Last month, a reader asked why his cows might have shorter gestations than he expected. I am not sure that I can provide the exact reason for his cows, but it is a great opportunity to talk about gestation length.
The average length of gestation in Holsteins is just about 278-279 days, with heifer gestation times being slightly shorter. Other common dairy breeds have longer gestations, with Brown Swiss being about the longest at around 287 days. Like all biological events, averages may not give much indication of how much natural variation there is around that mean value. A recently published study on Holsteins found the range of gestations was from 256 days to 296 days.
Gestation length is a heritable trait in cattle. In countries that have seasonal milking, like New Zealand, gestation length is included in some bull selection indices. There are a bunch of factors other than genetics that can affect gestation lengths. Cows that conceive in the fall had slightly shorter pregnancies than cows successfully bred in mid-winter. Pregnancies with twins are on average just about a week shorter than single births. With single births, the gestation period for bull calves tends to be about one day longer. As a general rule, average gestation length increases with the number of lactations. Summer heat can lead to shorter gestations. This shortening of gestation can be offset if evaporative cooling is provided to late pregnancy cows during hot weather.
As already mentioned, genetics plays a role in gestation length. Some bulls will give consistently shorter gestation lengths than average. This has a heritability of
33 % to 36%.
So what difference does having a shorter or longer gestation length really make? Even though gestation length is heritable, there does not seem to be a good body of evidence to determine what is the best gestation length for cow productivity, longevity or profitability. Cows with extremely short or long gestations tend to have poor calf survival rates. There is evidence that they can also be less productive in total milk, fat and protein. Cows with longer gestations are more likely to develop infections of their uterus.
A recently reported study of cows in Florida looked at the impact of three gestation periods: Shorter than 270 days, between 270 and 282 days, and longer than 282 days. Cows with pregnancies between 270 and 282 days had better health, better production and better reproduction.
Cows that had either longer or shorter gestations were more likely to have difficult births, stillbirths or uterine infections. The impact of a shorter pregnancy occurred in both first-calf heifers and cows but the impact of increasingly shorter gestation was more severe in cows than in first-calf heifers.
Cattle that had a shorter gestation were almost 40 % more likely to be culled earlier than cattle with an average length of pregnancy. There were similar adverse health consequences among cows with a longer gestation but those cows tended to have higher milk production. That production benefit was not seen in first-calf heifers with longer pregnancies.
Interestingly, these researchers found that the impact of gestation length extended to the calf born in that pregnancy too. Heifer calves born to cows that had gestation lengths in this average range were healthier (as measured by survival) and had better reproduction than heifers born after either a shorter or longer gestation period.
Dr. Robert Tremblay is a veterinarian for Boehringer-Ingelheim and lives near Guelph.