Estrus Cycle
During the breeding season, April - October in Wisconsin, mares will cycle every 21 days. In the diagram below you can see how the interval between ovulation is not always 21 days but varies from 20 - 23 days. Following each ovulation, the remaining follicular tissue develops into a corpus luteum and produces progesterone for approximately 14 days. The progesterone causes the diestrus behavior seen at this time. The corpus luteum will then regress, progesterone decreases, estrogen increases (not shown) and the mare comes into estrus for 4-8 days. Ovulation is triggered by LH, and the cycle repeats itself until day length decreases during the fall. In Wisconsin, most mares have their last estrus and ovulation in October or November. Some mares will however cycle all year so be careful if moving mares in with stallions during the winter.
