A mare is an adult female horse aged four years or older that has not been surgically sterilized. Below age four, a female horse is called a filly; at four and above she becomes a mare regardless of whether she has reproduced. A mare used specifically for breeding is called a dam relative to her offspring, though “dam” is a relational term describing her role, not her age class. A spayed mare — uncommon but surgically possible — is sometimes called a “ridgling mare,” though no universally agreed terminology exists for this in current equestrian usage.
Mares are seasonally polyestrous, meaning they cycle reproductively during the long-daylight months of spring and summer in the Northern Hemisphere, typically February through October, with cyclicity suppressed during winter months (seasonal anestrus) in response to reduced photoperiod. The estrous cycle averages 21 days, with a follicular (estrus) phase of 5 to 7 days during which the mare is receptive to a stallion, and a luteal (diestrus) phase of approximately 14 to 15 days during which she is unreceptive. Behavioral signs of estrus include a relaxed hindquarter stance, frequent urination, and a characteristic “winking” of the vulva, though individual expression varies considerably. Mares in transitional periods between seasonal anestrus and full cyclicity may show irregular behavior that can be misread as attitude problems by inexperienced handlers.
In competitive equestrian sports, mares compete alongside geldings and intact males in most disciplines without sex-based restriction, though hormonal cycling can affect performance consistency in some individuals. Supplementation with light therapy (artificial lighting to advance the breeding season) or hormonal management (progestins, GnRH) is used by breeding farms to control cycle timing. Mares are subject to the same dental care, vaccination, and deworming programs as other horses, and their body condition score should be maintained at a score of 5 to 6 for optimal reproductive performance and longevity.
Further Reading: Mare (horse) on Wikipedia; Routine health care of horses via the Merck Veterinary Manual.