Summertime Temperatures Shorten Gestation Length In Fall-calving Cows And Heifers

Fall-calving time is approaching rapidly, with its start in some herds sometimes beginning 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.”

OSU’s Division of Agricultural Sciences and Natural Resources recommends 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 OSU research first reported back in 2004.

The OSU physiologists studied early fall (August) and late fall (October) calving cows. Data from two successive years were combined for 60 Angus X 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 AI 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 degrees Fahrenheit for the “early” fall group. The average maximum temperature the week before parturition in the “late” calving group was 66 degrees Fahrenheit.”

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

The average gestation length for the “early” cows was six days shorter (279 days) as compared to the “late” cows (285 days) in the first year of the study.. The average gestation length for the “early” cows was four days shorter (278 days) as compared to the “late” cows (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 United States report a similar pattern, according to research published in 2009 in the Journal of Dairy Science. Holsteins bred in January and February – calving in October and November – averaged two days longer gestation than did 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.”

Source:okstate.edu