This year’s heat continues to cause problems for cattle producers as cows are losing embryos and coming back into heat.

Pregnancy losses are due to several reasons, says Scott Poock, University of Missouri Extension veterinarian. "The first is increased internal temperature of the cow," he says.

In July, a northern Missouri beef herd owner saw his bull breeding cows that were known to be pregnant from an earlier pregnancy check. Poock thought it was probably heat stress.

To find out more, the veterinarian took an ultrasound device to the field to recheck pregnancies. "Roughly, we saw 20% open in herds on average,” Poock says. “There are a few outstanding exceptions, but they bred early in April prior to May heat."

At the MU Foremost Dairy, Poock found up to 25% loss of pregnancies after early pregnancy diagnosis (30-32 days of gestation). He also found dead embryos from artificial insemination or AI breeding from mid-May through June.

"I am getting lots of calls on this," Poock adds.

He explains that the early embryo is sensitive to temperatures above normal body heat. However, at six to eight days the embryo becomes heat-tolerant. Early heat stress could lead to embryo loss right away. Those cows come back into heat on schedule, according to Poock.

High temperatures also disrupt ovarian and uterine functions. That affects the quality of the egg, with oocytes being compromised. Fertilization occurs, but the fertilized egg does not develop normally. The embryo dies later.

"Those cows return to heat at strange intervals," Poock says. "I have reports of beef cows showing heats at 30 to 50 days after timed AI. These cows likely conceived but then lost their embryo."

Heat stress also affects bulls with cow herds. "Heat decreases sperm quality, which leads to decreased pregnancies," he says.

Herd owners ask what to do with non-pregnant cows.

"With the lack of grass and hay, these open cows rise to the top of the list for culling," Poock says. "In a normal year, I might evaluate genetics of individual cows to see whether to move them to the fall-calving herd."

This has not been a normal year. With drought-stressed grain crops, feed costs may rise. Forage prices also increase.

The result may be fewer calves next spring.

Source: University of Missouri Extension