If you're breeding horses as a business endeavor, knowing about foal heat -- your mare's first heat after giving birth -- is important for several reasons, depending on your breeding goals and your mare's health. Her foal heat cycle can begin as early as six days after giving birth and as late as 12 days. Ovulation will typically begin a day or two after the cycle begins. Successfully breeding your horse during foal heat is the only way to ensure annual foal production.
Seasonal Heat Cycles
Unless your mare has health issues that interfere with heat cycles, she will have multiple heat, or estrus, cycles beginning with the increased daylight hours of spring. The term for having these multiple cycles is polyestrous. Her ovaries become active, meaning they develop follicles and she begins ovulating. After her first ovulation of the season, she should ovulate about every 21 days, with estrus occurring for two to eight days during this period. It's only during estrus that she is receptive to a stallion, and typically estrus is longer early in the spring. By the time summer rolls around, estrus might last only a few days. The times during her cycle when she is not receptive to breeding is called diestrus. During the winter her ovaries become inactive, with no follicles. This is called anestrus.
Foal Heat Particulars
If you breed your mare while she's in foal heat, she may become pregnant a few days faster than if you wait and breed her during her next month's estrus. Also, some mares will go into foal heat, followed by, for whatever reason, a period of antestrus, and then resume estrus. In that case it may be a few months before you can breed her. Some mares have delayed estrus cycles because they are nursing their babies, in which case you'll have to wait until the current foal is weaned. Many babies get diarrhea during their mothers' foal heat period. This, however, has nothing to do with the estrus affecting milk quality, so don't wean the baby prematurely because of estrus.
Annual Breeding Considerations
Your mare's pregnancy, or gestation period, lasts about 11 months -- 333 to 345 days -- so if you don't breed your mare again in foal heat, you will miss out on annual foal production. For recreational or amateur horse owners, this is typically not an issue, but it is for professional breeders who need to maximize breeding cycles. If you breed through artificial insemination, remember that stallion owners typically collect semen only during certain periods. In these cases you have to schedule the semen shipment for the times when your mare is receptive to breeding. The typical predictability of the foal heat is another reason it is so desirable. In some cases horse owners need to consider a projected foal's birth date when making breeding decisions (for example, if the birth date will occur during winter or if there is a buyer waiting for the foal within a stated time period).
Mare Breeding Health
If you breed your mare outside of foal heat, your vet may perform a uterine culture to ensure she doesn't have a uterine infection that can reduce her chances of getting pregnant. During foal heat, her odds of having bacteria in her uterus are actually higher but, unlike other estrus periods, will typically not affect her fertility. If your mare had any complications while or after giving birth, talk to your vet about whether you should breed her during foal heat. Depending on the severity of the complications, it might be difficult for her to become pregnant or it could be too hard on her body so soon after foaling.
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