Mare Heat Cycle: How Long It Lasts, Signs of a Mare in Heat, Foal Heat and Breeding Timing

A mare’s heat cycle (her estrous cycle) runs about 21 days from one ovulation to the next, and she is in heat, receptive to a stallion, for roughly 2 to 8 days of it (Merck Veterinary Manual). Most mares cycle like this during the long days of spring and summer under natural light in the Northern Hemisphere; through most of the winter their ovaries are quiet, and the first heats of spring are long, erratic and often come with no egg released at all.

So the honest answer to “how often does a mare come into heat?” is about every three weeks, in season. If you are watching a mare to breed her, the useful skills are spotting the heat, knowing that she ovulates near the end of it, and writing the dates down so the next one does not surprise you. Once she is bred, the free Creatures horse gestation calculator turns the breeding date into an estimated foaling date with no account needed, and a breeding record on her profile keeps that date, the stallion and her later checks together.

This guide covers the numbers behind the cycle, the breeding season, the signs of a mare in heat, foal heat after foaling, how heat timing relates to breeding, and when heat behavior is a reason to call your veterinarian.

How long is a mare’s heat cycle?

The full estrous cycle is about 21 days. Merck’s equine reproduction chapter describes it as a “21-day interovulatory” cycle, meaning the clock runs from one ovulation to the next (Merck Veterinary Manual, The Reproductive Cycle of Horses). The Ontario Ministry of Agriculture’s mare reproduction fact sheet gives the same figure with a spread of about three days either way (OMAFA, Anatomy, Physiology and Reproduction in the Mare).

That 21 days splits into two phases:

• Estrus (heat). The days she will accept a stallion. Merck gives 2 to 8 days. Ontario says heat usually lasts about 6 days but can run 4 to 10. The sources overlap rather than agree exactly, which is a fair picture of real mares: some are in and out in a couple of days, others linger for most of a week or more.
• Diestrus. The rest of the cycle, when she is out of heat and will reject a stallion. Ontario puts it at normally about 15 days, ranging from 12 to 18. Because the overall cycle stays near 21 days, a mare with a long heat tends to have a shorter diestrus.

The detail that matters most for breeding: ovulation usually happens 24 to 48 hours before the end of heat, not at the start (Ontario). On a veterinary ultrasound, the dominant follicle is typically 30 mm or more across when it ovulates (Merck). So the first day you notice a mare in heat is usually several days before the egg is released.

Why the length of heat changes through the year

Heat is not the same length all season. Merck notes that heats early in the breeding season are longer, and that around the summer solstice they shorten to 2 to 3 days. If your mare stood for a week in April and only two days in June, that pattern is normal.

When do mares come into heat? The breeding season

Mares are seasonally polyestrous: they cycle again and again while the days are long, and stop when they are short. Merck describes them as cycling “when the length of daylight is long”, and in the Northern Hemisphere the natural cycling season starts around April. Ontario’s fact sheet puts normal cycling from late April or early May until August, and says that from September through March very few mares cycle normally. Puberty usually arrives between 12 and 18 months of age (Ontario).

The year breaks into four stretches:

1. Winter anestrus. The ovaries are inactive. A mare who shows no heat at all in January is usually just being a horse.
2. Spring transition. As days lengthen, mares go through several long, irregular periods of receptivity before they settle into a pattern. Merck’s horse owner guide describes 3 to 4 of these over 8 to 10 weeks, and the veterinary chapter notes that ovulation does not occur during these early waves. Ontario adds that mares may be in heat for 20 to 30 days or more at this stage. Breeding a mare in a spring transition heat often wastes the cover or the semen.
3. The ovulatory season. Regular 21-day cycles with ovulation, from late spring through summer.
4. Autumn transition. Merck notes that similar long, irregular periods of receptivity can happen again as days shorten, before the mare goes quiet for winter.

Using lights to bring the season forward

Owners who want foals early in the year use artificial lighting to start the season sooner. Merck’s guidance on light programs (The Reproductive Cycle of Horses):

• Mares need 16 hours of light per day in total.
• The light must be at least 10 foot candles (107 lux).
• The extra light has to be added at dusk; adding it in the morning before dawn does not work.
• Mares take 8 to 10 weeks to respond, so for a season starting February 15, lights go on around December 1.

Veterinarians can also use prescription hormone treatments to shorten, time or suppress a mare’s cycles. Those are decisions to make with your vet for your mare, and this guide does not cover drugs or doses.

Signs of a mare in heat

A mare in heat does several of these at once, and the clearest show when a stallion is near. Merck describes mares in estrus that “raise their tail, squat, urinate, evert the vulvar lips” and finally accept the stallion. Purdue Extension’s guide to estrus detection lists the same primary signs, with accepting the stallion as the main one, plus secondary signs such as seeking the company of other mares, geldings and handlers (Purdue Extension, Estrus Detection in Farm Animals).

Signs a mare is in heat:

• Winking: repeated opening and closing of the vulva, exposing the clitoris.
• Squatting or lowering the hindquarters.
• Lifting the tail.
• Frequent urination in small amounts, especially near another horse.
• Seeking out the stallion (Extension Horses) and the company of other horses and people (Purdue).

Signs a mare is out of heat (diestrus): Merck’s owner guide says a mare who is not receptive usually squeals, kicks, bites and rejects the stallion’s attempts.

Teasing: how breeders check

Because heat signs are strongest around a stallion, breeders “tease” mares: a stallion is brought near her, usually across a solid partition or fence, and handlers watch her response. Merck’s owner guide recommends presenting the mare to a teaser stallion daily or every other day during the breeding season. Extension Horses describes both individual teasing and group teasing in purpose-built pens, and notes that most managers combine teasing with veterinary palpation or ultrasound (Extension Horses, Estrus Detection of Mares). Teasing involves a stallion and a mare in close contact, so it is a job for experienced handlers and safe barriers.

Why some mares show quiet heats

Some mares cycle normally but show little outward sign. Extension Horses lists the usual reasons: a mare protective of her foal, a maiden mare who has never been bred, a timid mare, stressful weather, and the early part of the season. For these mares, a veterinary ultrasound or palpation exam can show where she is in her cycle when her behavior does not.

Foal heat: the first heat after foaling

Mares come back into heat quickly after giving birth. Merck’s owner guide places this “foal heat” at 5 to 11 days after delivery and says breeding can happen then, but that success rates are higher when breeding is at least 10 days after delivery. Whether to breed on the foal heat or wait for the next cycle about three weeks later is a call to make with your veterinarian after a postfoaling check.

Foal heat is also worth knowing about if you are not breeding: a mare who seems restless a week after foaling may simply be in heat. If you are still waiting on the foal, our foaling guide covers the signs of labor and the stages, and the mare foaling preparation timeline lays out the veterinary milestones before the due date.

Timing a breeding to the heat cycle

Since ovulation comes late in heat, the goal is to get viable sperm to the mare shortly before she ovulates. Ontario’s fact sheet says breeding within about 12 hours of ovulation gives the best chance, and describes a traditional alternative for live cover without ultrasound: breeding on day 2 or 3 of heat and then every other day until she goes out (OMAFA). With shipped or frozen semen, timing is tighter, and your veterinarian will usually track the growing follicle by ultrasound to decide when to inseminate. The horse stud fees guide explains how that monitoring adds to the cost of breeding a mare.

After she is bred, the cycle becomes your first pregnancy check. A mare who does not return to heat 2 to 3 weeks after breeding is probably pregnant (Merck owner guide), but “probably” is not enough with horses. Merck’s chapter on pregnancy determination recommends an ultrasound at days 14 to 18 to confirm pregnancy and check for twins, with a recheck at days 25 to 30 (Merck Veterinary Manual, Pregnancy Determination in Horses). Our guide to how long horses are pregnant picks up from there, and if you have already chosen a stallion, the horse pedigree papers guide explains how to check his registration papers and DNA records.

When heat behavior needs a veterinarian

Most heat behavior is normal, even when it is inconvenient. Some patterns are not, and they deserve a veterinary exam rather than a guess:

• A mare who seems to be in heat all the time during the season, or never seems to go out.
• No heat at all through late spring and summer, in a mare who is not pregnant.
• Stallion-like behavior in a mare: mounting other horses, aggression toward other mares, or acting like a stallion at the fence.
• A sudden, large change in temperament that you or your trainer are putting down to “being in season”.

Ovarian tumors are one reason vets look closely at these mares. Granulosa cell tumors are the most common ovarian tumor in mares, and affected mares may show prolonged anestrus, persistent heat or stallion-like behavior (Frontiers in Veterinary Science, 2025). Only a veterinary examination can tell a tumor from an ordinary long heat, so do not try to sort it out from behavior alone.

Behavior under saddle is another common question. Plenty of owners find a mare more distracted or touchy when she is in heat, but pain and other problems can look similar. If the change is big, sudden or lasts beyond a heat or two, start with your vet rather than with a cycle chart.

Tracking a mare’s heat cycles on Creatures

A paper calendar works for one mare until a spring transition runs for a month, or the farrier visit lands on the day she finally ovulates. Creatures keeps her heats, the breeding and the checks that follow on her own profile, so next season you can look back instead of starting from memory.

Start with the free calculator (no account). On the horse gestation calculator, enter an exact breeding date or, if she ran with a stallion, the exposure window. It adds 340 days for an estimated foaling date and shows a 320 to 370 day planning window, along with a pregnancy timeline from the early ultrasound to the last weeks. The gestation calculators page covers other species too. For how the tool works, see using the gestation calculator.

Give her a profile and log her heats (free account). Add your mare, then from her profile choose Add record and Breeding & Reproduction. Under Reproductive Category, choose Heat Tracking (the option reads Estrus / Heat Tracking until the record is tied to a mare), enter the date you first saw her in heat and a short description: what she did, how she teased, and anything your vet found on ultrasound. Leave Track as breeding cycle off for a heat that is just a note. Creatures does not predict her next heat from these records, so if you want a nudge, add a reminder for about three weeks later; reminders can repeat on an interval you choose (reminders and upcoming care). Over a season, those dated entries show you her pattern: how long her heats run, when she started cycling, and whether her foal heat came on time.

Record the breeding. When she is bred, add another Breeding & Reproduction record and turn on Track as breeding cycle. Choose the service method, pick the stallion from your animals or type his name if he lives elsewhere, and enter the service date. If she was with a stallion over several days, enter a start and end date and Creatures widens the likely window to match. The steps are in recording a breeding. Her Breeding tab then shows the active cycle, the likely foaling window and milestones, and if your vet pins down the ovulation date you can adjust the projection rather than starting over (tracking pregnancy and progress).

If she comes back open. Record the pregnancy check result on the same breeding. When a check shows she is not pregnant, Creatures offers to end that breeding, which clears its remaining reminders, and to set a reminder to rebreed about when her next heat is due, counting forward from the service date in whole cycles of about three weeks.

Watch every mare on one screen. The Breeding dashboard lists every mare you manage with an active breeding under Expecting, Due soon and Overdue. Overdue only means the window has passed with no foaling recorded; it is a reason to check on her, not a health assessment. When the foal arrives, record the birth and Creatures closes the cycle and lets you create the foal’s own profile. Before then, the horse coat color calculator can show the colors a pairing could produce, and the free horse pedigree chart maker builds a pedigree for the foal without an account.

If you also run cattle, the same records work for cows: our sister guide to the cow heat cycle covers their much shorter standing heat. For the paperwork side of horse ownership, see the Coggins test guide, and if a small horse is on your list, how much a miniature horse costs.

Frequently asked questions

How often does a mare come into heat?
About every 21 days during the breeding season, which in the Northern Hemisphere runs roughly from spring through late summer. Most mares do not cycle normally through the winter.

How long is a mare in heat?
Merck gives 2 to 8 days; Ontario’s fact sheet says about 6 days on average, ranging from 4 to 10. Heats are longer early in the season and can be as short as 2 to 3 days around midsummer.

When does a mare ovulate during her heat?
Usually 24 to 48 hours before the end of heat, so near the end rather than at the start. Your veterinarian can track the follicle by ultrasound to time breeding more closely.

Do mares come into heat in winter?
Most do not. Their ovaries are inactive through the short days, and Ontario notes that very few mares cycle normally from September through March. Some owners use a lighting program, 16 hours of light a day started 8 to 10 weeks ahead, to bring the season forward.

How soon after foaling does a mare come into heat?
Foal heat comes 5 to 11 days after foaling, according to Merck’s owner guide, which notes higher success when breeding is at least 10 days after delivery. The next heat follows about three weeks later.

Can a mare be in heat and pregnant?
Not returning to heat after breeding is a good sign but not proof of pregnancy, and outward behavior can mislead in both directions. A veterinary ultrasound at days 14 to 18 is the reliable answer and also checks for twins.

What are the first signs of a mare in heat?
Usually interest in other horses, especially a stallion, followed by raising the tail, squatting, frequent small urinations and winking of the vulva.

Do this next on Creatures

Whether you are watching a mare for her first heat of spring or counting down to a foal, these are the most useful next steps. More about horses in general is on the Creatures horse page.