Published December, 2009

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.

A theoretical method for predicting weight gain in pregnant mares was suggested by the National Research Council in 2007. This method accounts for the development of other "products of conception," such as the placenta. The body weight gained by the mares in this study was almost the same as predicted from the theoretical equation.

The results of this study and the theoretical equation developed by the NRC suggest mares will increase their body weight by about 13% to 14% by the end of gestation. Therefore, a 1,100 pound mare will gain about 150 pounds while a 1,400-pound mare will gain about 190 pounds during gestation.

The goals of this study were to estimate total weight gain during gestation and document the amount of weight gained as mares progressed through the early, mid, and late stages of pregnancy. Estimates of typical weight gains in pregnant mares at different stages of gestation can be useful to horse owners who are monitoring the progress of a mare's pregnancy.

It has been reported that most fetal weight gain occurs in the last three months of gestation. Therefore, it would have been expected that body weight gain in mares would be concentrated in the last three months of gestation in this study. However, the results of this study showed this was not the case. Instead, most of the weight gained during gestation occurred between the fifth and ninth months of gestation.

At the end of the fifth month of gestation, mare body weight had increased by about 3%, and by the end of the ninth month, the increase was 11% of the initial weight. A previous study in which researchers studied Quarter Horses showed that mares tended to gain more weight during the middle of a pregnancy than during late pregnancy.

In this study there was no way to determine what types of tissues were being deposited when mares gained weight in early and mid-pregnancy. It is likely that the development of the placenta and the enlargement of the uterus occur during early and mid-pregnancy, but they would not account for the 11% increase in body weight that was observed. Instead, it is possible that the mares increased their own body mass during early pregnancy.

Mid-pregnancy in these mares would have coincided with the fall season (a time of year when many species augment body stores in preparation for winter). By increasing body stores in mid-pregnancy, mares might have been able to utilize those stores in late gestation to meet the nutrient demands of fetal development. It is also possible that the weight gained by mares during early and mid-gestation was retained in the mare after foaling. An increase in nonpregnant weight could occur due to growth in young mares, or to an increase in body fat content in mature mares.

It is also possible that those body stores are not used in late pregnancy, but are mobilized at foaling to meet the needs for initial milk production. The nutrient needs for milk production are much greater than the needs for pregnancy. Most mares will increase food consumption after foaling, but this increase may take several days and it may lag behind the nutrient needs for milk production.

Mares were weighed in the month after foaling to determine post-foaling weight, which was compared to the initial body weight. In this study, the post-foaling weight of young mares (younger than 8 years old) was greater than their initial weight. However, in mature mares (8 years old and older), post-foaling body weight not different from initial body weight. The younger mares increased total body weight by about 15% by the end of pregnancy, while the mature mares increased body weight by 13%.

Body condition is an important determinant of reproductive efficiency. Mares that are thin at foaling are more difficult to get back in foal. Therefore, routine body condition scoring of pregnant mares is an important reproductive management tool.

The most common body condition scoring system uses a 1-to-9 scale. A condition score of 5 is considered moderate; the horse's ribs can be easily felt but not seen. Mares that enter the breeding season with a condition score of 5 or greater have higher reproductive efficiency than mares that begin the breeding season with a condition score below 5. Therefore, pregnant mares should gain enough weight during gestation so they foal at a body condition of at least 5.

Allowing mares to gain weight in mid-gestation may be a way of ensuring they have adequate body condition at foaling.

Ultimately, further study is necessary to understand the optimal amount of weight a mare should gain during pregnancy.

Laurie Lawrence, PhD, is a professor in the University of Kentucky's Department of Animal and Food Sciences.