Choosing to Breed During Foal Heat

If a barn full of people were asked their opinion about breeding a mare during foal heat, a barn full of opinions would result. This topic has been controversial for a long time. There is debate about whether it is harmful to a mare's health, whether it negatively impacts fertility, and whether it causes an increase in fetal loss during pregnancy. Unfortunately, there have only been a handful of good-sized scientific studies published that thoroughly evaluate the practice.

So, is there any reason to breed during foal heat?

What is Foal Heat?

"Foal heat" is the term used to describe the first estrus or heat that occurs after a mare delivers a foal. During this time the mare ovulates and can become pregnant again. Typically, estrus occurs every 21 to 22 days in the mare and lasts four to seven days. A large follicle develops on an ovary, growing to a size of 35-55 mm or more in diameter. Most mares will show behavioral signs of heat throughout estrus, including signs of interest in a stallion, squatting to urinate, and "winking" to show the clitoris. It is not until the last 24-48 hours of estrus, however, that the follicle ruptures and ovulation occurs. Only then is the mare able to conceive.

The process of foaling stimulates the mare to begin cycling within a matter of days. The typical interval from the time of foaling to the beginning of foal heat is eight to 12 days. Some mares enter foal heat sooner. Interestingly, season of the year plays a role here. Mares that foal early in the year, from January to March, tend to experience a longer interval; they reach foal heat about 15 days after foaling. In comparison, mares foaling later in the season, in April and May, tend to have the shortest interval, about six to seven days. Research has shown that optimal conception rates are achieved when mares are bred at least 10 days after foaling. Therefore, if a mare enters foal heat too early, ovulation might occur before this 10-day point and before breeding is recommended.

Why Breed On Foal Heat?

The reason to breed during foal heat is simple--economics. Each mare in a commercial breeding herd can produce one foal every year if successfully bred within 20 days of foaling. Delivering one foal per year maximizes the number of foals a mare produces over a reproductive lifetime. Breeding during foal heat minimizes the interval between foaling dates from year to year, which is particularly important for certain breeds such as the Thoroughbred. (The longer the foal has to grow before sale, the better. Late yearlings usually are smaller at sales.) Foal heat breeding is an important tool for successful money management of large commercial farms that are in the business of producing and selling yearlings. The use of this tool, however, requires intensive management of each individual mare if it is to be used successfully.

The goal of breeding at foal heat is not to get each mare to conceive at all costs. When working with a commercial herd, the individual mare must be considered as a part of a group. How quickly that one mare becomes pregnant will impact the economic success of the entire herd. In other words, the overall goals of the herd must be considered. These goals are to get as many mares pregnant with the fewest possible number of matings, and to get live foals from as many pregnant mares as possible.

Research has shown that conception rates achieved during foal heat are slightly lower than those at later heats. Therefore, if a mare is chosen for breeding during foal heat, it is critical that every effort be made to maximize her chances of conception. Not every mare will be chosen for foal heat breeding; only those mares physically ready for another pregnancy should be bred at that time.

Choosing a Candidate

Mares that experience a difficult foaling should not be considered for breeding on foal heat. They need time to recuperate, as do mares that have post-foaling complications, such as retained fetal membranes or endometritis (inflammation of the innermost lining of the uterus). Even if these particular mares were able to conceive during foal heat, the increased chances of embryo resorption or abortion later in the pregnancy make the risk of foal heat breeding too costly for the herd. A better choice is to allow these mares to cycle through foal heat and breed them at a later estrus.

Another consideration for choosing candidates is gestation length of previous pregnancies. Gestation length tends to be repeatable in broodmares, to within several days. In other words, if a group of mares has a history of short gestation length (less than 330 days), there might be no need to breed this group at foal heat. These mares might be well-suited to breeding at the second estrus after foaling and still produce one foal per year.

Another example of a group of mares that might not be candidates for foal heat breeding are older mares that have produced many foals year after year. These mares' reproductive tracts undergo changes as they age, including a natural increase in the time it takes to recover from foaling. The uterus and cervix take longer to involute (return to normal size after enlargement during parturition), making it more difficult for these mares to successfully begin a new pregnancy at foal heat.

An invaluable management practice at top-producing commercial breeding farms is that of evaluating all broodmares about seven days after foaling. A thorough physical examination ensures that each mare is recovering as expected after foaling, and it allows personnel to schedule a date for breeding if everything is normal.

The Postpartum Physical

In an intensively managed breeding operation, all foalings are attended. If a mare requires assistance, someone is there. Unfortunately, when humans intervene in the foaling process, even when it is needed, the chances of the mare becoming injured increase significantly. For this reason, the first part of the postpartum breeding-readiness physical examination involves a visual inspection of the perineum, vulva, and vestibule (the area just inside the vulva). The veterinarian looks for any lacerations, hematomas (blood clots), or abscesses that might have occurred as a result of foaling. Even a small laceration can cause big problems if left untreated.

The veterinarian passes a speculum into the vagina as the examination continues. Lacerations are noted, as are the color of the membranes and the shape and appearance of the cervix. Cloudy cervical mucus can indicate endometritis and signals the need for uterine culture and cytology prior to breeding.

Next, the veterinarian performs palpation of the reproductive tract per rectum so she can evaluate the size and shape of the uterus, ovaries, and supporting tissues. This is usually followed by an ultrasound exam, during which she looks for free fluid in the uterus, hematomas in the uterus, ovaries, and pelvis, and tumors in the ovaries. Granulosa cell tumors, for example, tend to grow more rapidly during pregnancy and, therefore, they often are found during postpartum examinations.

Finally, the ovaries are examined for growing follicles and the veterinarian measures the largest dominant follicle to help predict the onset of foal heat, if it hasn't arrived already. The overall findings of the examination help determine whether the mare is physically ready for breeding during foal heat.

Foal Heat or Later?

If everything looks normal during the postpartum physical examination and a mare is considered a good candidate for foal heat breeding, the next step is to monitor follicular development, tease the mare, and prepare her for breeding. Again, the mare is typically in estrus for four to seven days, and she ovulates sometime near the end of that period. Research studies have shown that the greatest success in achieving conception, whether during foal heat or later, comes when breeding is timed with ovulation. Therefore, breeding is begun when the mare shows signs of standing heat, and it is continued at intervals throughout the heat period.

The important difference when breeding during foal heat is the timing of ovulation. If the mare ovulates before the 10th day after foaling and the veterinarian confirms on ultrasound that the follicle has ruptured, the chances for conception are significantly decreased. Therefore, the recommendation is not to breed her if she ovulates before the 10th day. This way, the reproductive performance of the entire herd will be improved.

The main reason we wait until Day 10 to breed foal heat mares is to allow the uterus to return to normal. In a normal mare the uterus has involuted by Day 14 post-foaling. Mares bred on Day 10 will most likely ovulate on Day 11. The embryo does not enter the uterus until Days 5.5-6 post-ovulation. Therefore the embryo shouldn't enter the uterus until Days 16-17 post foaling. This allows time for the uterus to recover and provide a healthy environment for the embryo.

Waiting for the next estrus in a mare that ovulates early during foal heat might seem like a waste of time. Normally, the estrous cycle lasts about 21 days, so if she ovulates early, at Day 7 for example, another 14 days must pass until that cycle will end. Then, 10 days must pass until she ovulates "on time," totaling 24 days. Luckily, there are methods of manipulating the estrous cycle to shorten the interval from one ovulation to the next.

Manipulating the Cycle

When the decision is made not to breed during foal heat and to wait for the next heat, the veterinarian can shorten the mare's next estrous cycle with the use of prostaglandins. After ovulation, the remnant of the follicle becomes what is called the corpus luteum. This structure secretes the hormone progesterone. About six or seven days after ovulation, if a mare is injected with prostaglandin F2-alpha, the corpus luteum will break down, decreasing progesterone levels in the blood and shortening the cycle by seven days. Therefore, the 24-day waiting period is shortened to 17 days.

Some commercial farms prefer to intervene and delay foal heat, especially in mares with a history of early ovulations. There are two methods to achieve this. Both involve the use of progesterone analogues. These treatments must begin on the first day after foaling, or they will not be successful.

In the first method, a combination of progesterone and estradiol is administered in the muscle daily for as long as estrus is to be delayed. When the injections are stopped, the mare will come into heat. In the second method, which is less precise, altrenogest is given orally each day. Again, when the medication is stopped, the mare comes into heat.

After Breeding

Regardless of when a mare is bred, whether during foal heat or at a later estrus, personnel at intensively managed farms make it a practice to examine all mares the day after breeding. This is done to ensure that ovulation has occurred and that the uterus is clear of fluid that could cause endometritis. If there is any free fluid in the uterus, the mare is treated with oxytocin to stimulate the uterus to contract and expel the fluid. This is especially important in mares that are bred during foal heat, because the uterus is less forgiving of fluid and debris during this first heat after foaling. Every step must be taken to prepare an optimal environment for embryo implantation.

Take-Home Message

Foal heat breeding is a practice that is not intended for every horse breeder or every mare. There is simply no reason to breed back most mares during the first heat after foaling. There is data to support a decrease in conception rates during this first heat, and without a thorough postpartum examination, there is reason to be cautious about rushing forward into another pregnancy.

Foal heat breeding has been adopted by the commercial breeding farm establishment as a tool for economic gain. The industry uses this tool as a means of minimizing the interval between foaling dates in its broodmares from year to year. Successful commercial farms recognize that the use of foal heat requires intensive management of individual mares in order to maximize the reproductive health of the entire herd. h


  1. Loy, RG. Characteristics of postpartum reproduction in mares. Veterinary Clinics of North America: Large Animal Practice, 2(2), 345-359, 1980.
  2. Robinson, N. Edward, ed. Current Therapy in Equine Medicine 5. Philadelphia, PA: W.B. Saunders, 2003.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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