Most printed gestation tables for beef cattle will tell you calving takes between 283 and 285 days. What those tables don't mention is how temperature can shorten up gestation times in both cows and heifers.
Glenn Selk, Oklahoma State University emeritus Extension animal scientist, says OSU physiologists looked at data from 60 Angus/Hereford crossbred cows, and found a definite impact on calving times tied to temperature. Their observations were done over two calving seasons.
Fall-calving cows and heifers were all been artificially inseminated in early November, or early January. Semen from the same sire was used for all. They were all exposed to a single cleanup bull for 35 days (four days post-AI). Weather prior to calving was significantly different for the two groups.
Average maximum temps the week before calving was 93 degrees F for what were considered the early fall group (August). For the late fall group (October) average maximum temperature was 66 degrees F, nearly 30 degrees difference.
Both groups had 100% survival rates and very high re-breeding rates (90% and 92%, respectively). But the length of gestation was nearly a full week shorter (six days) in the August-calving crowd the first year (279 days versus 285 days); and four days shorter the second year (278 days versus 282 days). Selk stresses the gestation times are averages, meaning about half of the cows actually calved earlier.
The takeaway, says Selk, is that producers with fall-calving cows should expect to see calves coming several days ahead of those textbook gestation tables. He recommends doing routine heifer and cow checks seven to 10 days ahead of that expected first calving date.
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