by Heather Smith Thomas

Heat stress in cattle can affect next year’s calf crop, in regions with hot summer weather and high humidity—if cows are bred in summer. Dr. Gary Williams, Animal Reproduction Laboratory, Texas Ag Experiment Station, Texas A & M, says that along the Gulf Coast and in most of the southern U.S. “people often use cattle with Brahman or zebu influence, since Bos indicus cattle have more heat tolerance. Bos taurus (British and European breeds) are heat stressed more readily in hot environments particularly where humidity is high.”
Light colored animals have an advantage over dark ones, being able to reduce the amount of heat absorbed, though they may have more incidence of sunburn, says Williams. Dark colors absorb more heat because they have less ability to reflect it, he explains.

Dr. Don Spiers, Associate Professor of Animal Science, University of Missouri, Columbia, says if you have dark-hided cattle, they tend to have a higher body temperature — up to 104 degrees F. during hot weather, as opposed to lighter colored cattle that will be about 10l to 102 degrees, which is normal.

Affects on Reproduction
Williams says, “We always advise ranchers to avoid breeding during July, August, or September, and have the breeding season during cooler times of year. The impact of high heat, thermal loads, and high heat index—the result of high humidity—can have a marked effect on embryonic survival. Cows may become pregnant but will lose the pregnancy. This is particularly dramatic in dairy cattle, to the point that dairymen in this part of the country generally don’t try to breed cows during the hottest seasons because of the tremendous adverse impact on fertility.”

The most common time for heat stress to cause embryonic loss is in the first week after mating (the embryo fails to attach to the uterus), though in some circumstances a pregnancy can be lost up to the first month of gestation. More rarely, heat stress can cause pregnancy loss in late gestation. Anything that stresses the cow or fetus at that time can trigger premature birth, which usually results in death of the calf.

Heat stress can also cause infertility in bulls. Under normal conditions, testicles are kept a few degrees cooler than the body because heat interferes with proper sperm development, but during hot weather the body’s methods for keeping the testes cool are not adequate. “Semen quality declines markedly during extreme temperatures of mid to late summer and early fall. There are carry-over effects, since the spermatogenic cycle is about 60 days from the time the cell is produced until it is out of the system (either through the process of ejaculation or through death and resorption). So you might have problems with infertile bulls even up to two months after the hot weather ends,” says Williams.

Bulls are frequently semen tested in late October or early November in the University’s fall-calving herd, to be ready for the fall breeding season. “We see effects in semen that we believe are due to the high heat load the bulls experienced earlier—called secondary abnormalities. In the bull, semen quality is directly impaired during the period of high heat load (and up to 60 days later), and embryonic mortality is higher in cows,” says Williams.

Dennis Maxwell of McNay Research Farm, Iowa State University, recalls a project a few years ago at the University of Missouri in which calving season was moved to early summer, to calve in more natural conditions when there’s lots of green grass. “But that put the breeding season in late summer when temperatures were high, and the cows had a very poor conception rate (30 percent),” he says.

“Here in southern Iowa there were problems in several herds, including our own, with early embryonic losses, when we had an extremely hot period in mid July. There can be several kinds of reproductive problems with a late breeding season. Many producers have such a problem with mud and calf health in early spring that a lot of us have been moving our calving season later. But then we get into a situation where we are dealing with hot weather during the breeding season,” says Maxwell.

“We also have a fall calving herd. Those cows start calving the middle of August. We have some hot weather then. If it’s extremely hot and humid, with no shade, it’s hard on baby calves.

If it’s so hot they don’t feel like nursing, they can become dehydrated in hot weather, especially if they get sick. Shade and water are very important, and the color of the calves also makes a difference. If a black calf is out in the middle of a pasture with no breeze and it’s 100 degrees, that can be a problem. Plan your breeding season for a time of year that works best for your region and situation,” says Maxwell. ©