Advantages and Disadvantages of Breeding on Foal Heat

Mares have a gestation period of about 340 days. Because of this relatively long pregnancy, they must become pregnant again shortly after giving birth in order to produce a foal at about the same time each year. If an owner waits a month or two to rebreed, the mare’s foaling date will slip later in the spring each year until she will eventually need to take a year off to avoid producing a summer foal.

Most mares come into heat about a week after foaling, but this can happen as early as 5 or as long as 14 days following parturition. This first cycle is known as foal heat, and many breeders take this opportunity to breed the mare in order to keep her on schedule for the following year. There are some advantages and some disadvantages to this practice.

The advantage is that a mare that conceives after being bred on her foal heat will produce a foal about the same time the next year. Mares that settle in foal can be turned out with their newborn offspring and can be taken off the broodmare manager’s list for breeding later in the spring. For many mares, this pattern is followed year after year with no problems.

Potential disadvantages include questions about the mare’s readiness to breed, the availability of the stallion, and poor conception rates. Especially in cases where the mare has had a difficult birth or a retained placenta, she may have inflammation, infection, or a buildup of fluid in the uterus. Any tissue damage occurring during the birth will also need time to heal before the mare is rebred.

Conception rates are usually better in mares that have somewhat later rather than earlier foal heats. Mares that begin to show signs of heat as early as five days after foaling are less likely to become pregnant than those that wait until ten or more days because the passage of time allows the uterus to shrink, clear fluid, and become less inflamed. Turning the mare out to pasture leads to better uterine recovery than keeping the mare stalled after foaling. In general, younger mares are more likely to conceive and less likely to lose the pregnancy than mares that are older.

Mares that foal very early in the spring may not have a foal heat. These mares will generally have their first post-foaling heat after about 30 days and then will resume a 21-day cycle. Keeping mares under lights for several months before they foal will encourage mares to have a normal foal heat. Owners with early-foaling mares should consult a veterinarian about the possibility of short-cycling (inducing ovulation by administering hormones) to bring the mare into breeding condition more quickly.

For Thoroughbreds and other horses produced by live cover, scheduling a stallion should be addressed when the mare shows signs of foal heat. A veterinarian can examine the mare by ultrasound to determine when ovulation will take place. The best rates of pregnancy occur when the mare is bred within a few hours of ovulation. For mares that are to be bred by artificial insemination, choosing a stallion with excellent fertility will increase the chance of conception at foal heat.