Mare Heat Cycle: Signs, Length and Foal Heat 2026

The mare is a seasonal polyoestrous breeder. In the breeding season she repeats an oestrous cycle of about 21 days until she is pregnant or the days shorten. Each cycle includes a heat of roughly 5-7 days, and she ovulates in the last 24-48 hours of that heat. Knowing this rhythm is the foundation of every covering and insemination plan in 2026.

What is the mare's oestrous cycle?

The cycle has two main phases, driven by hormones. During oestrus (heat), oestrogen is high and progesterone is low: the mare accepts the stallion and a follicle matures on the ovary. After ovulation, the corpus luteum forms and progesterone rises. This is dioestrus, when the mare refuses the stallion.

In practice:

• Total cycle length: about 21 days (some sources say 22).
• Oestrus: about 6 days on average, with a normal range of 5-7 days and extremes of 3-8 days.
• Dioestrus: about 14-15 days.

In France, the breeding season runs from the end of March to early October, according to a French veterinary clinic fact sheet on the mare's cycle. Colorado State University's Equine Reproduction Laboratory gives the natural breeding season as April to October. Outside this window, most mares in the northern hemisphere stop cycling. Ireland and Italy follow the same long-day logic, with local variation by latitude and individual mare.

How long does a mare's heat last, and what does each phase look like?

The table below summarises the phases a breeder meets across the year.

A dominant follicle usually grows 2-3 mm per day before ovulation, according to Oklahoma State University Extension. Growth can reach 5 mm per day, or stall for three to four days. This is why a scan tells you more than behaviour alone.

What are the signs of a mare in heat?

A jument en chaleur, as French breeders say, shows a recognisable set of signs, described by veterinary sources such as The Horse:

• Receptivity to the stallion: she stands, accepts him and tolerates his approach.
• Winking of the vulva (rhythmic eversion of the clitoris).
• Frequent urination in small amounts.
• Tail raised or held to the side.
• Squatting and a relaxed, softer attitude.

The opposite picture is typical of dioestrus: ears flat, squealing, kicking and clear rejection of the stallion.

Behaviour that looks like heat but does not fit the cycle needs a closer look. Constant grumpiness, stallion-like behaviour or heat signs at odd times can point to a persistent follicle, a granulosa cell tumour of the ovary or a non-reproductive problem such as a urinary infection. A normal ovary is only a couple of inches long, while a tumour can enlarge it dramatically. A vet can check with palpation, ultrasound and hormone testing, including anti-Müllerian hormone.

How do you tease a mare to detect heat?

Teasing means presenting the mare to a stallion, usually over a barrier, and scoring her reaction. Extension sources describe a simple 1-5 score, where 1 is strong resistance to the teaser and 5 is clear, strong signs of heat.

Good practice:

1. Tease at the same time each day, with the same teaser and the same set-up.
2. Observe each mare for about 2-5 minutes. Less time gives unreliable results.
3. Record the score every day, with the date.
4. Have a vet check any mare scoring 3 or higher by palpation or ultrasound.

Teasing tells you the mare is receptive, not when she will ovulate. Palpation or ultrasound is what links behaviour to the follicle.

When does the mare ovulate, and when should you breed?

Ovulation usually happens 24-48 hours before the end of oestrus. The best conception rates are obtained when the mare is covered from about 36 hours before ovulation up to ovulation itself, according to extension guidance on teasing.

The type of semen changes how tight the timing must be, based on Colorado State University's Equine Reproduction Laboratory:

To time ovulation, a vet may use an ovulation-inducing drug such as hCG or deslorelin once the follicle is mature. In French practice, hCG is given when the follicle exceeds 3.5 cm, and ovulation follows within 48 hours. Repeated hCG use can lead to antibodies that reduce its effect. These treatments are always veterinary decisions, taken after a gynaecological exam.

For an open mare with no reproductive history, the Colorado State team suggests the most efficient clinic visit is the second day of oestrus, with covering 2-3 days later.

What is foal heat, and should you breed on it?

Foal heat (chaleur de poulinage) is the first heat after foaling. According to IFCE (equipedia), it starts 5-12 days after foaling and lasts 2-7 days, 4 on average. It follows 91% of foalings.

Whether to use it depends on timing and on how the foaling went:

• Ovulation before day 10 after foaling: the uterus is often not fully recovered, fertility may be lower and the risk of embryonic loss is higher. Do not breed.
• Ovulation after day 12 with an uneventful foaling: fertility and resorption risk are comparable to other cycles.
• Difficult foaling or complications: skip the foal heat.
• Mares that routinely carry 11 months or more: do not breed on foal heat, as it pushes foaling later each year.

Colorado State University sets clear criteria for breeding on foal heat: foaling without dystocia, placenta passed within 3 hours with no retention, no excessive trauma and vaginal discharge limited to the normal 1-3 days. The mare is examined 7 days after foaling.

If you skip the foal heat, a vet can give prostaglandin 6-8 days after ovulation to bring the mare back into heat sooner, or examine her a few days before the heat around day 30. If you did not see the foal heat, restart teasing or a vet check from day 20 after foaling. IFCE recommends aiming to use at least three heats per season.

What happens in the transition seasons?

Mares are long-day breeders. As days lengthen in spring, the mare leaves winter anoestrus. A mare coming out of anoestrus typically takes about 60 days of transition before her first ovulation. During this time, heats can be long, irregular or weak, and follicles grow without ovulating. This is normal, and not a reason to cover yet.

In autumn, the reverse happens: heats become irregular, then stop as days shorten.

If you need an early foal, artificial light is the most reliable tool:

• Extend the total day length to 16 hours.
• Allow 60-90 days between starting the lights and the first ovulation. Starting in early December can bring first ovulation to mid-February (Colorado State University).
• Keep light intensity adequate, around 2 foot-candles, which is roughly one 200-watt bulb in a 12 x 12 ft (3.6 x 3.6 m) stall.
• Use a timer so the day length stays constant.

Mares kept without lights should generally be brought in for a first reproductive check from mid-March to early April.

What is silent heat?

A silent heat is a cycle in which the mare ovulates normally but shows few or no behavioural signs. It is more common in timid mares, in mares with a foal at foot and in the early or late season. The mare is cycling, you just cannot see it.

Teasing in a poorly suited set-up can also miss heat. Ultrasound is the reliable way to confirm follicle growth and ovulation when behaviour is unclear.

A related issue is a persistent corpus luteum. A corpus luteum that lasts beyond its normal 12 days can persist for up to 80 days, and the mare stays out of heat. A vet can treat it with prostaglandin, which only works after a refractory period of about 5 days post-ovulation.

When should you call the vet for scanning?

Call your vet in these situations:

1. Before the season: for a breeding soundness exam and to plan monitoring.
2. Early in heat: to scan ovaries and follicle size, especially with cooled or frozen semen.
3. After covering: for a pregnancy scan 14-16 days after ovulation (14-18 days at some clinics). This window also lets the vet find and reduce twins, which is only possible early, between about 14 and 17 days after ovulation.
4. At 24-25 days: to see the heartbeat, with another check at 30-35 days.
5. If your mare shows no heat for weeks in season, shows constant heat, or behaviour changes suddenly.
6. If the foal heat is not seen, from day 20 after foaling.

Older mares need closer follow-up. Healthy mares up to about age 15 have a 50-60% chance of conceiving per cycle, and fertility declines steadily from 15 and more sharply after 20.

How do you keep track of covering dates?

Once the mare is covered or inseminated, the date becomes the anchor for the rest of the season. Record it in the Gestation Tracker. Breedio follows each mare through the roughly 340 days of pregnancy, computes the gestation phases (early, approaching, optimal, attention, overdue), sends notifications at key milestones and keeps a full history per mare. It also works offline, which is useful in the stable. You can find all the free tools on the Breedio tools page.

Key takeaways

• The mare cycles about every 21 days in season, with 5-7 days of heat and ovulation in its last 24-48 hours.
• Tease daily for 2-5 minutes per mare and confirm with ultrasound.
• Foal heat starts 5-12 days after foaling: avoid it if ovulation falls before day 10 or after a difficult foaling.
• Transition takes about 60 days, and silent heats exist: do not rely on behaviour alone.
• Scan 14-16 days after ovulation for pregnancy and twins.