Extreme heat can make it difficult for cows to conceive. It can also make them give birth faster.
Producers need to be prepared for those early, underweight calves. Extension regional livestock specialist Eldon Cole at the University of Missouri Southwest Center in Mt Vernon said some calves arrived a month early, and initial reports indicated high death losses. “We were spoiled by good weather without high temperatures the last couple of Augusts,” Cole said. “Breeders began pushing for earlier calving dates.”
If the momma cow delivers the calf in full sunlight, there could be a dehydration risk. “Cows don’t always pick an ideal spot to drop their calf; they may not be under a shade tree,” Cole said. “When calving in winter, farmers pick up calves and put them under the heater in their pickup truck. They may need to put them under the air conditioner now.”
Dr. Glenn Selk, professor emeritus of animal science at Oklahoma State University, said a two-year study several years ago at OSU found cattle bred to deliver in the heat of the summer produced calves early – four days early on average in one year, six days in the other – compared to a control group of cows bred to calve later in the fall. “So there looks to be quite an influence of what heat does to gestation length in the latter part of the pregnancy,” Selk told Ozarks Farm & Neighbor.
And there’s no question that if a cow gives birth several days early – and the four to six day figures were averages; some had even shorter gestation lengths – the calf will be a little lighter than if it were delivered full term. Those figures also vary, but Selk said another study at OSU, done with A.I. drawn from the same sire, found cows bred to deliver in the fall had calves with birth weights on average 4.5 pounds lighter than those that were bred to calve in cooler, spring months.
“I think this sort of pattern would be expected anytime you would make that comparison,” Selk said. “We think some of the rationale as to why this could be happening is when cattle gestate in very hot weather, the blood flow in the cattle is going to be shunted more towards the extremities in order to dissipate heat, as compared to cattle that gestate in very cold weather where the blood flow is concentrated in the core of the animal to maintain body temperature. And therefore you would have, in the case of those that are gestating in this hot weather, less blood flow going towards the fetus, as opposed to more blood flow in those calves that are born after a cold winter.”
Selk said it’s not clear whether there’s a cutoff point at which the temperature is so high it will shorten the pregnancy, or whether there’s a more gradual relationship between ambient air temperatures and gestation length. But there is anecdotal evidence. “In visiting especially with producers that try to have a calving season in late summer in the Gulf Coast region of Texas where both heat and humidity have potential environmental impacts on late pregnancy, those people reported some rather significant drops in birth weight and calf viability in those environments,” he said. “All I can do is extrapolate from that that the combination of heat and humidity increase the stress – that those seem to impact, especially, birth weights more than what we see here generally in Oklahoma.”
Selk said if the hotter and drier summers persist, cow/calf producers may want to start their breeding seasons a week later than in the past. “Some people are looking at moving the start of the breeding season back to the first of December, and then therefore moving the start of the calving season back another week,” he said. “What we traditionally have done with some fall calving operations is start the breeding season the Monday of the week of Thanksgiving. That, by the book, would give us a starting date of Sept. 1, but we know these shorter gestation lengths – and just a few cattle that will always calve a little early – pushes more of them up into the August time frame, which gives us a better chance of exposing them to some really hot, dry weather.”


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