31 Mar It’s Breeding Time Again

Monitor your mare’s heat cycle.

By Ann E. Dwyer, DVM
Have you noticed that your mare’s behavior has been less erratic this winter than last spring? You are observing a fact of nature called “winter anestrus”—a three- to four-month period during which your mare’s hormones quiet down and she stops exhibiting the vulvar “winking,” squatting, raised tail and urination that are typical signs of a heat period.

After winter dormancy, the mare starts to produce reproductive hormones, and she enters the “spring transition” part of her annual cycle. For several weeks in late February and March, her ovaries produce multiple fluid-filled structures called follicles that stay less than 20mm diameter and fail to ovulate (release an egg into the uterus). Mares may be very receptive to the stallion during this time, teasing for days on end, but mating is unlikely to lead to conception.

The transitional period typically wanes by early April, and the period of “true estrus” begins. For the next six months, the mare’s hormones, ovaries and uterus coordinate to produce regular sequential heats. The entire estrous cycle of the horse takes, on average, three weeks, and is composed of 15 to 16 days that the mare is OUT of heat, and unreceptive to the stallion and 5 to 6 days that the mare is IN heat. During the time that a mare is “in,” her ovaries produce one or more follicles that grow to a diameter of at least 35mm. On average, the follicle(s) ovulate around the fourth day of observed heat behavior. Once a follicle has ovulated, the egg travels down to the fallopian tubes where fertilization can take place. Sperm that enter the fallopian tubes live about 72 hours, but the egg survives less than 24 hours, so the goal of any natural or artificial breeding program is to breed the mare as close to ovulation as possible to optimize the chances of conception.

As spring gives way to summer, most mares cycle quite regularly and experience peak fertility rates in June. Then, as the leaves start to fall and the days grow shorter, hormone levels and ovarian activity become irregular and the mare goes through a “fall transition” period that mirrors the spring. By mid-November most mares have stopped cycling and have entered “winter anestrus” again.

Veterinary research has proven that the annual cycle of mares is dependent on day length. Thus, mares are often “fooled” into entering true estrous in February or early March by exposing them to 16 hours of artificial light every day starting December 1 of the previous year. Veterinary science has also characterized the hormonal events that trigger follicular development and ovulation. This understanding has led to a variety of products (e.g., Regumate, Lutalyse, HCG, Deslorelin and Ovuplant) that manipulate individual heat cycles, delaying or accelerating ovulation. Many advances have occurred that aid our ability to control heat cycles and optimize fertility of mares.

If you are breeding this year, give us a call now to find out how we can help your mare conceive. If you are considering breeding artificially, we recommend that you take our AI short course.