Weather and Nutritional Effects
on Birth Weight
Factors other than genetics can affect birth weight.
Calving ease expected progeny differences (EPDs) are useful when trying to minimize dystocia. Other factors play a role, as well. Ken Olson, South Dakota State University Extension beef specialist, says nutrition of the cow is a factor.
“We know that if we underfeed the cow, this can result in a small reduction in birth weight, but we don’t want to do that. Some producers have tried this to have smaller calves and fewer calving problems, but the data shows that you actually increase calving difficulty if you set yourself up to have a weak cow. The calf might be smaller, but her ability to push it out is less,” he explains.
“Even worse, you set yourself up for a weak calf. The cow will have poorer colostrum, and won’t pass adequate immunity to the calf. Then you have a weak calf that is slower to get up, slower to suckle and, when it finally does suckle, it gets inadequate colostrum (less amount, poorer quality). Thus you end up with a weak calf with poor immunity that never gets going as well,” he says. The underfed cow will not rebreed as quickly after calving. All in all, shortchanging the cow is never a good idea.
The other extreme is overfeeding. This can happen in a cold winter when ranchers increase feed to help cows through cold weather.
“When a cow is overly fat, not only is she fat on the outside, but she is carrying excess internal fat. One place internal fat is deposited is in the birth canal, making it smaller,” he says. This can lead to difficult calving, along with the fact calves may be slightly larger from overfeeding the cow.
During cold weather, cattle try to conserve body heat and reduce heat loss by routing more of the blood supply around the internal organs and less to the skin and extremities. This means there is more blood supply to the uterus and more nutrients channeled into growth of the calf, especially during the last trimester when the fetus is growing the fastest. Calves tend to be larger at birth when their dams have just gone through a cold winter.
“Research data is scanty; this is mainly anecdotal information, but I am convinced that it matters. When cows are trying to maintain core body temperature, there is increased nutrient flow to the uterus. Many producers have experienced this. Without a change in genetics, birth weights are higher after a colder winter,” Olson says.
“It won’t make a huge change in birth weight, but can increase it a few pounds. Most producers, if the rest of their management is what it should be, won’t find it enough of a problem to create serious calving difficulties. We might see a small increase in birth weight, and a few more cows or heifers that need assistance, but it shouldn’t be a severe change,” says Olson.
Editor’s Note: Heather Smith Thomas is a freelancer and cattlewoman from Salmon, Idaho.