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Producers can influence gestation length and birthweight through genetics

by Wyoming Livestock Roundup

Several factors affect birthweight in calves, including breed, genetics of sire and dam, length of gestation, age and size of the dam, sex of the calf and environmental factors as well as the nutrition and health of the dam.  

Bull calves, compared within breed, tend to be larger than heifer calves. This is partly because male cattle are larger than female cattle and partly because male calves tend to be carried a day or more longer than heifer calves. 

Often, if a cow goes past the due date, she has a bull calf. On the contrary, if she calves a few days early the calf is often a heifer.

Heritability of gestation length

Some breeds and family lines within breeds tend to have gestation lengths slightly shorter or longer than the average 283 days. Low birthweight cattle often have a shorter than average gestation length, and high birthweight cattle tend to have a longer than average gestation length.  

Dr. Michael MacNeil, a retired animal genetics research scientist who now operates Delta G, a cattle consulting business, says gestation length is an interesting trait. Years ago he participated in a number of research projects focusing on gestation length and calving ease.  

“Intermediate optimum is better than either extreme, he explained. “If gestation length is too short, the calves are born too small, may be premature or not fully competent and may die.”

“If calves are carried too long, they get too big and this is also a problem. There’s actually a narrow window of acceptable gestation length,” he added.

Outliers present challenges

            “Mother Nature does not allow us to breed cattle outside of the acceptable window or she will kill them,” said MacNeil. 

Calves could die if they are premature, but they won’t be born easily if they are too big. There are advantages to having calves in the lower end of average and born easy rather than on the high end of average where producers are pulling 120-pound calves or delivering them by C-section.

            “Heritability for gestation length is moderate,” said MacNeil. “Estimates of heritability are about 0.3, which means it’s just a bit more heritable than weaning weight.”

However, MacNeil noted this information is misleading because weaning weight can be changed greatly by selection, but there is not much room for change when selecting for gestation length. 

“Producers can keep selecting for bigger weaning weights almost indefinitely, and this is what many breeds have done in the past 50 years,” he shared. “On the other hand, if producers select for shorter gestation length, they can only make gestation length shorter by a small amount and still have a healthy viable calf.”

Selection decisions

            “If the goal is to minimize calving difficulty, producers are better off to select on direct and maternal calving ease expected progeny differences (EPDs),” he explained. “In reality, one might not care as much whether the gestation length is 279 days or 286 days, as long as the calf is born easily.”

MacNeil continued, “There’s only about a 10-day window which is actually useful, though a calf carried 278 days is often born easier than one carried 288 days. Gestation length should be appropriate, but not outside this acceptable window.”

            The dam doesn’t have as much influence on gestation length as the sire, though the cow will have an influence on how big the calf gets before it’s born. A large cow tends to have a larger calf than a small cow, and the nutrition the fetus obtains from the dam can make a difference in how fast the calf grows.  

Actual gestation length is determined mainly by the calf’s growth and fetal maturity, which triggers labor. This is a function of genetics via the sire. Some bulls consistently sire calves with longer gestation.  

“The sire of the calf is the important piece in this equation,” said MacNeil.

            “One factor which partly explains the difference in birthweight between the sexes is bull calves are carried a little longer and calves get bigger by the day,” he explained. “Rate of growth will be determined partly by breed, and calving ease can be affected in some breeds, such as Charolais, by other factors including double muscling.”

            MacNeil shared the most important trait for selection is calving ease. 

“Gestation length may change if producers select for calving ease, as a correlated response, but a rancher will make the most progress by selecting for it indirectly,” he said. 

“By selecting for certain targets in calving ease, birthweight, gestation length, pelvic area, etc., simultaneously to improve calving ease, very few breeders will get the correct emphasis on these various EPDs to actually make the progress they want,” he said. “It is a significant challenge to make decisions in the correct relative importance.” 

Selecting for multiple traits can help producers attempt to manipulate calving ease, but most of the calving ease evaluations consider at least the birthweight information simultaneously with other parameters, according to MacNeil. If producers are selecting for calving ease, they wouldn’t need to put any more emphasis on birthweight than what is already in the EPD evaluation for calving ease – it’s already factored in and the math has already been done.

Calving and gestation records

            Many producers don’t measure pelvic area and most of them don’t record or measure gestation length. Gestation length can only be measured if producers have a breeding date, such as when the cow was artificially inseminated or by monitoring the breeding pastures and recording when each cow is bred.  

World-renowned South African Beef Scientist Jan Bonsma once said, “Cattle producers must recognize the fact without measurement they won’t make progress. They can’t compare things which aren’t measured.”

            Heather Smith Thomas is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to

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