In most cases, a mare is a female horse over the age of three, and a filly is a female horse three and younger. In Thoroughbred horse racing, a mare is defined as a female horse more than four years old. The word can also be used for other female equine animals, particularly mules and zebras, but a female donkey is usually called a "jenny". A broodmare is a mare used for breeding. A horse's female parent is known as its dam.
Mares carry their young (called foals) for approximately 11 months from conception to birth. (Average range 320–370 days.) Usually just one young is born; twins are rare. When a domesticated mare foals, she nurses the foal for at least four to six months before it is weaned, though mares in the wild may allow a foal to nurse for up to a year.
The estrous cycle, also known as "season" or "heat" of a mare occurs roughly every 19–22 days and occurs from early spring into autumn. As the days shorten, most mares enter an anestrus period during the winter and thus do not cycle in this period. The reproductive cycle in a mare is controlled by the photoperiod (length of the day), the cycle first triggered when the days begin to lengthen. As the days shorten, the mare returns to the anestrus period when she is not sexually receptive. Anestrus prevents the mare from conceiving in the winter months, as that would result in her foaling during the harshest part of the year, a time when it would be most difficult for the foal to survive.
However, for most competitive purposes, foals are given an official "birthday" of January 1 (August 1 in the Southern hemisphere), and many breeders want foals to be born as early in the year as possible. Therefore, many breeding farms begin to put mares "under lights" in late winter in order to bring them out of anestrus early and allow conception to occur in February or March. One exception to this general rule is the field of endurance riding, which requires horses to be 60 true calendar months old (5 years) before competing at longer distances.