The estrous cycle is the interval from the onset of estrus until the next estrus. Estrus or heat is when the mare is receptive to the stallion. Diestrus is the "cold" phase of the estrous cycle in which the mare will respond with total disinterest in the stallion to complete resentment with attempts to kick and bite the stallion. Mares most commonly do not cycle because of winter anestrus. That is, it is common for mares not to cycle from as long as October to March. The mare is a long day breeder; consequently, under natural conditions in North America, most mares' peak sexual period is April through September. Another reason for mares not cycling is due to lactation anestrus, that is, mares nursing foals. Usually, such a mare will have her foal heat and then will not come back into heat for a long period. These mares have a persistent corpus luteum as a result of foal heat ovulation, or they may be extremely possessive of their foal and not want to approach the stallion. Some of these mares may be cycling but not showing signs of estrus. The only way to tell is to monitor ovary and uterine development by palpation based on foaling dates. Mares coming off the race track often take six months to one year to get into a normal cycling situation. Therefore, do not expect the mare coming straight off the track to cycle and breed normally during the breeding season.

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