identify the cows that need to be inseminated,” she says.

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