Heat Stress Affects Fertility
by Heather Smith Thomas
Heat stress in cattle can affect next
year’s calf crop, according to Dr. Gary Williams with the Animal
Reproduction Laboratory, Texas Ag Experiment Station.
“We always advise ranchers to avoid breeding during July, August,
or September, and have the breeding season during cooler times of
year.” Williams says. “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.
Don’t forget about the heat’s affect on baby calves. Fall
calving herds that start calving the middle of August can suffer.
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.
