Breeding Soundness Examination of the Stallion

The breeding soundness examination (BSE) of the stallion is a complete reproductive evaluation that should be performed in a systematic and thorough fashion. This examination typically consists of the following procedures:

A veterinarian that has experience in evaluating stallions should perform the BSE. Optimally, a board certified theriogenologist, a veterinarian skilled in the discipline of animal reproduction, should perform the evaluation, but non-certified private practitioners with extensive experience and training in working with stallions are also qualified to do the evaluation.

The BSE is NOT a direct evaluation of fertility and only serves as a guideline to ascertain whether a stallion displays any obvious problems that may compromise his fertility potential. The only true measure of fertility is to breed a mare or group of mares with appropriate breeding management, then check for pregnancy.

The BSE results are only valid for the day of the evaluation; an insult or damage to the reproductive tract after the BSE is performed may change completely the results previously obtained. Additionally, immature stallions with poor results in a given BSE may improve over time, but this can only be monitored by repeating the examination 6 to 12 months later.

Stallions with congenital defects of possible genetic origin such as cryptorchidism and wobblers should not be used for breeding and should not pass a breeding soundness evaluation. Most veterinarians will not and should not issue a BSE report from a stallion with such congenital defects.


Optimally, all maiden stallions entering a breeding program should receive a thorough breeding soundness examination (BSE) in order to detect any problems that may compromise their fertility potential or the size of the book of mares that they can sustain. Other reasons to perform a BSE include:

Subfertility basically means that pregnancy rates resulting from breeding fertile mares to a given stallion are suboptimal, at least in regards to expectations. It is difficult to define a percent pregnancy rate that should be considered normal or optimal since this depends greatly on the particular population of mares (fertile vs. subfertile, aged vs. young) being bred to the stallion. One should remember also, that although the cause for this subfertility may be the stallion himself, farm, mare and breeding management are important contributors to the pregnancy rates achieved by a particular stallion in a given season.

Under good breeding management conditions with a random group mares one should expect a normal stallion to achieve seasonal pregnancy rates of at least 60 percent (that is, at least 60 percent of mares bred to the stallion should be pregnant by the end of the season if mares were bred at the appropriate time during their cycle). This is only a minimum expectation assuming that a stallion is going to service a full book of mares. Hence, a stallion has a full book when breeding 45 mares by artificial insemination or 120 mares by natural cover during a 5-month breeding season.

Infertility means lack of fertility. True stallion infertility may be untreatable. In general terms, until proven otherwise, the only true infertile stallion is not a stallion, but a gelding.

History and Identification

A stallion should always be identified accurately prior to a BSE. A veterinarian may not perform a BSE in a stallion that cannot be identified through proper branding, tattoos and/or breed registration records.

The breeding history is very important. If a stallion has already sired foals, this is the best proof of his potential fertility.

Conversely, if a stallion has been previously used for breeding and has not been able to sire foals, the BSE may become more involved and more specific tests may be needed in an attempt to ascertain the possible cause for infertility. As previously stated, younger stallions may need further evaluations until their reproductive system has completely matured. However, older stallions may have irreversible infertility due to aging changes in the testicular tissue (testicular degeneration). We should always keep in mind that inappropriate breeding management may result in suboptimal pregnancy rates from an otherwise perfectly normal stallion.

Physical Examination of the Reproductive Tract

A complete breeding soundness examination (BSE) should include a thorough general physical examination and a thorough examination of all the parts of the reproductive tract (penis, scrotum and accessory sex glands). A stallion with severe musculoskeletal or neurologic problems may have excellent semen quality and yet be incapable of mounting a mare and breeding, rendering him technically infertile.

The testicles are palpated for size and consistency. This serves to detect any signs of atrophy (decreased size due to disease or advanced age), hypoplasia (decreased size of congenital origin) or degeneration, in addition to the possible presence of masses or fluid within the scrotum that could compromise sperm production within the testicle.

Measurements of each individual testicle (height, length and width) and total scrotal width should also be recorded. Presently these measures are often taken by scanning the scrotum and each individual testicle with an ultrasound machine. Estimated sperm production efficiency can be calculated using a specific formula through the individual measurements obtained from each testicle. In general, a stallion should have a minimal total scrotal width of 8 cm to pass a BSE.

Ultrasonography can serve as an additional tool to evaluate the appearance of the testes or presence of fluid or masses (tumors or cancer) within the scrotum. This is also a very important tool for transrectal evaluation of the accessory sex glands (ampullae, seminal vesicles, prostate and bulbourethral glands). Optimally, ultrasound examination of the accessory sex glands should be performed in all stallions, but especially in those showing evidence of infection or blockage once semen collection has been attempted.

The penis can be evaluated during washing prior to semen collection. Inspection of the penis may reveal the presence of infection, tumors or scar tissue resulting from the placement of stallion rings to prevent masturbation. Masturbation is a normal behavior in all stallions that does not reduce semen production or performance in the breeding shed, and thus the use of devices to prevent such behavior is strongly discouraged and can be harmful to the stallion.

A BSE includes a minimum of three cultures taken from the dorsal urethral fossa (small orifice right above the urethral opening) and the urethra, before and after ejaculation. Additional cultures of the penis shaft, prepuce or semen can be taken if any problems are suspected, and depending on the preferences and experience of the evaluator.

Not all bacteria cultured during a BSE result in disease for the stallion or are transmissible to mares, and therefore the results of cultures must be interpreted with caution. It is common to obtain a positive culture of bacteria that are normal inhabitants of the penis or genital area and are not causing any problems. Furthermore, trying to eliminate these bacteria from the genitals of stallions with the use of topical disinfectants or antimicrobial drugs may actually cause irritation and enhance the growth of infective microorganisms.

We typically worry about potential infection when we obtain positive cultures of Pseudomonas spp. or Klebsiella spp. These types of bacteria have been associated with infections of the internal reproductive organs (accessory sex glands), penis and prepuce, yet their transmission and ability to cause endometritis (uterine infections) in bred mares has not been proven.

Observation of Libido and Mating Ability

Libido (sexual drive) and mating ability are important in order for a stallion to perform adequately during a long fully booked breeding season. The age and experience of the stallion should be taken into account when evaluating these parameters.

The breeding or semen collection process (time from entering to leaving the breeding shed) with an experienced stallion is usually accomplished within 3 to 5 minutes, without rushing. Inexperienced and/or immature stallions may take longer time to achieve an erection, mount, intromit and/or ejaculate.
Semen Collection and Evaluation

Semen from stallions is typically collected by the use of an artificial vagina. The most commonly used types of artificial vaginas used in the United States are the Missouri, Colorado State University, Polish and the Nishikawa or Japanese model. Each of them offers different advantages and disadvantages, and the selection of one over the other depends on individual as well as stallion preferences and experience.

In general, an artificial vagina consists of a tube (plastic, rubber or metal) with a water jacket. This jacket must be filled with hot water for a final temperature of 107.6 degrees to 118.4 degrees Fahrenheit (42 to 48 degrees Celsius), for adequate stimulation of the penis during collection. One should avoid contact of the artificial vagina with spermicidal substances like detergents or disinfectants, and the artificial vagina should always be clean and dry prior to use. Additionally, the inside of the artificial vagina should be lubricated with a non-spermicidal water-soluble sterile jelly prior to semen collection. To avoid damaging the sperm during processing procedures, precautions should also be taken for all devices (collection bottles, liners, microscope slides, insemination pipettes, syringes, pipettes) that will come into contact with the semen, and all should be kept in an incubator at body temperature (98.6 F or 37 C).

A BSE consists of the collection of a minimum of two ejaculates one hour apart. More ejaculates may be collected if deemed necessary during the evaluation procedure. When only two ejaculates are collected, the second ejaculate should provide approximately half the number of sperm from the first one. The number of sperm in this second ejaculate is considered a rough estimate of the potential daily sperm output of the stallion. The daily sperm output is the number of sperm that a stallion is capable of producing in a daily basis and will allow us to determine whether a stallion can sustain a full book of mares during the breeding season.

After collection, semen is taken to the laboratory and evaluated for gross appearance. The volume of semen, concentration of sperm in the sample, percentage of total and progressive sperm motility and the total number of sperm are recorded. Additionally, a drop of semen placed in formalin or stained is evaluated for sperm morphology under the microscope at high magnification. Morphological evaluation of sperm allows us to determine whether the sperm produced by the stallion have abnormal shapes such as large or small heads, bent or coiled tails or missing parts.

Based on all the results obtained during the breeding soundness examination (BSE) a stallion may be classified as a Satisfactory, Questionable or Unsatisfactory Prospective Breeder.

To be considered a Satisfactory Prospective Breeder, a stallion must pass all the tests performed during the BSE, and must be producing a minimum of 1 billion progressively motile, morphological normal sperm in the last ejaculate collected during the BSE.

A Questionable Prospective Breeder is a stallion that presently displays some problem that precludes his classification as Satisfactory. However, this stallion may pass a BSE performed at a later date. An example would be a young stallion that is expected to improve with age or a stallion that was recently sick and his semen quality has been affected, but is expected to improve over time.

An Unsatisfactory Prospective Breeder has a problem that is not expected to improve. Examples of this would be stallions with genetic abnormalities or very old stallions with advanced testicular degeneration.

If you are planning to purchase a breeding stallion, you should always request that a BSE be performed, and then only buy the stallion if classified as Satisfactory. You may consider purchasing a stallion that is classified as Questionable only if you fully understand why the stallion did not pass the evaluation at this time, and optimally with a purchase agreement contingent to the stallion passing the exam at a future evaluation.

Common Problems