Figure 1. The seasonal reproductive cycle of a typical mare.
The natural peak in breeding activity occurs in May, June and July.

It is often not practical to wait until May to breed mares because many breeds recognize January 1 as the official birthday for all foals born in a calendar year. This means, for example, that a Thoroughbred foal born in May after a June breeding the previous year is the same age for racing purposes as one born in January. In reality, the foal born in January is almost one-half year older and will have a decided advantage when racing as a two- or three-year-old.

If your foal is to be used for a purpose such as racing, where its real age relative to its official age is important, you should have your mare bred early in the season. Since mares generally are not ovulating (that is, they are anovulatory) in February, March and April, special measures must be taken if you want to breed your mare at this time.

The most common approach to early breeding is to place the mare under supplemental light starting in late November or early December (Figure 2). Essentially, you must convince your mare that spring has arrived by exposing her to artificially increased day-length. Light can be supplemented in the afternoon and evening to give a total of 16 hours light each day. (Consult your veterinarian for alternative lighting regimens.) A dark period should not interrupt the transition from daylight to supplemental light and the lighting program should be consistently followed each day. The light should be bright enough for you to read the small print of a newspaper throughout the mare's housing area. Most mares will begin ovulatory cycles after about 60 days of light supplementation and will be ready to breed in February and March. The lighting should be continued until your mare is confirmed pregnant. Your veterinarian may recommend that a progesterone product be used in conjunction with lights to avoid the pattern of long or irregular estrus typical of the spring transition from the anovulatory to ovulatory season.

Figure 2. Starting in late November, providing 16 hours of light each day will advance the onset of the ovulatory season so that breeding can begin in February.

Scientists are investigating alternative methods for inducing mares to ovulate outside the normal ovulatory season. A device that can be implanted under the skin and programmed to slowly release a hormone which stimulates the ovary is being studied. It is likely that in the not too distant future it will be possible for a single treatment to predictably induce ovulation in seasonally anovulatory mares.

Breeding Your Mare

Poor management of breeding is the single most important cause of reproductive inefficiency in the horse. Most of the problems result from a failure to appreciate either the seasonal nature of reproduction in the horse, or the fact that mares have a variable estrous cycle pattern even during the ovulatory season. A thorough understanding of basic breeding management and careful attention to the details of a breeding program should maximize your chances for the timely birth of a healthy foal.

The mare's estrous cycle, the period between one ovulation and the next, is about 21 days long. Estrus, the time of sexual receptivity, occupies about seven days of the cycle (Figure 3). Unfortunately, the duration of estrus is extremely variable and can last anywhere from two or three days to two or three weeks. Diestrus, the period between two successive estrus periods, is more consistent in length and lasts about 14 to 16 days.

Figure 3. A typical estrous cycle lasts about 21 days. However, the length of estrus itself (the period of sexual receptivity) is highly variable. Ovulation is more closely linked to the end than to the beginning of estrus.

The systematic teasing of mares to monitor their estrous cycles is the most important part of a sound breeding program that does not rely on drugs to synchronize estrus. Teasing is the process of determining a mare's reproductive status by exposing her to a stallion and observing her behavioral reaction. Mares that do not readily display estrus may need to be teased by more than one stallion. Teasing should be done every day during the breeding season. Mares with a foal require special precautions to insure the safety of the foal and the people handling the animals. Teasing mares over a teasing rail is the safest arrangement for most mares. It may be more convenient to tease large numbers of mares individually in their stalls or as a group over a fence, but more vigorous teasing of an individual mare is difficult under these circumstances. Teasing while leading or riding the stallion through a group of mares is not recommended.

Mares that are in estrus usually stand calmly, squat and evert the labia of the vulva (wink) in the presence of a stallion. They often raise their tail and urinate. Mares that are not in estrus tend to be more agitated in the presence of a stallion. They are restless and may lay their ears back, squeal and kick. There is great variability in these signs from mare to mare and the results of teasing should be carefully recorded for each mare so that changes in her behavior throughout the estrous cycle can be appreciated. A sample form that you and your veterinarian can use to record teasing and other breeding management information is found at the end of this publication. Teasing should continue until the end of the breeding season, even for those mares that have been confirmed pregnant.

In the absence of a good teasing program (for example, in a situation where artificial insemination with transported semen is to be used and there is no stallion on the farm with the mare), the estrous cycle of the mare may be monitored by other means. This is usually accomplished by rectal palpation or ultrasonography of the cervix, ovaries and uterus. These techniques are also useful for mares with foals or mares that fail to show behavioral estrus for whatever reason. They are generally more successfully used as a complement to a teasing program than as a replacement for it. Consult your veterinarian for programs to synchronize estrus.

Ovulation occurs about one to two days before the end of estrus. Ideally, breeding should occur close to the time of ovulation and no more than a day or two before. Breeding more than a few hours after ovulation is usually not very successful. A useful rule of thumb is to breed (cover) mares on the third day of estrus and every other day until they go out of estrus. This will require three breedings of a mare that stays in estrus for seven days.

Because estrus is so variable in length, and because ovulation is more closely associated with the end of estrus than with the beginning, deciding when and how often to breed your mare can be very difficult. A mare that remains in estrus for 13 days would require six breedings by our thumb rule. This will be very expensive if transported semen is used for each breeding. Under these circumstances, hormonal induction of ovulation with human chorionic gonadotropin (hCG) under the supervision of your veterinarian can be very helpful. Properly selected mares will ovulate within 24 to 48 hours after hCG treatment. Prostaglandin and progesterone are two additional hormones that can be used by your veterinarian to regulate the onset of estrus in mares during the ovulatory season. However, neither prostaglandin nor progesterone influence the length of estrus nor the timing of ovulation during estrus.

There are a number of specific causes of subfertility in the horse that are capable of foiling even the best breeding management. Infection of the reproductive tract is a commonly identified cause of clinical subfertility in the mare. Treatment with antibiotics may correct the problem, but oftentimes, especially in older mares, poor conformation of the reproductive tract is a contributing factor. Your veterinarian can examine your mare and decide if problems with conformation are allowing contaminants to continually enter and infect her uterus. Successful breeding of these mares will generally require that an episioplasty (Caslick's surgery) or a breeding stitch be incorporated into the breeding management program.

Breeding Methods

Natural breeding and artificial insemination are the two methods used to breed mares. Natural breeding (or live cover) reduces the opportunity for human error to influence the outcome of the breeding. Artificial insemination (AI) requires collection, evaluation and dilution of semen in addition to insemination of the mares. Using frozen semen further complicates the AI process. Each step is an opportunity for human error to negatively affect the breeding outcome. Obviously, careful planning is required to implement an AI program for your mare.

There are, however, advantages of AI that offset the potential problems. For one, smaller semen doses are used. This results in more efficient use of stallions because more than one mare may be bred with each ejaculate. More importantly, the amount of contamination introduced into the uterus of the mare is reduced and an antibiotic can be added to the semen to counteract any infection in the stallion or mare that may cause subfertility.

With careful monitoring of the estrous cycle and the use of hCG to induce ovulation, a single properly timed insemination during estrus can result in a high pregnancy rate. This is the essence of the "minimal contamination breeding technique" that is often recommended for older mares or those with a history of subfertility. Another significant advantage of AI is that with proper facilities there is a reduced risk of injury to the horses and their handlers when compared with live cover.

Embryo transfer is a relatively new development and provides an option for obtaining a foal from an old or subfertile mare, or from the young athlete or show mare. In embryo transfer, the donor mare is bred and the embryo is flushed from her uterus about a week after ovulation. The embryo is then placed in the uterus of a fertile but less valuable recipient mare that is at the same stage of the estrous cycle. The recipient serves as the surrogate mother for the foal. The major drawback of embryo transfer is the increased cost of obtaining a foal.

Pregnancy Diagnosis

Pregnancy diagnosis is an essential part of your breeding management program. The goal is to identify mares that are not pregnant so that they can be bred again as soon as possible. Ultrasonography can be used to detect pregnancy as early as 10 to 11 days after ovulation, but most veterinarians prefer to wait at least 13 to 15 days. Diagnosis of pregnancy at this early stage will help with management of twin pregnancies and is especially important in breeds like the Thoroughbred where twinning is common. Twin pregnancies are a serious problem in the horse. They rarely result in live foals and cause a substantial loss of time and money.

The persistent absence of estrus in response to teasing is a fairly reliable indicator of pregnancy, especially two to three weeks after breeding when you would expect your mare to return to estrus if she were not pregnant. Mares that lose their foal more than 35 days after breeding will generally not return to estrus that season because of the formation of endometrial cups in the uterus. For this reason, pregnancy should be reconfirmed by palpation of the uterus at 30 to 45 days and again at 60 to 90 days after breeding. Ideally, a final confirmation of pregnancy should be made at the end of the breeding season.

Summary

Planning and executing a program for breeding your mare is the best way to insure success. The keys to successfully breeding your mare are:

1. Evaluating your mare and planning a health maintenance program for her.
   
   
2. Selecting a stallion.
   
   
3. Deciding when and how to breed your mare.
   
   
4. Monitoring your mare's reproductive cycles and breeding at the appropriate time.
   
   
5. Diagnosing pregnancy.

Your veterinarian can assist you with all aspects of your breeding program and should also be called whenever problems arise. Some problems that require veterinary attention include mares that show stallion-like behavior or remain in estrus for more than 15 days. Also, mares should be examined if they fail to show estrus within the first three weeks of the breeding season or if they keep returning to estrus after being adequately covered by a fertile stallion. Mares with abnormal discharges or genital lesions require veterinary attention as well. Once your mare is pregnant, careful maintenance and monitoring, especially around the time of foaling, will help you insure the safe delivery of a live foal.