The corpus hemorrhagicum luteinizes to form the corpus luteum, sometimes called the yellow body.Īs the corpus luteum develops, it starts to produce progesterone, which influences the pituitary gland and reproductive tract. The cavity left by the ruptured follicle becomes engorged with blood to form a corpus hemorrhagicum. The follicle contains a viscous fluid and when the follicular wall ruptures, this fluid flows out, carrying the ovum with it. As the follicle develops on the ovary, the ovum (egg) inside the follicle undergoes a number of changes to become capable of being fertilized by the sperm. The surge of luteinizing hormone causes the follicle on the ovary to rupture, resulting in ovulation. Estrogens are responsible for the clinical signs of estrus and act on the oviducts, uterus and cervix to prepare the reproductive tract for pregnancy. When blood estrogen reaches a certain level, a surge of luteinizing hormone is released from the pituitary gland into the blood stream. The developing follicle produces estrogens, which are released into the blood stream.Įstrogens have a number of functions in the body. Follicle-stimulating hormone is released into the blood stream and travels to the ovaries to initiate development of a follicle containing an ovum. As spring approaches, the pituitary gland is stimulated by increased daylight to enhance follicle-stimulating hormone production. The pituitary gland is located at the base of the brain and produces follicle-stimulating hormone and luteinizing hormone. This section describes some of the hormones involved in mare reproduction. Hormones are chemical substances created by the body that control various bodily functions. The estrous cycle is controlled by hormones ( Figure 1). Artificial lighting can be used to induce mares to cycle earlier in the year. The primary environmental factor causing mares to cycle normally is increased hours of light (photoperiod). During late March, April and May, most mares exhibit sexual receptivity, and from late April through August, most mares will cycle normally.įew early estruses result in ovulation thus it is not recommended to breed mares during this time without rectal palpation of the ovaries to determine follicular development. The mare's first estrus phases of each year are often erratic and prolonged, during which time mares may be in heat for 20-30 days or more. Breeding or insemination of mares, starting on Day 2 or 3 of estrus and continuing every other day throughout the estrus, is a practical means of achieving satisfactory pregnancy rates. Ideally, to maximize the chance of conception, breeding should occur within 12 hr of ovulation. However, it normally occurs 24-48 hours before the end of the estrus period. Ovulation, the release of the egg from the ovary, can occur at any time during the estrus phase. From September through March, very few mares will cycle normally, so conception is more difficult to achieve during these months. Diestrus is normally 15 days, but may vary from 12-18 days. Estrus usually lasts for 6 days, but can be 4-10 days, depending on the mare. The estrous cycle is composed of two phases: the estrous phase (in heat) and the diestrous phase (out of heat). During this time, the mare will have an estrous cycle of 21 days (☓ days). The estrous cycle in most mares starts to normalize in late April or early May until August - the normal breeding season for horses. This Factsheet presents information on basic anatomy, physiology and management techniques that can assist in improving reproductive performance in the mare. With an understanding of basic reproductive science, breeders can be better positioned to achieve their goals. Horses have the potential for high reproductive performance.
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