The Henneke Body Conditioning system (that uses a scale of 1-9 to describe fat coverage in horses, with a score of 1 representing an emaciated horse, while a 9 represents a grossly obese horse) was originally contrived to better describe differences in body fat cover in mares in relation to elements of reproductive efficiency. For example, Henneke et al., showed that mares in higher body condition score (i.e. more fat coverage) were more efficient at getting pregnant and maintaining pregnancy than thinner mares. Coupled with the general increase in the number of overconditioned and obese animals in the horse population, mares are also tending to be overweight in increased numbers.
Maternal obesity is problematic in other animals, resulting in increased birth weights and other health defects. In horses, maternal obesity was also expected to result in increased birth weight, which may pose problems for the foals, particularly with respect to incidence of orthopedic diseases. Drs. Smith, Marr, Dunnet and Menzies-Gow set out to determine The Effect of Mare Obesity and Endocrine Function on Foal Birthweight in Thoroughbreds, and this work was published in the Equine Veterinary Journal this month (doi: 10.1111/evj.12645). In the 66 mares studied, foal birthweight was indeed correlated to mare body condition score. Further work is required to determine if there are relationships between birthweight of a foal and health consequences during growth and maturity. Certainly larger, fast-growing foals are at risk for developmental orthopedic diseases (DOD), though the direct link between mare obesity and DOD has yet to be established.
Henneke, D.R., Potter, G. and Kreider, J.L. (1984) Body condition during pregnancy and lactation and reproductive efficiency of mares. Theriogenology 21, 897-909.