Fall-calving cows and heifers don't always read and follow the published gestation tables for their respective breeds, says Glenn Selk, Oklahoma State University Extension cattle reproduction specialist. It is has been reported on several occasions that fall calving cows have lighter birth weights than spring calvers. Part of the reason for those lighter births may be attributed to shorter gestation lengths.
He points to research where Oklahoma State University physiologists studied early fall (August) and late fall (October) calving cows. Data from two successive years were combined for 50 Angus X Hereford crossbred cows. (Source: Kastner, Wettemann, and co-workers. 2004 Southern Section of American Society of Anim. Sci. Abstract 77).
The "early” and "late” fall calving cows had been artificially inseminated in early November or early January, respectively. Semen from the same sire was used for all cows. All cows were exposed to a single cleanup bull for 35 days at 4 days after the AI season.
The weather prior to calving was significantly different for late pregnancy in the two groups. The average maximum temperature the week before calving was 93 degrees F. for the "early” fall group. The average maximum temperature the week before parturition in the "late” calving group was 66 degrees F. There was a 100% survival rate for calves in both groups and both groups of cows had very high re-breeding rates (93% and 96%, respectively). The average gestation length for the "early fall” cows was 3 days shorter (280 days) as compared to the "late fall” cows (283 days), says Selk.
"Producers with early fall-calving cows should expect calves to start coming several days ahead of the 'textbook gestation table' dates. They should begin their routine heifer and cow checks at least a week to 10 days ahead of the expected first calving date," he says.
For more information on calving management, check out Oklahoma State University Extension publication called "Calving Time Management for Beef Cows and Heifers” (E-1006).
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