Breeding Soundness Exam of the Mare

Horse breeding is a time-consuming and expensive business. Therefore, horse breeders must be able to assess the likelihood of a mare being able to conceive, carry a fetus through a pregnancy, and deliver a live foal. While unforeseen problems can always occur, there are procedures that help you decide if a mare is suitable for breeding. The suitability for breeding does not refer to the quality of the mare or her temperament; rather it is an assessment of the genital health of that particular mare. Collectively these procedures are known as the breeding soundness examination.

For any mare, it is important that she is properly prepared for the breeding season. This article will focus on what owners should do before the breeding season to help get their mares ready for a successful breeding and describe the breeding soundness examination of the mare.

Understanding the Mare's Breeding Season

The mare is a seasonal polyestrous breeder (her reproductive tract is active for multiple cycles during a particular season of the year), and environmental and other factors can exert a profound effect on reproductive function, particularly during the transitional period between winter anestrus (period of sexual inactivity) and the onset of cyclical activity in the spring. During the breeding season, a mare which is not pregnant will have recurring estrous or heat cycles.

In nature, the mare does not usually experience an estrous cycle during the winter and early spring months (although some mares cycle all year round). Throughout that time of year, her reproductive system goes into a rest period and she is acyclical. This is known as winter anestrus. On rectal palpation or transrectal ultrasound imaging, both ovaries will be small (less than 3x2x2 cm) (see Figure 1 on page 58), and in some mares there will be a number of small follicles. Plasma progesterone (a hormone, more on this later) concentrations are less than 1 ng/ml.

The mare doesn't suddenly begin to cycle regularly in spring, but has what is known as a transitional phase. This is the period between winter anestrus and normal cycling. Once the mare is truly in the breeding season, her estrous cycles last 22 days on average. Cycle length is very variable and the cycle tends to be longer in the spring.

The regulation of cyclical activity is under the control of the hypothalamus and pituitary gland (located in the brain) and ovaries. The hypothalamus produces gonadotropin-releasing hormone (GnRH) that stimulates the release of the gonadotropins from the pituitary gland. The two gonadotropins from the pituitary gland are follicle stimulating hormone (FSH) and luteinizing hormone (LH); these hormones are responsible for follicle growth and ovulation, respectively.

When nature has its way, the production of the hormones that "switch on" the mare cycling is controlled by the amount of daylight. During the short days of winter, there's limited production of hormones; in the longer spring and summer days, there is greater hormone production. The horse is a long-day breeder and daylight has an important influence on mare reproductive performance (see "How the Estrous Cycle Works" on page 56 for a description of hormonal control of the estrous cycle).

This effect of season on reproduction effectively results in foals being born in the spring and early summer. This is the time when environmental conditions should be optimal for foal survival in the wild. Therefore, the natural breeding season of the horse (in the Northern Hemisphere) is from May until August. This is the time when highest pregnancy rates are likely to occur.

Unfortunately, man has superimposed his ideas of when mares should foal on this natural pattern. Since the early 19th Century, when Jan. 1 was declared the official birth date for Thoroughbred foals (irrespective of their actual birth dates within that year), horse breeders have had problems attempting to breed mares in the winter and early spring so they would foal as early in the year as possible. For some types of horse, notably the Thoroughbred, an operational breeding season exists from Feb. 15 until the first week of July. This "man-made" breeding season means that many breeders try to get mares in foal from the middle of February onward.

It might not be clear to some readers why so much trouble is taken in trying to breed mares before they are in their most fertile period. The reason is that the two major racing breeds (Thoroughbreds and Standardbreds) and the majority of the breed societies use Jan. 1 as the official birth date. Some breed societies use the actual birth date, but Jan. 1 is used for show purposes. This means that a foal born on Feb. 1 and a foal born on Aug. 1 would both be considered one year old on the following Jan. 1. This is despite the fact that there is a half-year difference in actual age.

Artificial Lighting

Fortunately, something as simple as light can cause the mare to cycle earlier. Putting a mare under lights is very common in horse breeding, and it involves providing extra light at the end of the day.

Experience has shown that 16 hours of light stimulus (artificial plus natural) is adequate. This means providing light from 7 a.m. until 11 p.m. The extra artificial light is best added at the end of the natural daylight period, and works by suppressing the release of a hormone called melatonin. An area of the brain known as the pineal gland produces this hormone, which affects release of the gonadotropin hormones involved in follicle production and ovulation.

Alternatively, a one-hour "flash" of light from nine to 10 hours after nightfall can be used to achieve the same effect. This is more difficult for most breeders to achieve than simply providing light at the end of the day.

The light in the stall must be bright enough. This means a 150-watt clear bulb in the middle of a 13 foot by 13 foot (4 meters by 4 meters) stall. If a strip light is used, it should be 4.3 feet (1.3 meters) long (40 watts). Care should be taken to eliminate shadows, as they can prevent a good response. A practical method of checking that there is sufficient light is to see if it is easy to read a newspaper wherever you stand in the stall.

There are two important points to note with putting a mare under lights:

  • Extra lighting must be begun early. Generally a minimum of eight to 10 weeks is needed, so mares should be exposed to the lighting system by Dec. 1. This should mean that they will begin to cycle normally by Feb. 15. It is not good enough to decide to put a mare under lights and one week later expect the mare to begin cycling, since this does not happen immediately. Recently researchers in France found that much lower light levels might be effective. They reported 20 watts was sufficient to photostimulate mares and inhibit melatonin production. The exposure could be during late evening or early morning, and might only be necessary for five weeks. When the 20 watts was given as a "flash" in the middle of the night, it was only effective in 50% of mares.
  • Mares should be in good body condition and well-fed when they begin to have extra lighting. It is preferable to increase the mare's body condition, and this might mean extra feed. It is also important that mares are healthy, on an appropriate anthelmintic (deworming) program, and have had regular dental checks.

In aged mares, delayed initiation of normal cyclical ovarian activity might reduce the number of estrous cycles during the breeding season. Therefore, it is particularly important to prevent poor body condition from occurring in such animals. Prolonged anestrus can be prevented by good management.

Hormones for Manipulating Estrus

Several hormones can be given to the mare to induce ovulatory heats early in the breeding season in barren and maiden mares. This helps ensure that foals are born early and reduces pressure on the stallion by getting these mares serviced before foaling mares come on line to be bred. (See "Hormonal Control of Estrus" above for a description of the various hormones' function in controlling estrus.)

Progesterone/progestogen-withdrawal therapy has been used successfully in mares near the end of a two-month light program. Progesterone (a hormone that keeps the mare from coming into heat; it also maintains the uterine environment for pregnancy) can be administered for 1-15 days as an oil-based intramuscular injection, or orally as the synthetic progestogen altrenogest (Regumate). However, such therapy is effective only in anestrous mares which are already well into the transitional phase to the resumption of cyclical ovarian activity. Results are poor when used in mares with ovaries containing follicles less than 0.8 inches (20 mm) in diameter.

Synthetic GnRH analog Deslorelin (Ovuplant) implants have been used in anestrous mares, but only around 50% of mares in anestrus will respond and several implants might be needed. The mare might return to anestrus if she has an induced ovulation, but fails to become pregnant.

Twice daily injections of synthetic GnRH analog buserelin (Receptal) might induce ovulation in anestrus mares, but results are much better with transitional mares.

Transitional "Spring" Estrus

After a variable transitional period of up to two months, larger follicles (greater than 1.4 inches, or 35 mm) will develop and ovulate, usually heralding the onset of normal cyclical ovarian activity. During the transitional period, behavior is variable, ranging from total rejection of the stallion to interest but resistance to him mounting, to normal acceptance. These behavioral signs can be consistent or inconsistent.

During this stage, ultrasonic examination and rectal palpation reveal transitional follicles reaching a size of greater than 1.2 inches (30 mm) (see Figure 2 on page 58). Visual identification of a corpus luteum (CL) or progesterone levels above 4 ng/ml confirm that the first ovulation has occurred and hence the onset of normal ovarian cyclical activity.

Hormonal treatment of mares in the transitional stage is based on progesterone or progestogens, with or without the addition of estradiol esters, by one of several routes of administration. Progesterone exerts a negative feedback on gonadotropin secretion, which is followed by increased release of FSH and LH. When the source of progesterone is withdrawn or its effect wanes, there is follicular growth, maturation, and ovulation.

Progesterone treatment is more effective in mares in the late transitional stage, and is ineffective in mares with minimal follicular activity, particularly during deep anestrus. Currently, the most common treatment is oral medication with the potent progestogen altrenogest to mimic the luteal phase. This liquid should be added to the food once per day for 10 to 15 consecutive days; estrus should occur within six days and ovulation seven to 13 days after the last treatment. Because of the possibility of ovulation occurring during treatment, an injection of the luteolytic agent PGF2-alpha (a prostaglandin) on the last day of in-feed medication might be necessary to cause luteolysis (regression) of any CL that is present. Oral progestogen therapy works when the mare has estrogen competent follicles and an endometrial edema pattern (see Figure 3 on page 58).

The use of intramuscular injections of progesterone and estradiol-17 beta in oil for 10-15 days produces a similar response to altrenogest, but the interval to estrus is longer due to the suppression of follicular development by the estradiol.

There has been much interest recently in using GnRH or its analogs, administered by injection, infusion, or subcutaneous implant, to hasten ovulation in the transitional phase mare. I have successfully used buserelin given twice daily by intramuscular injection. It is expensive, as treatment is necessary for at least one to two weeks. The use of the short-term implant of the GnRH analog deslorelin has been reported. Its value in accelerating the first ovulation of the breeding season following seasonal anestrus would appear to be a real benefit.

Regardless of the hormones used, mares undergoing treatment early in the season need 16 hours of adequate light and good nutrition to ensure success.

During the transitional period before the year's first ovulation, mares demonstrate erratic estrous behavior of varying intensity. The presence of multiple large follicles, possibly as large as 1.2 inches (30 mm), makes detection of ovulation difficult by palpation alone. Even outside this transitional period, misinterpretation of ovulation--even by experienced palpators--has been as high as 50% in some studies. It is much easier to visualize the corpus hemorrhagicum/early CL with ultrasound (see Figure 4 on page 58) when the anechoic (fluid-filled) follicle is replaced by an intensely echoic (solid) area--the early CL (see "Diagnostic Imaging in the Broodmare Examination" on page 58 for more information on ultrasound use for reproduction examination).

It is recommended that the interval between matings be less than three days, although there have been no critical studies on the survival time of sperm in the mares' genital tract. It is important not to begin breeding too early, or this will result in the mare being mated many times (which is less efficient and increases the chances of normal post-mating endometritis lowering fertility). The appearance of uterine edema (fluid swelling) is an indication that the follicle should ovulate within a few days.

A key factor in the emergence out of vernal (spring) transition is the development of steroidogenic competence (the ability of the follicle to produce hormones) leading to an increase in circulating estrogen concentrations, which cause the release of LH from the pituitary due to a positive feedback mechanism. Estrogen is responsible for the appearance of uterine edema (in the absence of progesterone), which might be why the detection of uterine edema is important in signalling the emergence of the mare from the transitional period.

Normal Expectations of Fertility

Unlike many other domestic animals, horses have never been selected for fertility. Popular horses to breed from are those which can run fast, jump well, or look pretty. Unfortunately, these are not necessarily the most fertile horses. Additionally, how mares are managed has an enormous influence on pregnancy rates, so therefore only approximate guides are possible.

Of course, some mares lose their pregnancies through abortion and the live foal rate (the number of live foals born the following year) is lower than the pregnancy rate. It is important to make sure that an owner knows these figures so he/she is not under the impression of being guaranteed a live foal if a stallion covers the mare.

Possibly the two most important (or at least the two most-quoted) parameters in connection with reproductive efficiency are end-of-season pregnancy rate and live foal rate. From the veterinarian's point of view, pregnancy rate per estrous cycle is a more up-to-date indicator of efficiency of the breeding operation. A well-managed stud farm should typically achieve pregnancy rates (diagnosed at 15 days of gestation) per estrous cycle of 65%, an end-of-season pregnancy rate of 85%, and a live foal rate in excess of 75%. The figures need to be reviewed in light of the history and past breeding performance of each mare.

Is Your Mare A Healthy Breeder?

As well as using light to get your mare to cycle early, it is important to make sure that her reproductive tract is healthy. This is particularly important if she failed to become pregnant last year or lost the foal at some stage of pregnancy. Ideally such mares should be investigated in the autumn rather than the spring to allow time for any abnormalities to be treated and the reproductive tract time to "settle down" before the breeding season. If you did not have your mare checked out in the autumn, then at least have a thorough examination performed in the spring. Even if your mare is a first-time breeder, it is a good idea to have your veterinarian perform a thorough gynecological examination known as a breeding soundness examination. The aims are to detect any cyclical abnormalities, physical defects, or signs of inflammation and to assess the likelihood of a mare being able to conceive, carry a fetus through a pregnancy, and deliver a healthy foal.

The breeding soundness exam involves an external and internal examination of the mare. Using a systematic approach, the ovaries, uterus, and cervix are checked. Usually the veterinarian will use an ultrasound machine as well as palpation. The vagina and cervix will also be checked by looking at them through a speculum inserted in the vagina. A speculum is a narrow tube through which you can shine a light and see the inside of the vagina and the cervix.

Vulva--In the normal mare, the vulva provides the first effective barrier to protect the uterus from ascending infection. The vulval lips should be full, firm, and meet evenly in the midline with 80% or more of the vulval opening below the brim of the pelvis (see photo A on page 58). If the vulval seal is high (more than 1.6 inches, or 4 cm, of length dorsal to or above the pelvic floor) in relation to the pelvic brim, the vestibular seal is incompetent and pneumovagina (aspiration of air, bacteria, and contaminated material) can occur. The initial vaginitis (vaginal inflammation) can lead to cervicitis (cervical inflammation) and acute endometritis (inflammation of the endometrium, or uterine lining), resulting in subfertility.

Contamination of the caudal reproductive tract with bacteria during pregnancy can result in embryonic death, and in
late pregnancy contamination can cause placentitis and lead to abortion. The more severe conformational abnormalities are more likely to result in failure of the vulval seal and to increased fecal contamination since the vulva forms a shelf on which feces can collect (see photo B on page 58). The vulval lips might be angled at 25 or even 50 degrees to the vertical in these cases.

Dr. E.A. Caslick first pointed out the importance of this condition in relation to genital infection in Thoroughbred mares in the 1930s. Interestingly, it is most commonly found in Thoroughbreds, and, in the author's experience, is almost unknown in Shires and native ponies.

Defective vulval conformation can be congenital--which is very rare--or acquired. The latter can be due to vulval stretching following repeated foalings, injury to perineal tissue, or poor body condition (old, thin mares).

Older mares are more commonly affected with pneumovagina. However, young mares which are in work and have little body fat and/or poor vulval conformation can also develop pneumovagina. In some mares, pneumovagina only occurs during estrus when the perineal tissues are more relaxed. Some mares make an obvious noise while walking, but in other mares the diagnosis is more difficult.

Hyperemia (excessive blood) and a frothy exudate in the anterior vagina on speculum examination are pathognomic (characteristic of this problem). Rectal palpation of a ballooned vagina or uterus from which air can be expelled confirms the diagnosis. Real-time ultrasound examination of the uterus might reveal the presence of air as hyperechoic (white) foci (see Figure 5 on page 58). Cytological and histological examination of the endometrium might show significant numbers of neutrophils (pus cells) indicative of endometritis.

Treatment should be directed at correcting the cause of pneumovagina and treating the resulting endometritis. The former can be done surgically by Caslick's operation, although in some cases increasing the physical condition and fat status of the mare might be sufficient.

The aim of the operation is to reduce the vulval aperture and thus prevent pneumovagina and fecal contamination of the vagina. The time of suture removal is not crucial; this is normally done approximately two weeks after surgery. However, the vulva must be re-opened by performing an episiotomy (a surgical incision into the perineum and vagina to prevent traumatic tearing during delivery) before the next foaling, otherwise major damage can result.

Mares which require natural mating subsequently might need to have an episiotomy performed if "tightly stitched." If not, the vulva can rip and/or the sutures can injure the stallion's penis. The episiotomy wound should be repaired soon after foaling or mating to prevent pneumovagina.

If repeated episiotomy followed by closure is not done very carefully, there can be considerable loss of vulval tissue, poor healing, and major problems for the mare.

Vagina--Urine pooling is the retention of incompletely voided urine in the vagina due to an exaggerated downward cranial slope of the vagina. Pneumovagina from defective vulval conformation also predisposes mares to the condition. Transient urine pooling, which is sometimes found in postpartum mares, usually resolves after uterine involution (return to normal sizes) has occurred.

Vaginal bleeding from varicose veins in the remnants of the hymen at the dorsal vestibulovaginal junction is occasionally seen in older mares, particularly during estrus. Treatment is not usually necessary as the varicose veins normally shrink spontaneously, although diathermy (surgical destruction of these veins) can be used.

Cervix--The cervix forms the important third (and last) protective physical barrier to protect the uterus from the external environment. The cervix must relax during estrus to allow intrauterine ejaculation or insemination of semen and drainage of uterine fluid. An inflammation of the cervix is usually associated with endometritis and/or vaginitis.

Anatomically, the cervix is a thick-walled sphincter. Expansion and contraction are possible due to the action of the longitudinal and circular smooth muscles, which are rich in elastic fibres. Two distinctive features of the equine cervix are its ability to dilate and the lack of rigid, annular constricting rings seen in farm animals.

The cervix is a dynamic organ that changes its size, consistency, and shape according to hormonal changes during the estrous cycle. During anestrus, the cervix is flaccid, dry, and might be partially open. During diestrus and pregnancy, elevated concentrations of plasma progesterone cause the cervical opening to close, although its lumen (cavity) can still be entered. The cervix in this stage is pale, tonic, and dry, and projects into the vagina. The cervix during estrus is usually moist and sometimes hyperaemic (bloody). The cervical opening is usually open, edematous, and rests upon the floor of the vagina.

Often an older maiden mare has an abnormally tight cervix due to fibrosis. In these mares, the cervix fails to relax properly during estrus, thus fluid is unable to drain and it accumulates in the uterine lumen. In many cases, this fluid does not have bacterial growth or neutrophils. Once the mare is bred, the fluid accumulation will be aggravated due to poor lymphatic drainage and impaired myometrial contraction compounded by the tight cervix. In many cases, these mares are susceptible to post-breeding endometritis even though they have never been bred before. To maximize the fertility of these mares, it is vital that the veterinarian is aware of the possibility of this type of cervical pathology.

Failure of the cervix to open during estrus can lead to unwillingness of the stallion to complete mating or intra-vaginal ejaculation. Artificial insemination has been used successfully in mares with an abnormally narrow cervix. Impaired cervical drainage of uterine fluid can predispose a mare to persistent endometritis. The good news is that a mare with a fibrosed cervix which becomes pregnant does not normally have any difficulties at foaling.

Failure of the cervix to close during diestrus can lead to persistent endometritis, failure to conceive, or early embryonic death. Failure to maintain closure during pregnancy can lead to gestational failure. Both surgery and exogenous progesterone have been tried in an attempt to encourage closure of the cervix. These treatment options can be successful when the damage to the cervix is not severe.

Assessment of the cervix must be a part of the routine pre-breeding assessment of a mare. The cervix can be examined by either direct visualization using a speculum per vagina (see photos on page 50)and/or by digital exploration.

Damage resulting in cervical incompetence or fibrosis is usually traumatic in origin and most often occurs during parturition. This is especially the case if a fetotomy (removal of a dead foal in pieces) is performed by an inexperienced clinician and without adequate instrumentation. Damage to the cervix can also occur during vigorous mating by an over-sized stallion, especially if the mare was not in full physiological and behavioral estrus. Usually the damage is not too severe in these cases. The cervix can also be damaged by irritant chemicals such as povidone-iodine. Adhesions of the cervix can be broken down manually, but this must be done daily for as long as two weeks to prevent recurrence. If severe, adhesions can contribute to the development of pyometra (pus accumulation in the uterus).

Endometrial Swabs

Infection is a risk in breeding horses, and swabs (cultures) taken as health precautions are an important part of ensuring your mare is ready for the breeding season.

The terms "swabbing a mare" or "has she been swabbed" are very commonly heard in the horse breeding industry. There are two parts of the reproductive tract that are commonly swabbed--the clitoris and the uterus. A swab is a small piece of cotton wool twisted onto the end of a thin plastic rod, like a long Q-tip that is rubbed on something to get cells for analysis. A swab of the uterine lining will be taken to see if there are any bacteria present or signs of an inflammation of the uterus known as endometritis. Mares should be free from endometritis before they are covered by a stallion or artificially inseminated.

A diagnosis of endometritis can be made by collection of concurrent endometrial swab and smear samples during early estrus for bacteriological culture and cytological examination, respectively. This allows time for resolution prior to mating and maximizes the chances of pregnancy. To reduce the risk of contamination, the use of guarded swabs is advised.

Cultures should be examined at 24 and 48 hours. An air-dried smear is differentially stained and then examined for the presence of inflammatory and endometrial cells; the latter confirms contact of the swab with the endometrium (see Figure 6).

A positive culture result, with no evidence of inflammatory cells in the smear (usually neutrophils), is likely to be due to contamination during collection. Diagnosis of acute endometritis is based on the presence or absence of significant numbers of neutrophils in the smear. Very rarely, neutrophils can be detected, usually at the "foal heat" or the first estrus of the breeding season in maiden mares, although there is no endometritis.

Uterine Biopsy

In some cases, a small piece of the uterine lining might be taken for examination under a microscope. This procedure is termed a biopsy. The technique involves passing a biopsy instrument through the cervix and into the uterus for sampling.

If the uterus appears normal on palpation, the sample should be taken from one of the areas of embryo fixation, i.e. the uterine horn-body junction on either side. Single samples are usually representative of the entire endometrium. If the uterus is abnormal on palpation per rectum, biopsy samples should be taken from both the affected area and a normal area.

When a biopsy of the uterus is looked at under a microscope, certain features allow the biopsy to be categorized according to the likelihood of the mare becoming pregnant and having a foal. This information is useful to decide whether to try and get the mare pregnant. Mares with a lot of scar tissue (fibrosis) in their uteri have a poor chance of getting in foal.

Detecting Intraluminal Uterine Fluid with Ultrasound Imaging

Transrectal ultrasonography provides a non-invasive method of assessing the uterus for the presence of fluid (see Figure 7 on page 58). Significant accumulations of fluid might indicate susceptibility to endometritis.

Careful examination of the mare well in advance of scheduled breeding will help ensure that any problems are detected and corrected in time for breeding. Manage your mare accordingly, and you will have maximized your chances of having a healthy, pregnant mare on time. 


Asbury. A.C. Evaluating Reproductive Health in Mares. The Horse, April 2002, 71-76. Article Quick Find #3423 at

5 TIPS: Broodmare Exam

  1. Getting a breeding soundness exam performed on your mare early enough to treat any problems is key.
  2. Manipulating hormone levels and the number of hours a mare is exposed to sufficient light can induce estrus during winter months.
  3. "Transitional" spring and fall estrous cycles are erratic and fertility is variable.
  4. Pregnancy rates at any one heat vary from 60-80% on a well-managed farm.
  5. The breeding soundness exam involves an external and internal examination of the mare using ultrasound, palpation, speculum examination, and uterine culture/biopsy.

About the Author

Jonathan F. Pycock, B. Vet. Med, PhD, DESM, MRCVS

Jonathan F. Pycock, BVetMed, PhD, Dipl. ESM, MRCVS, operates Equine Reproductive Services, a first opinion and referral private equine practice based in Yorkshire, England. He has published many papers and book chapters on a variety of equine reproductive topics, and edited the book Equine Reproduction and Stud Medicine. His main interests include ultrasonography, breeding the problem mare, and artificial insemination. Currently, he is evaluating the use of oxytocin and depot oxytocin as a post-breeding treatment for mares.

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