Reproductive Physiology and Breeding Management of the Mare

Breeding a mare at the right time in her cycle is imperative to ensure adequate pregnancy rates. Most pregnancy failures are due to inappropriate breeding management resulting in not only disappointment but also increased breeding costs, such as longer periods of boarding time at a breeding facility, veterinary services or repeated shipments of semen for artificial insemination. In order to breed successfully, it is important to understand the mare's reproductive system.

The period of time between two successful heats is termed the estrus cycle, which lasts for about 21 days: approximately one week in heat or estrus, when the mare is receptive to a stallion, and two weeks not in heat, termed diestrus. This cyclicity between estrus and diestrus is the result of a complex interaction between hormones secreted by the brain, ovaries and endometrium (lining of the uterus). Understanding this is fundamental, not only to breed mares at the right time of their cycle and hence optimize pregnancy rates, but also to allow manipulation of the cycle with administration of hormones in order to synchronize breeding timing.

Additionally, mares are considered a seasonal species because estrus cycles occur only at a particular time of the year, which we call the breeding season. Increased day length is the main cue for estrus cyclicity in mares, and therefore most mares cycle during the late spring and summer. This seasonal limitation puts more pressure into appropriate breeding management for optimal pregnancy rates.

Veterinarians and breeding managers must work as a team to manage broodmares and stallions, and to detect possible problems that may jeopardize pregnancy rates in a horse breeding program.

Basic Anatomy of The Reproductive System

The reproductive tract of the mare is located within the pelvic area and therefore is easily evaluated through the rectal wall by palpation or ultrasound, a tool commonly used in breeding management of mares.

The reproductive tract is composed of the following parts: vulva, vestibule, vestibulo-vaginal sphincter (tissue separating the vestibule and vagina), vagina, cervix, body of the uterus, uterine horns, oviducts (or Fallopian tubes) and ovaries.

The vulva, vestibulo-vaginal sphincter and cervix are important barriers for preventing contamination or infection of the uterus. The vagina and the uterus are sterile in the normal, reproductively sound mare. This is an important consideration, and whenever bridging the reproductive tract of the mare for an examination or for insemination, the vulva should be thoroughly scrubbed to avoid introducing an infection into the vagina or uterus. Furthermore, vaginal exams should always be avoided at any time during pregnancy.

The uterus of the mare is Y-shaped with a body and two uterine horns. During pregnancy, the embryo develops within one of the uterine horns. The entire uterus enlarges to fit the foal as pregnancy advances.

Mare ovaries have a typical kidney bean shape. Eggs develop within blister-like structures called follicles. These follicles can become very large before ovulation and bulge over the surface thus increasing the overall size of the ovary, making them easily felt (palpated) or seen on ultrasound exam through a rectal exam performed by a trained veterinarian.

During the breeding season, a mare ovulates one follicle (occasionally two) at each heat period. If the mare is bred during that heat, fertilization may occur within the oviduct and the mare may become pregnant. The oviducts or Fallopian tubes (one for each ovary) are responsible for transporting the fertilized egg into the uterine horn for further development into a fetus during pregnancy.

Mares are seasonally polyestrus. This means that successive estrus cycles are displayed only during a particular time of the year. For the mare, regular estrus cycles begin during the late winter or early spring, after a period of increasing daylight, and continue during the summer and sometimes through part of the fall. Some mares (up to about 10 percent) may cycle year round.

Mares may show signs of heat in the so-called transition period between the winter and breeding season, but this does NOT mean that they have started cycling regularly or that they are ready to be bred. It is not uncommon to confuse these early signs of heat with regular cycles and mares are often wrongly bred at a time when they have not yet started ovulating. They will NOT become pregnant if bred at this time.

During the breeding season, mares begin an estrus period and ovulate approximately every 21 days. This period of 21 days is termed the estrus cycle. The average duration of heat or estrus is five to seven days, although estrus length is extremely variable among mares, and can last anywhere from three to 10 days. Furthermore, in the same mare, estrus length tends to be longer at the beginning of the season and becomes shorter as the peak of the season approaches in June and July. Estrus is the period of the cycle during which the mare will accept a stallion if teased or bred by natural cover. Ovulation occurs near the end of the estrus period.

The period of the estrus cycle between heats is termed diestrus. The length of diestrus is usually about two weeks.


The 21-day pattern of cyclicity during the breeding season results from a complex interaction of hormones secreted by the brain, uterus and ovaries. The hypothalamus is a structure in the brain that secretes into the bloodstream gonadotropin-releasing hormone or Gn-RH. Gn-RH acts on another structure near the brain, the pituitary gland, inducing secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In general terms, FSH and LH stimulate the ovaries and are responsible for follicle growth and ovulation, respectively.

In turn, growing follicles in the ovaries are responsible for estrogen secretion, the dominating hormone during estrus. Under estrogen influence, and in the absence of the hormone progesterone, mares display signs of heat.

Towards the end of estrus, usually one follicle will ovulate in response to LH secreted by the pituitary. Then, the fluid filled follicle will be replaced by a solid structure termed corpus luteum. The corpus luteum secretes progesterone, the dominating hormone during diestrus and pregnancy, that keeps the mare out of heat.

Follicles may grow in the ovaries and even ovulate when the mare is in diestrus. It is a common mistake to breed mares during diestrus just because they have a large follicle on one of the ovaries that is detected during a palpation or ultrasound exam. If teased, these mares will not show signs of heat while in diestrus since progesterone always dominates over estrogen. If bred by artificial insemination at this time, there is a small possibility that the mare may become pregnant, but there is also an increased chance of developing a uterine infection. Therefore, breeding during diestrus when a large follicle is present in the ovaries should be strongly discouraged.

After breeding and ovulation progesterone levels will remain high if the mare is pregnant, so the mare will not come back into heat. If the mare does not become pregnant in that cycle, toward the end of diestrus, or about 13 days after an ovulation, the hormone prostaglandin F2-alpha will be released by the lining of the uterus. Prostaglandin is responsible for eliminating the corpus luteum, which results in cessation of progesterone secretion and return to heat.

Breeding management can be done with or without the help of a veterinarian. Degrees of intervention vary from turning horses out in pasture for natural cover to breeding a mare by artificial insemination with frozen-thawed semen. In most breeding farms there is veterinary intervention at some level. The veterinarian can perform one or several of the following tasks depending on the particular breeding situation:

Heat Detection

Mares can only be bred at the appropriate time of their cycle through good heat detection. The most common way to detect heat in mares is to tease them consistently, that is three or four times per week, by presenting a stallion in-hand. When a stallion is not available, teasing may be attempted with a gelding, but it is often inaccurate and should not be trusted as the sole means of heat detection. Alternatively, in small farms where the individual mare is being bred by artificial insemination with cooled or frozen semen and a stallion is not available, the veterinarian must rely on accurate palpation and ultrasonography of the reproductive tract to ascertain when the mare is ready to be bred. This requires expertise and accurate knowledge of the anatomy and physiology of the mare's reproductive tract.

Nevertheless, teasing with a stallion in-hand remains as the most accurate way to detect heat in mares. There are several teasing methods to perform this procedure safely and accurately:

Mares in heat will show one or several of the following signs:

Mares in diestrus (not in heat) will not tolerate the stallion's advances and may:

When teasing mares for estrus detection it is important to realize that each mare is an individual and the array of signs of heat in each mare should be recognized through routine teasing and accurate record keeping. Furthermore, maiden mares never exposed to teasing may feel threatened by the advances of a stallion at first and may need some acclimation period before they feel comfortable displaying signs of heat. Most mares readily tolerate teasing and display clear signs of estrus or diestrus once acclimated to the routine.

Breeding The Mare by Natural Cover

Systems where the intervention of the veterinarian is minimal are usually those where the stallion and mares are located in the same facility or stud farm, and bred by natural cover when in heat. At present, this only happens consistently in registered thoroughbred and mini-horse farms.

Breeding by natural cover can be performed by turning out the horses in a pasture or enclosure, resembling a feral situation, or by bringing mare and stallion in-hand into a breeding shed. Pasture breeding is still performed in some breeding farms and in some research situations, and should not be underestimated as a valuable form of breeding management. Pasture breeding is less labor intensive than other breeding management techniques, since mares and stallions do not have to be handled individually for heat detection or cover. Stallions are usually much more efficient breeders when freely allowed to breed a group of mares in a pasture situation. However, this breeding system is not applicable to all situations and some mare or stallion owners do not feel comfortable turning them out to pasture for breeding.

For in-hand natural cover programs, mares must be teased consistently to ascertain when they come into heat. Once in heat, mares can be covered every other day until they come out of heat. If a stallion has a large book of mares this breeding frequency may be impractical, and a veterinarian can check mares in heat by palpation or ultrasound to ascertain the optimal time for breeding, as close to ovulation as possible. The use of hormones to induce ovulation can also narrow the timing for optimal breeding.

The main advantage of natural cover breeding programs is that pregnancy rates are usually the greatest, considering that both stallions and mares in the program are fertile and that breeding management is appropriate.

Disadvantages of natural cover versus artificial insemination programs include the following:

Artificial Insemination

Artificial insemination programs require more intensive management of the mare, especially when transported or frozen semen is used. Similar to natural cover programs, mares can also be teased for heat detection if a stallion is available. The optimal time for breeding is usually ascertained by your veterinarian through palpation or ultrasound exam of the reproductive tract. Hormones are often administered to narrow the window for insemination in relation to ovulation.

Pregnancy results for artificial insemination programs with fresh or transported (cooled) semen can equal those obtained in natural cover programs, as long as breeding management is optimal. However, pregnancy results with frozen-thawed semen are inconsistent and there is a high variability amongst stallions.

Advantages of artificial insemination programs include the following:

The main disadvantages of artificial insemination programs are that they need specialized equipment and training and are also more labor intensive.

Administration of Hormones

There are basically three groups of hormones used to control the mare's estrus cycle:

The main principle to consider when giving these hormones to mares is that they will only work when given at the correct time during the estrus cycle.

To Synchronize Breeding

A mare or group of mares may be synchronized to come into heat at a time when the stallion is available or for embryo transfer purposes. For estrus synchronization the most commonly used hormones are a combination of progesterone, given alone or with estrogen, and prostaglandin F2a. These regimes require daily injections of progesterone/estrogen for about nine days and are reserved for special situations where tight synchronization between mares is of utmost importance, such as with embryo transfer. A prostaglandin injection is usually given in the last day of the progesterone/estrogen series. These regimes have also been used with variable success to advance the start of the breeding season. Mares usually come into heat three to five days after the regime ends and ovulate at variable times after that.

Progesterone-like hormones can also be given by mouth, avoiding the impracticality of daily injections. However, these preparations are less effective for estrus synchronization, although encouraging results have also been attained in advancing the start of the breeding season.

Another synchronization regime consists of two injections of prostaglandin given to a group of mares approximately 10 days apart; this will synchronize estrus in approximately 70 percent of the mares in the group. Mares in this regime will also come into heat around three to five days after the second injection of prostaglandin.

To Short-Cycle The Mare

Short-cycling the mare consists of bringing the mare back into heat sooner than we would expect in her own cycle. For this purpose, natural prostaglandin F2a and similar synthetic hormones are given to cause demise of the corpus luteum in mares in diestrus. To be effective, prostaglandins must be given in a mare that is at least six days from her last heat (ovulation). In such case, we can expect a mare to come back into heat around three to five days after the treatment.

To Induce Ovulation

Two main hormones are given to mares to induce ovulation:

To be effective, both hormones must be given in a mare that is in heat and has a follicle at least 33 mm in diameter. Therefore, it is crucial to examine the mare by ultrasound and determine the size of the follicle. Smaller follicles will not respond to the hormones. When given at the right time, both hormones will induce ovulation between 24-48 hours later. This allows breeding the mare very close to ovulation time, without having to perform repetitive inseminations and rely on her own spontaneous ovulation.

In summary, good knowledge of the mare's reproductive system paired with application of old and modern breeding tools and the collaboration of your veterinarian can make your breeding season a total success.