A recent study conducted at Texas A&M University looked at reproductive function in mares following foaling. Each mare was scored for body condition using a 1-9 scale, with mares having low body condition scores being thin and those with the higher numbers being fatter. The mares used in this study were divided into two groups: moderate condition mares with a body condition score of 5 to 6 and fat mares with a body condition score of 7 to 8. The results of this study revealed no significant differences in average time from parturition (foaling) to foal heat ovulation (14.41±1.07 and 16.18±1.06 days), foal heat ovulation to second postpartum ovulation (22.91±1.07 and 24.33±0.93 days) and conception rates (91.67% and 83.33%) between the two groups. The results of this study indicate that mares maintained at a high level of body condition (body condition of 7 to 8) are not prone to reproductive dysfunction or lowered levels of fertility. This study is of value to the horse breeder since it gives an indication of the level of fatness a mare can have and still be reproductively efficient.

Reference: C. Cavinder et al., 2005 

Reproductive parameters of fat vs moderately conditioned mares following parturition. 19th Equine Science Society Symposium, p. 65.

Body condition score pictures modified from: Japanese Feeding Standard For Horses
(1998).