Following the initiation of puberty (onset of the first ovulatory estrus), the female will exhibit distinct periods of receptivity and nonreceptivity to approaches for mating by the male. In domestic farm animals, the estrous cycle is defined as the number of days from initiation of behavioral estrus expression to onset of a subsequent estrus. Mares and sows are both polyestrous (exhibit repeated estrous cycles). However, unlike the sow that can exhibit cycles throughout the year, the mare is a seasonal breeder that initiates estrus activity as day length increases during the early spring, usually becoming anestrus (no estrus expression) as day length decreases following the summer solstice. Length of the estrous cycle in both the sow and mare averages 19 22 days. Cycle length in the mare can also be affected by type (pony, horse, donkey) and time of year, as length of the estrous cycle tends to be longer in winter and early spring compared to late spring and summer. Estrous cycle of the mare and sow is characterized by four distinct stages called proestrus, estrus, metestrus, and diestrus. Classifications of the stages are based on ovarian, hormonal, and behavioral changes that occur during the estrous cycle.
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