COLUMBIA, Mo. – Prolonged heat stress this year may bring a smaller calf crop next year. Herd owners are seeing cows known to be pregnant coming back into heat to be rebred.

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

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 said, “It’s probably heat stress.”

To find out more, he took his ultrasound device to the field for rechecks of pregnancies. “Roughly, we saw 20 percent open in herds on average. 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 percent loss of pregnancies after early pregnancy diagnosis (30-32 days of gestation). He also found dead embryos from AI breeding from mid-May through June.

“I am getting lots of calls on this,” Poock added.

“The early embryo is sensitive to temperatures above normal body heat,” he says. 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.”

High temperatures also disrupt ovarian and uterine functions. That affects 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 shot up with lack of baled hay.

At the MU Thompson Farm, Spickard, cow herd pregnancy checks have been near normal, says Jon Schreffler, farm manager. The heifers were slightly lower this year compared to 2017. But last year pregnancies were above normal.

“We’ll do final ultrasounds in late September,” Schreffler adds.

The MU herd was bred mid-April into May. They missed high heat at artificial-insemination time.

Research at the MU herd led to protocols used in the Show-Me-Select Replacement Heifer Program. Part of that protocol calls for ultrasound within 90 days after start of timed AI. Cleanup bulls are turned in with AI-bred heifers. They catch any missed conceptions. That is done before summer heat arrives.

Show-Me-Select protocols give high conception rates and calving-ease births. Fixed-time AI leads to short calving seasons and uniform calf crops.