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You might not have any problem determining when your mare is in heat, if she tends to behave like a Jekyll and Hyde. Some female horses don't behave in a "mareish" manner, so signs are more subtle. A normally placid mare might become more anxious. Mares with more difficult temperaments might tend to bite or kick, if that's not an issue when they're not in heat.
The Estrus Cycle
In North America, mares usually go into their estrus cycle between April and September, although much depends on the individual horse. The average estrus cycle lasts three weeks, but your mare only shows strong evidence of heat for about five of those days. Mares normally ovulate on about the fifth day of the cycle. To predict when your mare will next come into heat, the Louisiana State University School of Veterinary Medicine recommends counting the day the mare goes out of heat, usually two days after ovulation, and count forward 14 days when she'll come into heat again.
If your mare doesn't undergo mood swings, there are other, obvious behavioral changes. When you groom her, she might become very sensitive to brushing, especially around her belly and flanks. She might constantly squirt urine, "winking" her vulva, especially if there's a gelding -- let alone a stallion -- in the vicinity. If she's turned out with a herd, you might have trouble removing her from her herdmates without a lot of drama on her part, such as constant calling. If you're riding or training her, you might find her easily distracted and not paying attention to your aids.
If your mare becomes difficult during her cycle and you don't intend to breed her but want to continue riding or competing, ask your vet about supplements that might alleviate some of her symptoms. If she's truly a handful during heat, you've got other options. Ask your vet about hormonal therapy, which consists of giving her progesterone or other hormones to fool her body into thinking it is pregnant. In a worst-case scenario, if your mare is absolutely impossible during heat and you don't plan to breed her, your vet can spay her.
If you do intend to breed your mare, determining the right time for conception to occur is crucial to getting her in foal. That's true whether you're using live cover or artificial insemination. Your vet can perform an ultrasound to detect ovulation, inserting AI semen at the proper time. If your mare is at a breeding facility, a teaser stallion is used to determine the mare's receptivity to copulation. These stallions aren't the ones actually breeding the mares, but their presence and behaviors stimulate mares who are ready to breed, while those who aren't receptive make that clear.
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