You can tell if a horse is in heat by watching her behavior and checking her body for physical signs. A mare in heat, or in estrus, shows specific actions and changes that breeders and horse owners look for. These signs help confirm when it is the right time to breed.
Deciphering the Equine Reproductive Calendar
To spot when a horse is ready to breed, we first need to grasp the basics of the estrus cycle in horses. Horses are seasonal breeders. This means they breed more often when the days are long and light.
The Horse Heat Cycle Timeline
The heat cycle in a mare has two main parts: estrus and diestrus. Estrus is the “in heat” time when she accepts a stallion. Diestrus is the non-receptive time.
When is a mare in heat? Typically, during the breeding season (spring and summer), mares cycle every 21 to 23 days. The heat period itself lasts about five to seven days.
Frequency of horse heat cycles changes with the seasons. In winter, mares often have longer periods where they don’t cycle, called anestrus.
Stages of the Estrus Cycle
| Stage | Duration (Approx.) | Mare’s Behavior | Ovarian Changes |
|---|---|---|---|
| Estrus (Heat) | 5–7 Days | Receptive to the stallion | Follicle grows and matures |
| Diestrus (Not in Heat) | 14–16 Days | Rejects the stallion | Corpus luteum forms after ovulation |
Identifying a Horse in Heat: Key Indicators
Knowing the signs of mare in heat is vital for timing artificial insemination (AI) or natural breeding. These signs fall into two main groups: visible behaviors and physical changes.
Behavioral Signs of Equine Heat
Behavior is often the first thing owners notice. A mare acts differently when she is receptive. These are key behavioral signs of equine heat.
Approaching the Stallion
When a mare is receptive, she shows clear interest in a male horse.
- She may walk toward the stallion frequently.
- She might stand quietly when the stallion approaches her rear end.
- She might whinny or nicker softly toward the stallion.
The “Flagging” Action
One of the most telling behaviors is “flagging.” The mare lifts her tail high and away from her hindquarters. This movement exposes the vulva, making it easy for the stallion to approach for mating.
Squatting and Urination
Mares in heat often adopt a specific posture when a stallion is near or when being teased. They may:
- Hunch their hindquarters slightly.
- Spread their hind legs a little apart.
- Urinate small amounts of urine frequently. This urine often contains hormones that signal readiness.
Aggression to Other Mares
During estrus, a mare might become irritable or aggressive toward other mares. She may swat her tail at them or try to kick them if they come too close to her, especially if a stallion is present nearby.
Physical Signs of Mare Estrus
Besides what she does, what you can see on her body also gives clues. These are important physical signs of mare estrus.
Vulvar Changes
The vulva often swells during estrus. It looks fuller, puffier, and may appear more moist than usual. This swelling is caused by increased blood flow to the area.
Discharge
A mare in heat often produces a thin, watery, or slightly yellowish discharge from the vulva. This is different from the thick, mucus-like discharge seen during other times or if an infection is present.
Increased Sensitivity
If you gently touch the mare’s back near her tail base, she may react quickly. This sensitivity is a physical sign she is ready to stand for breeding.
Methods for Confirming Estrus
While observing behavior is helpful, sometimes you need more concrete proof. This is where specialized techniques come in.
Teasing a Mare: The Best Way to Know
Teasing a mare is the process of introducing her to a stallion or a “teaser” stallion (a gelding with high libido, often kept safely separated) to check her receptivity. This is the most reliable way to confirm heat before moving her to the breeding shed or pasture.
How to Safely Tease a Mare
Safety is the number one priority when teasing. The mare and stallion must always be physically separated by a sturdy barrier, like a breeding chute or strong gate.
- Preparation: Have the mare restrained by a handler in a safe area.
- Introduction: Bring the teaser stallion close, allowing him to sniff the mare’s hindquarters safely behind the barrier.
- Observation: Watch for the mare’s reactions described above (flagging, squatting, winking).
If the mare shows strong receptive signs, she is likely in peak heat. If she kicks, pins her ears, or moves away aggressively, she is likely not in heat (diestrus).
Using Veterinary Tools for Precision
Veterinarians use tools to pinpoint the exact time of ovulation, which is the goal when identifying a horse in heat.
Rectal Palpation
A vet inserts a gloved, lubricated arm into the mare’s rectum to feel the ovaries.
- In Heat: The dominant follicle, which holds the egg, will feel large, smooth, and fluid-filled, like a small blister on the ovary.
- Not in Heat: The ovaries might feel smaller, or the major structure might be the corpus luteum (the structure left after ovulation).
Ultrasound Examination
Ultrasound is the gold standard for precise timing. A vet inserts an ultrasound probe into the rectum.
- The ultrasound clearly shows the growing follicle size in millimeters.
- Signs of ovulation in mares are confirmed when the follicle reaches a certain size (usually 40–55 mm) and then collapses on the image shortly after breeding.
Hormone Testing
Blood tests can measure levels of Luteinizing Hormone (LH) and Estrogen. High estrogen levels usually mean the mare is approaching heat, and a surge in LH often signals impending ovulation.
Factors Affecting When a Mare is in Heat
The timing of the estrus cycle in horses is not fixed year-round. Several factors influence when is a mare in heat.
Sunlight and Seasonality
Light exposure is the biggest trigger. Mares need long days (14–16 hours of daylight) to start cycling regularly. This is why most mares begin cycling in early spring.
- Spring: Mares begin transitioning out of anestrus. Cycles can be erratic or short initially.
- Summer: Cycles are regular and predictable.
- Winter: Most mares enter anestrus and do not cycle regularly due to short daylight hours.
Age and Health
Young fillies usually have their first heat (foal heat) between 12 and 18 months old. Older mares might show a decline in fertility, and cycles can become less regular as they age. Poor body condition or illness can also interrupt normal cycling.
“Foal Heat”
A special event is the “foal heat.” This is the first heat cycle a mare has after giving birth. It usually occurs about 5 to 12 days after foaling. While convenient, many breeders wait until the second or third cycle to breed back because the chances of conception are lower during the first post-foal heat.
Differentiating Heat from Other Behaviors
Sometimes a mare shows behaviors that look like heat but are not. It is crucial to separate true estrus from other issues when identifying a horse in heat.
Prolonged Diestrus vs. Estrus
If a mare is not cycling but has spent time with a stallion, she might be “sore” or moody. If you tease her and she shows mild tail lifting but doesn’t allow breeding, she is likely in diestrus. True heat involves a strong willingness to stand and be bred.
Uterine Infections (Endometritis)
A mare with a uterine infection might have a foul-smelling discharge and swelling. However, she will be aggressive toward a stallion, unlike a mare truly in heat who welcomes him.
False Heat
Sometimes, especially early in the breeding season, a mare might show heat signs for only a day or two and then stop. These short cycles are common as the mare’s body prepares for consistent cycling. This is why repeated teasing is necessary.
Managing Breeding Schedules Based on Estrus Signs
Accurate recognition of mare estrus signs allows breeders to maximize pregnancy rates. Timing is everything.
Optimal Breeding Window
The best time to breed a mare is near the end of her heat period, right before or immediately after ovulation. Sperm survive better than the egg, so breeding a day or two before ovulation is ideal.
If you rely only on behavioral signs, you might breed too early or too late. This is why vets use ultrasound to find the exact moment the follicle is about to rupture.
Synchronization Strategies
For large breeding operations, vets may use drugs to synchronize cycles. This brings many mares into heat around the same time. Even with synchronization, careful observation of behavioral signs of equine heat is still necessary to fine-tune breeding appointments.
Summary of Essential Signs Checklist
When trying to confirm if your horse is ready, use this quick checklist focusing on key indicators.
| Sign Type | Check For: | Receptive (In Heat) | Non-Receptive (Diestrus) |
|---|---|---|---|
| Behavior | Tail Position | Lifted high, flagged | Held low, swishing normally |
| Behavior | Stance | Squatting slightly, standing firm | Moving away, kicking, biting |
| Physical | Vulva Appearance | Swollen, moist, relaxed | Dry, normal appearance |
| Teasing Response | Reaction to Stallion | Winking (opening/closing vulva), urination | Ignoring stallion or aggressive display |
| Discharge | Presence | Thin, clear to watery white | Absent or thick/colored |
Final Thoughts on Recognizing Estrus
Accurately identifying a horse in heat requires patience and consistent observation. Combining daily behavioral checks with regular professional evaluations, especially ultrasound, gives the best chance for a successful pregnancy. Keep detailed records of when signs start, how long they last, and when ovulation occurs to predict future cycles effectively. Mastering the signs of mare in heat is a core skill for any horse breeder or owner involved in reproduction.
Frequently Asked Questions (FAQ)
How long does a horse stay in heat?
A mare usually stays in heat for five to seven days. This is the period when she is willing to accept a stallion.
What is “winking” in a mare?
Winking is when the mare rhythmically opens and closes her vulva lips when receptive. It often happens when she is posturing for a stallion or during urination. This is a strong behavioral sign of equine heat.
Can a mare show signs of heat when she is pregnant?
Sometimes, a mare that is already pregnant may show very mild heat signs around the time she would normally cycle (about 21 days after breeding). This is often called “heat behavior” but is not true estrus. A vet can confirm pregnancy via ultrasound to rule this out.
What is the best time of day to check for heat?
It is best to check at least twice a day, usually morning and evening. Mares might show more pronounced signs when stimulated, such as after being teased or after exercise. Consistency in checking helps you catch the peak window of receptivity.
If a mare is not cycling, is she infertile?
Not necessarily. If it is winter, she is likely in anestrus due to low light levels. If she is not cycling in the summer, there might be an underlying health issue or hormonal imbalance that requires veterinary attention to restore normal estrus cycle in horses.