Heat stress shortens summer, fall gestation

Cattle Market & Farm Reports, Editorials
Aug 29, 2005
by WLJ

A two-year study by Oklahoma State University professor Bob Wetteman shows that heat stress can significantly shorten gestation length in early fall calving cows and heifers.
Wetteman monitored gestation length for a group of 50 Angus-Hereford cross “early” (August) and “late” (October) fall-calving cows. Wetteman found that when subjected to the stress of hot days in late summer, cows tended to have shorter gestation lengths than cows bred to the same bull due to calve later in the fall.
The average maximum temperature for the early calving group was 93 degrees during the week prior to calving. Wettemann found that cows in this group calved an average of three days early, an average gestation period of 280 days.
Cows in the second group were subjected to average maximum temperatures of 66 degrees during the week prior to calving. Wetteman found this group had an average gestation length of 283 days.
“Early parturition is a result of maturation in either the brain of the calf or adrenal stimulation. We aren’t sure which of these factors causes the shortened gestation, but I believe it is likely the adrenal stimulation of the fetus, brought on by heat stress,” said Wetteman.
Despite the combination of added stress and early birth, calves in the study had a survival rate of 100 percent.
Cows in the study experienced similar success in their re-breeding rates. Cows in the early calving group bred back at 93 percent, while the late group experienced a re-breeding rate of 96 percent. Both sets of cows were AI bred and allowed 35 days of exposure to a clean-up bull.
Wetteman attributed much of the cow and calf success to the availability of good forage in the period leading up to the calving and the following breeding season. Cows who have better quality forage available exhibit better body condition prior to calving. That improved condition contributes to better calf health and survival and ultimately earlier breed-back times.
Glenn Selk, extension cattle specialist for Oklahoma State University, who has participated in similar gestation studies, also discovered differences between “early” and “late” fall calving cows. Selk and fellow researchers found that in addition to shortened gestation, another difference between early and late fall calving periods was a lighter birth weight for calves sired by the same bull.
“Early fall calves averaged about 4.5 pounds less than spring calves,” said Selk.
Selk attributes the decrease in birth weight to the physiology of the cow. “On hot days, blood flow of the cow is directed toward the skin and outer extremities, reducing blood flow to the calf, which lowers average birth weights,” he said.
Wetteman has been working with cow-calf operations in the state to convince producers to calve first calf heifers in the fall. “We have had good success calving heifers in the fall, we pull a lot fewer calves than in the spring,” he said.
When anticipating fall calves, both Wetteman and Selk stressed the need for producers to keep an eye on the calendar.
“Ranchers who have a target calving date of Sept. 1 can find as much as a third of their calf crop on the ground by that date when temperatures are high,” Selk said.
Studies by both researchers has shown that producers should start their routine herd checks at least a full week ahead of time when high temperatures prevail before the calving period. — John Robinson, WLJ Associate Editor

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