Gestation Length Variation in Mares

Oh, how mare owners envy swine farmers! Why? Sows are far more predictable in their gestation length than mares. Whereas sows farrow within a few days of a set length, 114 days, normal gestation length of mares can range from 320 to 360 days or longer. Could conception date, or even geographical location, affect length of pregnancy of mares?

Researchers in New Zealand studied gestation length in Thoroughbred mares, with an eye on determining variation by conception month and geographical area*. Sixteen years’ worth of breeding records (2005 to 2015) were mined for the necessary data. To pinpoint geographical data for each foaling farm, Google Maps was used.

Analysis of the data uncovered that gestation lengths were longer for mares bred early in the season compared to mares bred later. Few regional differences were noted in gestation length, though researchers feel more fieldwork is necessary to determine if environmental factors, some of which affect pasture growth, may cause differences in gestation length between regions.

Kentucky Equine Research found differences in growth rates in different regions of the world.

Mares in New Zealand are often kept on pasture year-round. In addition to high-quality pasture, mares require a supplemental source of essential nutrients. “For many mares, this means a well-formulated concentrate fed at the manufacturer’s recommendation,” explained Catherine Whitehouse, M.S., a nutrition advisor with Kentucky Equine Research.

“Some mares, however, do not need the calories afforded in these feeds and require only a balancer pellet or a vitamin/mineral supplement for optimal nutrition,” she said. Balancer pellets usually can be purchased where other concentrates are sold, and high-quality vitamin/mineral supplements such as Micro-Max (Gold Pellet, Nutrequin, or Perform in Australia) are available worldwide.

Are you unsure about how best to feed a broodmare as she advances from early pregnancy to her foaling date? The nutrition experts at Kentucky Equine Research can help ease your concerns. Contact a nutrition advisor today!

*Roca, J., E.K. Gee, and C.W. Rogers. 2018. Regional variation in gestation length of Thoroughbred mares at stud in New Zealand. In: Proc. Australasian Equine Science Symposium 7:28.