The Council for Dairy Cattle Breeding (CDCB) released a new trait, Gestation Length (GL), that allows dairy farmers to more accurately pinpoint the time of calving. According to CDCB, the new trait offers three benefits:
1. For grazing herds or seasonal herds, it will allow farmers to more tightly concentrate calvings.
2. In all operations, farmers should be better able to time dry dates, pinpoint expected calving dates and better manage maternity pen usage and flow.
3. Researchers would better understand other traits such as calving ease, stillbirths and age at first calving.
On a more practical level, it’s likely dairy farmers would not use the new trait in any of their selection indexes. Kent Weigel, a dairy geneticist from the University of Wisconsin, says: “I would continue to use sire calving ease to avoid dystocia and pick mates for yearling heifers, and I would use the gestation length info to adjust expected calving dates (as a fresh cow/heifer management tool).”
Les Hansen, a University of Minnesota dairy geneticist, agrees: “The goal of the new PTA for gestation length is for day-to-day management information to improve estimation of predicted calving date. The new trait isn’t included in Net Merit. Calving ease is a trait unto itself, although there is a relationship between calving ease and gestation length (at least within breed, maybe not across breeds).”
“Many, many years ago, beef breeds got into trouble by selecting for gestation length—they inadvertently selected for the dwarf gene. For dairy cattle today, shortened gestation length also has a relationship with stillbirth. Therefore, selection should be directly for calving ease and stillbirth and not for gestation length,” he says.
GL PTA will be available for all six breeds and even crossbred animals if parentage of the female is known.
There is breed variation in gestation length. Below is the average gestation length by breed for the base year (2010).
Note: This story appeared in the September 2017 issue of Dairy Herd Management.