Burris said spring-calving cows really need to conceive early in the breeding season, preferably before late June, for best results.

The UKREC conducted a trial several years ago where they separated similar cows into three breeding periods of 45 days each on high-endophyte fescue. Cows exposed to bulls from June 19 to Aug. 4 had a pregnancy rate of only 59 percent. The average maximum daily temperature reaches 90 degrees Fahrenheit by June 20 at the UKREC. The elevated temperature, coupled with the endophyte present in most fescue pastures, likely contributed to that decreased performance.

“We have also measured the alkaloid levels in high-endophyte fescue at this location,” Burris said. “The primary culprit in toxicity of high-endophyte pastures seems to be ergovaline. After our July 10 measurement, the ergovaline levels dramatically increased.

“So this toxicity, coupled with high temperatures, appears to mean that breeding will not occur at acceptable rates in July, August and September. Therefore, we believe cows need to be pregnant by the end of June for best results.”

He explained that ergovaline levels differed greatly by pasture and that could make it possible to avoid the “hot” pastures, those with higher endophyte levels, during the summer months, especially during breeding and heat stress.

There are several other keys to a successful breeding season. Obviously, fertile bulls are extremely important and breeding soundness evaluations are essential. Producers should supply a complete mineral supplement on a year-round basis. If producers use artificial insemination, they will need to manage the details of artificial insemination and estrous synchronization protocols.

“In the short-run, don’t let cows lose body condition as the breeding season nears,” Burris emphasized. “Lush, watery grass might not support regaining condition after calving, peak milk production and rapid re-breeding. Do whatever it takes to get ‘em bred and bred early.”

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