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A mare is in heat for typically 5-7 days. Signs such as winking vulva, tail rising, squealing, or unpredictable behavior could mean a mare is in heat.

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This is something you need to be able to work out if you want to have your mare served by a stallion at a stud. It is also something that may be able to explain certain behaviours in your mare. Or it may simply be something you are interested in understanding...

Mares are what is known as seasonally polyoestrous. This means that they will cycle a number of times each year, but will have a period of time when they are cycling and a period of time when they are not. Cycling is dependent upon day length. So as the hours of daylight each day get longer, mares will start to cycle. They will usually cycle from around mid spring until the end of summer, but some mares will start cycling earlier than this and some will cycle for longer.

The normal oestrous cycle in the mare is 19 - 22 days long with most cycles lasting 21 days. The duration of oestrus (when the mare is in heat) can vary but will usually last 5 - 7 days. Oestrus tends to last longer when the hours of daylight are less and become shorter as the hours of daylight get longer. The most consistent part of the cycle is dioestrus (when the mare is not in heat). Dioestrus tends to last 14-15 days. So the best way to determine when your mare will next come into heat is to count 14-15 days from the day she comes off heat.

Signs of your mare coming into heat can include some or all or the following:

A winking vulva

Tail raising, frequent urinating and a subsequent wet tail and hind end

Squatting down, dropping her hips and generally adopting a breeding position

Increased interest in stallions and mature geldings

Squealing, having her ears back, kicking and aggression

Looser bowel motions

Being difficult to handle or ride

Nervousness or anxiety

Unpredictable behaviour

But the only way to determine 100% that your mare is on heat is by ultrasound.

Ovulation is usually said to occur on around day 5 of the mare's cycle, or approximately 24 - 48 hours before the end of oestrus. No matter how long she is in heat for, ovulation will only occur shortly before the end of oestrus. So if you are breeding your mare then it is necessary to breed her shortly before she comes off heat, as sperm will only live in her reproductive tract for about 24 hours. In order to get your mare in foal it is recommended to breed her on the second day of her heat cycle and then every other day until she goes off heat.

Most mares are not intentionally bred until they are at least 3 years old, although mares will reach puberty earlier than that. Most will begin cycling soon after they are 1 year old. Age is a factor in mare fertility, and there tends to be a decline in the fertility of mares from around 12 years old. So if you are looking to purchase a broodmare, probably the ideal would be one between 4 and 10 years old that has successfully foaled at least once already.

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How do I Tell if My Mare is in Season? - equine9.co.nz

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This is something you need to be able to work out if you want to have your mare served by a stallion at a stud. It is also something that may be able to explain certain behaviours in your mare. Or it may simply be something you are interested in understanding...

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Typically, to collect semen from a stallion, a 'phantom mare' is used to reduce risk of injury. (Price, 121)

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Teasing can be very useful for identifying when the mare is in heat.

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Mark Chittick explains the use of the Teaser in preparing mares for service.

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According to Sexual Behavior of Mares, actions such as aggression, striking, and kicking towards the stallion occurs if he attempts to court a diestrus mare.

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The mare is seasonally polyestrus, having an anovulatory period during the short light days of late fall and early winter, and beginning to ovulate as...

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A stallion or gelding is used to tease the mare. Her behavior has to be watched closely each time the tease is brought out to ensure she is bred at the proper time.
“Teasing out the perfect time to cover your mare.” FarmIreland.ie, 29 Nov. 2012, www.independent.ie/business/farming/teasing-out-the-perfect-time-to-cover-your-mare-26725084.html.

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Horses are long day breeders. As day lengths increase, they come into heat.

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It is important that horse owners and breeders understand and recognize the basics of equine reproductive behavior for management purposes. This article discusses the main behaviors associated with horse breeding.
Ashley Griffin, University of Kentucky
An understanding of the basics of reproductive behavior is important. This understanding can lead to management applications that can improve reproductive success. It is important to recognize the behaviors associated with a mare in heat, a mare that is receptive to a stallion, mating, signs of foaling, normal behavior after foaling, and libido in a stallion.

Puberty is the attainment of sexual maturity. In fillies, this is usually at 12 to 15 months of age, but it can be as early as 9 to 10 months. Stallions are 15 months or older before they can successfully breed. Research has noted that both stallions and, to a lesser degree, fillies may exhibit sexual display before their reproductive tracts are physiologically mature. Pregnancy cannot occur until the respective reproductive tract matures at the time of puberty. Conversely, some fillies may cycle but not exhibit signs of estrus.

Estrus (Heat)

Estrus, or heat, is the period of the reproductive cycle when the mare ovulates and, if bred, is likely to conceive. Estrus is also the time when the mare is receptive and will accept the stallion. The average length of the estrous cycle, or the period from heat period to the next heat period, is 21 days, but the estrous cycle can vary from 19 to 26 days. The duration of estrus is five to seven days (actually about six days), but it can vary from two to 10 days. The first heat following foaling is referred to as foal heat. Foal heat typically occurs six to nine days after foaling, but it may be as early as five days or as late as 15 days.

It is important to recognize the behavioral signs of estrus. Some signs are general, including restlessness, hyperactivity, less time devoted to eating and resting, and more time "running the fences." Other signs more descriptive of estrus are frequent urination, straddling (squatting) posture, and clitoral "winking." Mares exhibiting strong heat will actually lay against a fence or teasing partition when exposed to the teaser, a stallion used to make mares exhibit estrus. Most mares will not exhibit overt signs of estrus without the presence of a stallion.

Courtship and Mating

Horses are referred to as "long-day breeders" because they come into heat as the days increase in length in the spring. Mares are also " seasonally polyestrous," meaning they have multiple estrous cycling throughout the spring and summer. The natural breeding season for horses in the Northern Hemisphere is the spring or summer. Light is the controlling factor in causing mares to come into heat in early spring. Most studies have indicated a tendency toward anestrus (not cycling) in the winter months; however, some mares may cycle during this time as well.

Mares will cycle several times during the breeding season if they do not conceive and become pregnant. The most intense estrus behavior occurs when the mare is most sexually receptive to the stallion. Intense estrus behavior lasts about three days.

A mare in heat may actively seek out and attempt to stay in the vicinity of a stallion. During the peak of estrus, the mare may sniff, lick, or nuzzle the stallion. A mare in heat will also urinate frequently, particularly if a stallion is teasing her to test her receptiveness. She is also likely to raise her tail and assume a breeding stance. The classic behavioral display of the stallion when it "checks" a mare is to lift its nose into the air and curl his upper lip. This is called the Flehmen response. The stallion will often be impatient, alert, hyperactive, and restless. Vocalization is common. The stallion will frequently nudge the mare, apparently to signal readiness and to assess her "firm stance" response. In addition to nudges, some stallions may smell and bite over the mare's body. Most behavioralists consider this display to be more important in the courtship process than odor recognition.
Establishing Dominance
Dominance patterns are very much a part of breeding behavior, particularly in wild horses. Dominance patterns are not as easily seen on most modern stud farms, where stallions are not allowed to run in groups with bands of mares. In a natural environment, one stallion will typically dominate the breeding of a band of mares, and competing stallions will be banished to form their own separate band. At some point, one of the banished stallions will become old enough, brave enough, or tough enough to defeat the dominant stallion. In modern breeding establishments with numerous, separately stalled breeding stallions, all the stallions are used for breeding. Dominance, nevertheless, is in evidence. Most breeding barn managers can tell you which stallion is dominant, or "the boss."

Libido is the term used to denote sexual drive or the degree of sexual urge in animals. A stallion with a high libido will exhibit an eagerness to mount and attempt to breed a mare. In natural situations, stallions exhibit a wide range of libido levels, from zero activity to extreme aggressiveness. Some stallions will have such a strong libido that they will sacrifice all other pursuits in favor of searching for and breeding mares in heat. An extremely high or low libido may cause problems. Young stallions are more likely to exhibit a wide range of libido. Young stallions with extremely low libido are hard to breed and require patience from those handling them. Young horses with very high libido require extreme caution by the handler and those working the breeding shed.

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It is important that horse owners and breeders understand and recognize the basics of equine reproductive behavior for management purposes. This article discusses the main behaviors associated with horse breeding. Ashley Griffin, University of Kentucky An understanding of the basics of reproductive behavior is important.

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"The mare typically raises her tail, exposing the clitoris, and often assumes a urination stance." (Price, 133)

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This mare is showing signs of estrus by raising her tail when a stallion is near.

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A stallion catches an odor, then curls his lip up to keep the odor trapped in the nasal cavity.
Biles, Deirdre B. “Flehmen Response in Horses.” BloodHorse.com, 23 Jan. 2001, www.bloodhorse.com/horse-racing/articles/194702/flehmen-response-in-horses.

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If a mare was brought off of the farm to breed then returned to the home stable, there is a 31% chance that mare would abort the foal. This is most likely because the mare wanted to preserve her energy for the stallion in the home group.
Bartoš, Luděk, et al. "Promiscuous Behaviour Disrupts Pregnancy Block in Domestic Horse Mares." Behavioral Ecology & Sociobiology, vol. 65, no. 8, Aug. 2011, pp. 1567-1572. EBSCOhost, doi:10.1007/s00265-011-1166-6.