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A mare is a female horse who is mature enough for sexual reproduction. As a general rule, a horse is considered a mare after the age of three, although some people prefer to wait until mares fully mature at age four or five to breed them. Under the age of three, a female horse is known as a filly. The counterpart of a mare is a stallion, a male horse, who is known as a colt when young; gelded males are known as geldings.
The term “mare” comes from the Old English mere, which is the feminization of mearh, meaning “horse.” Historically, mares have been considered extremely valuable, because they have the potential to be bred to generate more horses, and in some regions, mares have been preferred to stallions and geldings, because they are considered to be easier to handle and more suitable for some tasks.
Mares go into season once annually, triggered by the length of the days in their environment. After breeding, a mare's gestation period lasts around 11 months, at which point she typically produces a single foal, although twins do occur. In the wild, mares will nurse for a year or more, although many horse breeders prefer to wean at a younger age to get the mare fit again and to begin training the foal.
Some people in the horse community suggest that mares can be obstinate and difficult to handle, especially when they are in season. Others have suggested that this may not necessarily be true; a well trained mare handled firmly will generally be well behaved and less easily distracted, even when in season, than a stallion, whereas a poorly trained and badly handled mare will indeed act up while in season. Because of the popular belief that mares can be ill-mannered, some people medicate their mares with hormones to control their behavior, especially when the horses are being used in competition.
A mare can compete in any event open to stallions and geldings; famous mares have competed as racehorses, excelled at dressage, and shown their mettle as jumpers. Mares can also be used as trail and pleasure horses, and they can make excellent pasture companions as they age.
When mares are used exclusively for breeding, they are known as broodmares. A high-quality broodmare can be extremely expensive, especially if her bloodline includes famous horses, and she is considered to be extremely valuable, as she can only throw a limited number of foals during her lifetime, unlike a stallion, who could potentially contribute genetic material to thousands of horses.