Reproductive Management of the Open Mare EQM 120 – Introduction to Commercial Breeding

Mares are long day, seasonally polyestrous breeders…meaning in a natural environment they exhibit a distinct breeding season during the spring, summer and early autumn months in which normal, non-pregnant mares display repeated estrous cycles that last approximately 21 days. The estrous cycle is the interval from one ovulation (when a mature egg is released from the ovary and is available to be fertilized) to the next ovulation. It is essential to understand basic equine reproductive physiology to manage mares for maximum conception rates!! The mare’s reproductive functions, including the estrous cycle, are controlled by hormones produced in the glands of the endocrine system.

In the natural breeding season, mares establish estrous cycles due to increased amounts of sunlight during the spring and summer months because it triggers the production of higher levels of Gonadotropin-Releasing Hormone (GnRH) which stimulates the pituitary gland to produce Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). The mare’s reproductive cycles are dependent upon the length of daylight within each season and can be divided into three phases. The natural breeding season is referred to as the Ovulatory Phase, which is the period of time from the first ovulation in the spring until the last ovulation in the autumn. In response to reduced amounts of daylight exposure, the Anovulatory Phase (also known as Anestrus) is a period of ovarian inactivity during the winter months. An open mare in anestrus is best described as sexually dormant. A veterinarian can examine the ovarian activity through ultrasound and rectal palpation. During the anestrus phase, the ovaries are small and firm with no follicular activity. In response to increasing amounts of daylight, the Spring Transitional Phase is characterized by the re-establishment of endocrine function, erratic sexual behavior and follicle development without accompanying ovulation. Mares should NOT be bred during the transition phase as excessive breeding during this period may result in endometritis!! Increasing follicular activity causes the ovaries to enlarge and soften. At the start of the transitional phase, clusters of small follicles (approximately mm in diameter) are present on the ovarian surface. As the season progresses, mares develop 1 to 2 follicles that slowly progress to 25-30mm in diameter but remain firm then slowly regress without ovulating.

The repeating estrous cycles, which occurs during the ovulatory phase, consist of estrus and diestrus. Estrus, also referred to as the mare being in “heat”, is the period of sexual receptivity. It lasts approximately 4-7 days and during this time, one or more follicles mature and ovulate. Diestrus is the period of non-sexual receptivity, lasting approximately days. Ovulation, which usually happens at night, occurs 24 hours before the end of estrus and sperm is viable for 48 hours within the mare’s reproductive tract.

In the Thoroughbred Industry, all Thoroughbreds turn one year older on January 1 st regardless of their birth date. As a result, the Northern Hemisphere Thoroughbred Breeding Season is from February 14 th – June 30 th. The most effective method of stimulating the mare’s natural breeding season’s hormones is by exposing the mare to artificial light for the hours required to simulate the longer days of summer. Since there is a day delay from the onset of exposure to the first ovulation, mares planning to be bred in February or March should be put under lights by December 15 th of the year prior. Because the Thoroughbred breeding season starts during the mare’s natural anestrus phase, managers must utilize other methods to move the mare into the ovulatory phase.

Once a mare has entered the transitional phase, a manager can use hormonal treatments to move the mare into the ovulatory phase, but the effectiveness of hormone treatment depends on the stage of the transition period and follicular status at the onset of treatment. Prior to treatment, several mm follicles should be present on the ovaries. Administer a day oral dose of Regu-Mate, which is a synthetic form of progesterone. Regu-Mate “shuts down” the mare’s sexual receptivity. NOTICE: Women handling Regu-Mate should ALWAYS wear gloves!! On the last day of progesterone (Regu-Mate) treatment, a 2 cc dose of prostaglandin should be administered intramuscularly to bring the mare into heat. Lutalyse is the only approved drug in horses and is a natural prostaglandin which can cause side effects such as minor colic and sweating.

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