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:

  • Recording the breeding history and identification of the stallion
  • General and reproductive physical examination
  • Cultures of the reproductive tract
  • Observation of libido and mating ability
  • Semen collection and evaluation

    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:

  • Pre-purchase
  • History of subfertility or infertility in previous breeding seasons
  • Advanced age
  • Recent disease or injury that may have compromised sperm production in an otherwise normal stallion

    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.

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