Satué K and Gardón JC
With an understanding of basic reproductive science, veterinarians and breeders can be better positioned to achieve their goals. It is important to understand the heat or estrus cycle in order to maximize the chances of success when breeding the mare. Reproductive activity in horses is seasonally dependent, as it is primarily affected by the length of daylight. Therefore the mare has a seasonally polyestrous type of estrous cycle. This means they have breeding season in which they have multiple heat cycles, is receptive to the stallion and ovulates; and a period where they will not go into heat or anestrus. During the anestrus period, most mares show no behavioral signs of sexual receptivity and fail to develop follicles that ovulate. There are exceptions in that a small percentage of mares that do not express a seasonal pattern in that they stay both behaviorally and physiologically receptive to stallions throughout the year. During the ovulatory season, the mare is cycling, thereby exhibiting sexual receptivity to the stallion on a regular basis and is producing follicles that ovulate. The equine estrous cycle is commonly described as a combination of a follicular phase, or estrus, and a luteal phase, or diestrus. The endocrinology of the estrous cycle involves a balance between hormones produced by the pineal gland, hypothalamus, pituitary gland, ovaries, and endometrium. Growth of antral follicles in the ovary occurs in wave-like patterns, and is influenced by several factors such as stage of the estrous cycle, season, pregnancy, age, breed and the individual. In this article will describe the neuroendocrine mechanisms related to breeding seasonality, the hormonal changes that occur during the estrus cycle as well as the variations among mares in regards to the understanding the physiological mechanisms related with the estrous cycle in the mare