Clinical Cases in Equine Reproduction

The physiologic breeding season of horses extends from April to October in the Northern Hemisphere. Mares are polyestrous in that they exhibit repeated estrous cycles during the breeding season. In the winter months, 80% or more of mares enter a prolonged anovulatory phase referred to as winter anestrus. The onset of the transition period may be defined as the first significant increase in follicular diameter greater than that noted during deep anestrus. From a practical standpoint, transition begins in horse mares when a follicle first reaches 20–25 mm in diameter. In pony mares, the onset of transition occurs when the diameter of the largest follicle reaches 21 mm in anovulatory mares. The term “late transition” may be used when one or more follicles are 30 mm in diameter in horse mares. Mares may exhibit multiple waves of follicular growth and regression during the transition period. The total number of follicles present on each ovary increases during the transition, and ultrasound examination classically reveals a “grape-like” cluster of smallto medium-sized follicles. The transition period may last for 50–70 days or more prior to the first ovulation of the season. Once a mare has ovulated, she will generally continue to cycle at regular intervals. Behaviorally, transitional mares may display prolonged or irregular periods of estrus. This is likely caused by an extreme sensitivity to the low levels of estrogens being produced by developing follicles. The mean calendar date of the first spontaneous ovulation of the year in anestrous mares maintained under ambient light conditions in North America ranges from the middle of March to early May. Exposure of mares in deep anestrus to a stimulatory artificial photoperiod has been used for decades to advance the first ovulation of the year. Mares maintained under lights still go through a transition period of relatively normal length, but the transition begins earlier in the year. In most instances, the duration from onset of adequate light exposure to ovulation is 60–70 days. General recommendations for light treatment are: