Body condition can influence reproductive success; however, several factors must be considered.  In one large study, moderately fleshy to fat mares (with a body condition score 3-4 on a 0-5 scale) had cycled earlier in the year, had fewer cycles per conception, had higher pregnancy rate, and had maintained pregnancy more often than thin mares (with a body condition score of 2 or lower).  Thin mares with visible ribs and 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 conception and maintenance of pregnancy.  Mares that become pregnant but then 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 digestive problems 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 (with a body condition score of 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 them 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.