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Equine reproduction: Breeding soundness exams are critical for evaluating fertility in stallions

by Wyoming Livestock Roundup

“Given the high economic value of some stallions and the willingness to breed horses based more on their competitive potential than on their breeding soundness, many resources and expensive tests are sometimes used to evaluate breeding soundness,” states Dr. Sylvia Bedford-Guaus of the Peeksill-Cortlandt Veterinary Hospital in the Merck Veterinary Manual

            According to Sylvia, a traditional breeding soundness exam (BSE) of a stallion is made up of a history, a physical exam, an exam of the external and internal genitalia, a culture of urethral and penile swabs and a collection and evaluation of at least two ejaculates collected one hour apart for total sperm number, sperm motility and sperm morphology. 


            Sylvia says the history in a BSE should include the stallion’s prior fertility and type of management used for breeding. 

            “If less than 10 mares were bred, individual mare fertility should be considered if pregnancy rates were low,” she says. “If the stallion has been racing or training recently, he may have been receiving anabolic steroids or other drugs.” 

            She also points out stallions recently retired from a performance career may be evaluated again three to six months later if they fail the initial BSE. 

Physical examination

            “During the physical examination, the stallion should be evaluated for general body condition and the presence of any conditions that might interfere with breeding,” Sylvia explains. “Genetically inherited defects, including parrot mouth and cataracts, render the stallion unfit for breeding.” 

            She also mentions a few other defects which may render a stallion as an unsatisfactory breeding prospect, including blindness, lameness or ataxia and penile paralysis. 

Scrotal palpation and ultrasonography

             Next, Sylvia says both the external and internal genitalia need to be examined. 

            For the external exam, she says scrotal palpation should be performed when the stallion is relaxed.

            “Total scrotal width should measure greater than eight centimeters, as assessed with a blunt caliper across both testes together,” she states. “The testes should be firm, resilient and homogenous on palpation.”

            As for the internal exam, Sylvia notes ultrasonography is commonly used to evaluate scrotal contents and take individual measurements for each testis. 

            “Measurements of the width, height and length of each testis can be used to calculate testis volume and to estimate expected daily sperm output,” she explains. 

            She provides the formulas for these predictions. 

“Testis volume can be measured by multiplying the product of width times height times length by 0.5233 for each testis, while daily sperm output can be measured by multiplying testis volume by 0.024 then subtracting 0.76,” she explains. 

Penile evaluation

            Sylvia says a penile evaluation should then occur.

            “The penis is usually examined while it is washed before the first semen collection,” she notes. “It can vary in size with no effect on fertility and it should be freely distensible from the sheath without lesions.” 

            “Of the internal genitalia of the stallion, the ampullae, vesicular glands and lobes of the prostate gland are palpable per rectum,” she adds. “While performing palpation per rectum on a stallion, the internal inguinal rings should also be palpated to determine their size and presence of any abnormalities.”

Collection and evaluation of semen

            “Semen is collected from a stallion using an artificial vagina (AV) filled with water at 122 degrees Fahrenheit, which usually cools to around 107.6 to 113 degrees Fahrenheit by the time semen is collected,” explains Sylvia. 

            She further explains the stallion is teased with an estrous or ovariectomized mare, and once an erection is achieved, a culture swab of the sheath and fossa glands is obtained. 

            “The stallion is then allowed to mount the mare or the breeding dummy and the semen is collected,” says Sylvia. “A second swab sample is taken from the distal urethra immediately after ejaculation. The stallion should be given one hour of rest before the second ejaculate is collected.” 

            Sylvia notes the ejaculate should be evaluated for gross appearance, sperm concentration, sperm motility, percentage of morphologically normal sperm and percentages of specific spermatozoa morphologic abnormalities.

            “The ejaculate should be free of pus, urine and/or blood,” she explains. “Normal ejaculate may contain gel, a viscous cloudy material that originates from the seminal vesicles and forms the third and last fraction of the ejaculate.” 

            She further explains semen analysis only occurs on the gel-free fraction of the ejaculate, and concentration may be determined using a hemocytometer or properly calibrated photometric instrument.

            She points out sperm motility and morphology evaluations are performed similarly to that of a bull. However, the sperm concentration is much lower in stallions, so only individual sperm motility is assessed.

            “Assessment of motility should be performed with both raw semen and semen diluted with a good quality extender,” she says. “Semen should also be warmed to 95 to 98.6 degrees Fahrenheit before assessing motility.” 

            In addition to sperm motility, Sylvia notes sperm quality is assessed and total number of spermatozoa is calculated.

            “The total number of sperm in the second ejaculate is considered a rough estimate of the daily sperm output for the particular stallion,” she says. “The second ejaculate should have about the same or a slightly higher percentage of morphologically normal sperm, as well as the same or better motility than the first.” 

            If sperm numbers or quality differ from this guideline, Bedford-Guaus says a third ejaculate should be obtained.

            “The third ejaculate should have about half as many sperm as the second, with the same or better morphology and motility,” she explains.

            In a satisfactory breeding stallion, she says sperm numbers, after more than five days of sexual rest, should be around eight to 10 billion in the first ejaculate and greater than or equal to four billion in the second. 

            Additionally, Sylvia says total spermatozoa motility should be greater than or equal to 65 percent and progressive motility around 50 percent.

            “At least 50 percent of the sperm should be morphologically normal,” she notes. “A stallion is considered satisfactory if he produces at least one billion progressively motile and morphologically normal sperm in the second or third ejaculate.” 


            After performing a BSE, Sylvia says stallions are then classified as satisfactory, questionable or unsatisfactory. 

            “Satisfactory potential breeders should achieve a seasonal pregnancy rate of 80 percent or higher when bred to 50 mares via natural breeding or 120 mares through artificial insemination,” she says. “Questionable potential breeders may experience some difficulty in achieving the above guidelines. Typically stallions are placed in this category if they have a problem that might resolve over time or without treatment.” 

            Because of this she points out stallions classified as questionable are reassessed six to 12 months later.

            She continues, “Unsatisfactory breeders have problems that may profoundly reduce their fertility or have undesirable heritable traits that may be transmissible to their offspring.” 

            Hannah Bugas is the assistant editor for the Wyoming Livestock Roundup. Send comments on this article to

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