I hear it all the time: “My mare is impossible whenever she is in heat, and, in fact, she’s in heat most of the time!” or “Every time I want to do something with my mare, she’s in heat; in fact, she was in heat for every single show last year!” or “My mare has a split personality–most of the time she’s a witch. Every couple weeks she’s in season, and for those few days, she is the sweetest little angel,” or “My mare gets PMS,” or “Sometimes this mare just won’t run her best, sometimes she’ll pull up or stop. I think it’s when she’s in heat.”

Temperament and performance problems believed to be related to the estrous cycle in mares are among the most common complaints of owners and riders of mares. The problems come in a few varieties. Careful evaluation sometimes can identify the cause and lead to resolution, or at least a strategy for satisfactory management.

Rarely is the problem as simple as poor performance during estrus or prolonged estrus that interferes with top performance. For a small percentage of cases, the problem might be related to the reproductive cycle, but with the diestrous phase rather than the estrous phase of the cycle. In some of these, it turns out that the mare’s best behavior might be during estrus and the problem behavior associated with the diestrous phase of the cycle. In another small percentage of cases, the mare is found to have a reproductive pathology. The most common pathology is a hormone-producing tumor that can cause various forms of abnormal behavior. Most often, it turns out that the problem is not estrous behavior, and the root cause is not associated with estrus or the reproductive cycle of the mare