Gary Williams always advises ranchers in humid regions to avoid breeding from July to September. High heat, thermal loads and high humidity can take their toll on both cows and bulls, says the Texas A&M University (TAMU) Animal Reproduction Laboratory research leader.

Cows may become pregnant, but lose the pregnancy. Meanwhile, bulls may suffer reduced semen quality.

Dennis Maxwell of Iowa State University's (ISU) McNay Research Farm refers to Missouri research conducted several years ago on early summer calving, which delayed breeding until late summer. The result was a 30% conception rate, he recalls.

“Here in southern Iowa there were problems in several herds a few years ago with early embryonic loss when we had a hot period in mid-July,” he adds. “Many producers, and our research farm, had such problems with mud and calf health in early spring that we moved our calving season later. Then we got into a situation with hot weather in breeding season.”

The ISU facility also has a fall calving herd that begins calving in mid-August.

“If it's hot and humid with no shade, it's hard on baby calves,” Maxwell says. “If it's so hot they don't nurse, they become dehydrated. Shade and water are important, and calf color can also make a difference. A black calf in a pasture with no breeze and it's 100° F, can be a problem.

“Semen quality declines markedly during high summer temperatures,” Williams says. “And since the spermatogenic cycle is 60 days from the time that cell is produced until it leaves the system (through ejaculation or resorption), there are carryover effects. You could have problems with infertile bulls up to two months after hot weather ends.”

As a check for fall breeding, bulls in TAMU's fall calving herd are often semen-tested in late October, Williams says.

“We think the defects in semen are due to the high heat load that bulls experienced earlier,” he says. “When we check them again in late March, when they're getting ready for the spring herd, we don't see those defects.”

ISU animal scientist Curtis Youngs says bull fertility problems due to heat stress can be mild (bull has a normal libido, but more cows than normal come back into heat) or severe (complete wipe-out of all sperm in the testes and stored in the epididymis). This latter situation causes a true sterility for 45-60 days, depending on the severity and duration of the heat stress.

The long-term problems encountered are from sperm damaged during formation, Williams says. Damaged immature cells take the same length of time to get through the system but are useless once they get there.

“Sometimes, the first few matings after the heat stress are fine, followed by a long string of matings in which no cows become pregnant,” Williams says. “Once the heat-damaged sperm work their way through the system, the bull has normal sperm again and fertility picks up.

“Bulls should avoid physical activity during the heat of the day. If you have the time and facilities, night-only breeding can work, but it's labor-intensive,” Williams says. “Turn him with the cows in late evening, then entice him back in with a bucket of grain in the morning. Lock him away from the cows during heat of day in a shady place with good ventilation” — good bull disposition permitting.

Good body condition prior to breeding season will also help bulls weather the heat, Youngs points out. A thin bull won't have much appetite and could lose more weight than he can afford. Well-managed animals do a better job of tolerating short-term stresses, Youngs explains.