It’s generally known that a mare gains weight during gestation, but University of Kentucky (UK) researchers are studying how much weight a normal mare should gain for the health of the fetus.

A 1,100-pound mare will produce a foal that weighs about 110 pounds, so a normal mare would be expected to gain at least 110 pounds during gestation. However, anecdotal observations suggest that many pregnant mares gain more weight than can be attributed to the developing foal alone. Bryan Cassill, MS, and Laurie Lawrence, PhD, in the Department of Animal and Food Sciences at the University of Kentucky collaborated with Stephen Jackson, PhD, of Bluegrass Equine Nutrition, to examine body weight changes in broodmares on a Thoroughbred breeding farm. Preliminary observations were reported at the 2009 Equine Science Society meeting, which was held in Colorado.

The average initial weight of the mares at breeding was 1,272 pounds, but there was a large amount of variation among the mares. The smallest mares weighed about 1,100 pounds and the largest mares weighed more than 1,400 pounds at the beginning of gestation. A previous study reported by UK researchers found that the average Thoroughbred broodmare weighs about 1,300 pounds.

At the end of gestation, the average weight of the mares was about 1,448 pounds. Therefore, average weight gain was 176 pounds. The typical Thoroughbred foal weighed between 115 and 125 pounds, so the weight gained was more than predicted from fetal development alone