I was told by a stallion owner that my mare is “too fat” and won’t get pregnant at her present body weight. Can this be true?
Yes, it is possible for an obese mare to get in foal; however there are several factors to consider. Research conducted in the mid-1980s set out to determine how body condition influenced reproductive performance in mares.
In this large study, moderately fleshy to fat mares (body condition score 3 - 4, 0-5 scale) cycled earlier in the year, had fewer cycles per conception, had a higher pregnancy rate and maintained pregnancy more often than thin (body condition 2 and below) mares. Research also indicated thin mares (ribs visible) with foals by their sides do not have enough stored fat to support efficient reproductive performance.
Mares in marginal or poor body condition are more likely to skip a breeding year as their bodies use dietary nutrients for milk production rather than for reproduction.
This study also pointed out that mares that become pregnant but receive inadequate nutrition have a higher rate of embryo loss. Reproductive performance can be improved in thin mares when they are fed to gain weight. However, pushing a mare that has been nutritionally neglected can lead to problems with digestive upset if feed intake and weight gain increase too quickly.
Foaling difficulty and rebreeding problems have not been scientifically linked to obesity in mares. However, there is no reproductive advantage in having mares extremely fat (body condition score 5). Hormonal changes associated with obesity may limit reproductive success, but the additional feed expenses necessary to keep mares extremely fat should be reason enough to keep mares in lighter body condition. Absolute weight is not the only important criteria by which to evaluate the horse. Appearance and condition have always been used as indicators of fitness and health.