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Reproductive Physiology and Breeding Management of the Mare

By: Dr. Sylvia J. Bedford-Guaus

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Mares are seasonally polyestrus. This means that successive estrus cycles are displayed only during a particular time of the year. For the mare, regular estrus cycles begin during the late winter or early spring, after a period of increasing daylight, and continue during the summer and sometimes through part of the fall. Some mares (up to about 10 percent) may cycle year round.

Mares may show signs of heat in the so-called transition period between the winter and breeding season, but this does NOT mean that they have started cycling regularly or that they are ready to be bred. It is not uncommon to confuse these early signs of heat with regular cycles and mares are often wrongly bred at a time when they have not yet started ovulating. They will NOT become pregnant if bred at this time.
        
During the breeding season, mares begin an estrus period and ovulate approximately every 21 days. This period of 21 days is termed the estrus cycle. The average duration of heat or estrus is five to seven days, although estrus length is extremely variable among mares, and can last anywhere from three to 10 days. Furthermore, in the same mare, estrus length tends to be longer at the beginning of the season and becomes shorter as the peak of the season approaches in June and July. Estrus is the period of the cycle during which the mare will accept a stallion if teased or bred by natural cover. Ovulation occurs near the end of the estrus period.

The period of the estrus cycle between heats is termed diestrus. The length of diestrus is usually about two weeks.

Endocrinology

The 21-day pattern of cyclicity during the breeding season results from a complex interaction of hormones secreted by the brain, uterus and ovaries. The hypothalamus is a structure in the brain that secretes into the bloodstream gonadotropin-releasing hormone or Gn-RH. Gn-RH acts on another structure near the brain, the pituitary gland, inducing secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In general terms, FSH and LH stimulate the ovaries and are responsible for follicle growth and ovulation, respectively.

In turn, growing follicles in the ovaries are responsible for estrogen secretion, the dominating hormone during estrus. Under estrogen influence, and in the absence of the hormone progesterone, mares display signs of heat.
        
Towards the end of estrus, usually one follicle will ovulate in response to LH secreted by the pituitary. Then, the fluid filled follicle will be replaced by a solid structure termed corpus luteum. The corpus luteum secretes progesterone, the dominating hormone during diestrus and pregnancy, that keeps the mare out of heat.
        
Follicles may grow in the ovaries and even ovulate when the mare is in diestrus. It is a common mistake to breed mares during diestrus just because they have a large follicle on one of the ovaries that is detected during a palpation or ultrasound exam. If teased, these mares will not show signs of heat while in diestrus since progesterone always dominates over estrogen. If bred by artificial insemination at this time, there is a small possibility that the mare may become pregnant, but there is also an increased chance of developing a uterine infection. Therefore, breeding during diestrus when a large follicle is present in the ovaries should be strongly discouraged.

After breeding and ovulation progesterone levels will remain high if the mare is pregnant, so the mare will not come back into heat. If the mare does not become pregnant in that cycle, toward the end of diestrus, or about 13 days after an ovulation, the hormone prostaglandin F2-alpha will be released by the lining of the uterus. Prostaglandin is responsible for eliminating the corpus luteum, which results in cessation of progesterone secretion and return to heat.

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