Fall calving time is approaching rapidly, with some herds starting sometimes before producers expect.

“Most printed gestation tables predict that calving will take place 283 days — 285 days in some — after artificial insemination or natural breeding,” said Glenn Selk, Oklahoma State University Cooperative Extension emeritus animal scientist. “It is not unusual for cows and heifers that gestate in hot weather to calve a few days earlier.”

Oklahoma State's Division of Agricultural Sciences & Natural Resources recommends that herd managers begin their routine heifer and cow checks at least a week to 10 days ahead of the expected first calving date, based on Oklahoma State research first reported in 2004.

In that research, reproductive physiologists studied cows calving in early fall (August) and late fall (October). Data from two successive years were combined for 60 Angus/Hereford crossbred cows. 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, and all cows were exposed to a single cleanup bull for 35 days at four days after the artificial insemination season.

“The weather prior to calving was significantly different for late pregnancy in the two groups,” Selk said. “The average maximum temperature the week before calving was 93°F for the 'early' fall group. The average maximum temperature the week before parturition in the 'late' calving group was 66°F.”

There was a 100% survival rate for calves in both groups, and the cows had very high re-breeding rates of 90% and 92%, respectively, Selk noted.

The average gestation length for the early cows was six days shorter than the late cows (279 days versus 285 days) in the first year of the study and four days shorter than the late cows (278 days versus 282 days) in the second year of the study.

“Keep in mind, the listed gestation lengths are the average,” Selk said. “This means that about half of the cows calved earlier than that.”

Records from millions of Holstein dairy cows across the U.S. show a similar pattern, according to research published in 2009 in the Journal of Dairy Science, Selk said. The average gestation for Holsteins bred in January and February — calving in October and November — was two days longer than for Holstein cows bred in October and calving in July and August.

“Many of these would be in northern climes with less heat stress and more moderate temperatures in the summer months,” Selk said. “Here in the southern Plains, late-summer heat is more intense and persistent. Therefore, producers with early-fall-calving cows should expect calves to start coming several days ahead of the textbook gestation table dates.”