Horse Breeding - Estrous Cycle of The Mare

Estrous Cycle of The Mare

Also see Equine anatomy-Reproductive system

The estrous cycle (also spelled oestrous) controls when a mare is sexually receptive toward a stallion, and helps to physically prepare the mare for conception. It generally occurs during the spring and summer months, although some mares may be sexually receptive into the late fall, and is controlled by the photoperiod (length of the day), the cycle first triggered when the days begin to lengthen. The estrous cycle lasts about 19–22 days, with the average being 21 days. As the days shorten, the mare returns to a period when she is not sexually receptive, known as anestrus. Anestrus - occurring in the majority of, but not all, mares - prevents the mare from conceiving in the winter months, as that would result in her foaling during the harshest part of the year, a time when it would be most difficult for the foal to survive.

This cycle contains 2 phases:

• Estrus, or Follicular, phase: 5–7 days in length, when the mare is sexually receptive to a stallion. Estrogen is secreted by the follicle. Ovulation occurs in the final 24–48 hours of estrus.
• Diestrus, or Luteal, phase: 14–15 days in length, the mare is not sexually receptive to the stallion. The corpus luteum secretes progesterone.

Depending on breed, on average, 16% of mares have double ovulations, allowing them to twin, this does not affect the length of time of estrus or diestrus.