–Ron Torell, Nevada Livestock Extension Specialist

Everything is based on an average, but there is no average. For example, when was the last time we saw an average precipitation year? Gestation length on a beef cow is another one of those numbers where the average is seldom seen. The gestation length for all breeds of cattle averaged together is 283 days.

The range is 279 for Jersey and up to 292 for Brahman. On the average, the Continental breeds of Charolais, Simmental, and Limousin exhibit gestation lengths of 289 days. English-bred cattle such as Angus, Shorthorn, and Hereford exhibit, on the average, shorter gestation lengths of 281, 282 and 285 respectively. Within those breeds the average gestation length can vary an additional twelve days on either side of the average for the breed.

Gestation length is an issue because it is associated with dystocia and it affects the postpartum interval. Dr. Bob Bellows, retired Miles City, Montana researcher at the Fort KEOG Research Center states “During the last ten days of gestation, one to one-and one-half pounds of birth weight can be added to the size of the fetus. This means that for a calf that had a five-day extended gestation, you could be adding as much as eight pounds to the birth weight. This might mean the difference between an unassisted birth or a dystocia situation.”

Sally Northcut, director of genetic research for the American Angus Association feels another big advantage of short gestation bulls is increased postpartum interval and breed back of the cows. “Research clearly shows that young cows and cows that have difficult and slow deliveries require additional days of postpartum interval to cycle and re-breed. If a calf is born at 275 days gestation versus the breed average of 283, that cow will usually have an easier delivery and will automatically have an additional eight days postpartum interval advantage.”

Wayne Vanderwert of the American Gelbvieh Association agrees “The Gelbvieh Association is the only breed association to currently have a gestation EPD established for their breed. During the period we have had the gestation length EPD, the average gestation length of the Gelbvieh breed has been reduced from 289 to 284 days. Virtually all of ourA.I. sired registered cattle go into the data bank. The Gelbvieh Association’s effort to reduce gestation  length to the current levels parallels a strong genetic trend for reduced birth weight and dystocia in the breed during that same time frame.”

We hear about curve benders-those bulls that are small birth weight and calving ease yet defy the antagonisms of low birth weight and growth potential. Many of these curve bender bulls are short gestation. This would explain  why they have the smaller birth weights yet the calves explode and grow like a long gestation growth bull. Conversely, high growth bulls are oftentimes long gestation sires. This might partially explain the larger birth weights  often associated with growth bull sired calves.

If you would like to discuss this article or simply would like to talk cows do not hesitate to contact Ron at 775-738-1721 or torellr@unce.unr.edu.