Breeding Soundness Examination of the Stallion

The breeding soundness examination (BSE) of the stallion is an attempt to provide an estimate of a stallion's future fertility and is composed of the following:

Other factors, such as management of the stallion and his mares, may have a large impact on the overall fertility of the stallion. Therefore, the BSE is an attempt to provide an estimate of a stallion's potential fertility and should not be interpreted as an absolute measure of the stallion's fertility. Annual assessments for actively breeding stallions prior to the breeding season can help with management decisions and provide a baseline for comparison if a problem should arise.

At the time of semen collection, mating behavior and libido are evaluated, as is inspection of the penis. Bacterial cultures of the external genitalia are taken at this time. Aerobic bacterial cultures are obtained from the penis, fossa glandis, and urethra prior to ejaculation and the urethra after ejaculation. Potential pathogens are Klebsiella pneumoniae and Pseudomonas aeruginosa, but isolation does not always indicate a venereal problem.

Taylorella equigenitalis, causing Contagious Equine Metritis (CEM), is the only known bacterium that constantly produces venereal disease in the horse.

Two semen samples are collected from the stallion one hour apart. The two ejaculates are considered representative if volume of the ejaculates are similar and the second contains approximately one-half the number of spermatozoa as the first with comparable or increased sperm motility. A semen sample is generally obtained by use of a jump mare or phantom and an artificial vagina. If it is impossible to obtain a sample with an artificial vagina, a condom can be used, but it provides an inferior sample.

Immediately after collection, the sample is evaluated for color, clarity, and volume. The gel portion of the ejaculate should be removed by filtration. Next, the motility of the sperm should be assessed and sperm concentration determined using a densometer, hemocytometer, or spectrophotometer. Sperm concentration multiplied by gel-free semen volume will give the total number of sperm. Sperm morphology is also addressed as well as longevity of spermatozoa.

The internal genitalia (ampullae, seminal vesicles, prostate, bulbourethral glands) are not routinely evaluated unless there is a suspicion of problems. These are generally evaluated per-rectum utilizing ultrasound.

The testes should be evaluated and measured as well as total scrotal width. Sperm production is directly related to testicular size. The volume of each testes is calculated so that theoretical sperm production can be compared to actual production. Stallions with a total scrotal width of less than 8 centimeters should be suspected of testicular hypoplasia or degeneration.

The Society for Theriogenology has developed Guidelines for Classification of Breeding Stallions based on breeding 40 mares by natural service or 120 mares by artificial insemination. Stallions are classified as satisfactory, questionable, or unsatisfactory breeding prospects. To be classified as satisfactory, the following criteria must be met:

If a stallion fails these criteria, he is classified as a questionable or unsatisfactory. Many stallions are not used for 40/120 mares and may be able to settle a smaller book with adequate reproductive management.

Reassessment after 60 days may be in order. Many stallions breed larger books of mares (100+) and some breed in both the northern and southern hemisphere seasons. Ancillary testing to assess DNA integrity of spermatozoa, fertilizing capacity of spermatozoa, etc. are available for selected problem cases.


Dr. John Steiner, (859) 255-8741
Hagyard-Davidson-McGee Associates, Lexington, Kentucky.