When a racing or performance stallion is retired to stud, many owners choose to purchase first-season subfertility insurance, also called congenital infertility insurance, against the possibility that he might be infertile. Veterinary examination of the prospective breeding stallion is required and, generally based solely on the size of his testes, the veterinarian is asked to judge whether the stallion might have acceptable fertility and how many covers he should handle per day.

At the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., a discussion of the calculations a veterinarian can use to estimate a stallion's fertility and capabilities was presented by Terry Blanchard, DVM, MS, Dipl. ACT, professor of veterinary medicine at Texas A&M University. All of the calculations are based on testis size as measured by calipers or ultrasound (Blanchard's preferred method).

A practitioner might collect the following measurements from a stallion:

  •  Total scrotal width (across both testes);
  •  Left and right testicle widths, heights, and lengths;

These measurements can be compared with published reference values.

From these measurements, the practitioner can calculate or estimate:

  •  Testicular volume = (0.05233 x length x width x height);
  •  Daily sperm output, billions per day = (0.024 x total testicular volume in mL) - (0.76 to 1.26);
  •  Number of covers per day (from calculated daily sperm output divided by 1.5 billion sperm per cover);
  •  Maximum book size (using a table based on covers per day required for different book sizes, or how many breedings per season).


"Typically, semen collection and evaluation are not permitted to determine if libido, erection, mating ability, ejaculatory function, sperm output, or semen quality are normal," commented Blanchard. Thus, the veterinarian and insurance company must be aware that estimates of fertility and book size are only valid if significant fertility factors, such as libido, semen quality, etc., fall within normal parameters.

Another caveat is that testis size can be temporarily reduced by hard training, anabolic steroid usage, stress, past illness, and other testicular insults. If there is reason to suspect one of these events has reduced testis size, re-examination at a later date is recommended.

"It is possible that semen collection and evaluation may one day become an integral part of the veterinary examination required to obtain first-season subfertility insurance," Blanchard commented. "Until that time, however, the veterinarian will have to make a recommendation on whether or not a stallion should qualify for first-year subfertility insurance and whether or not the stallion can be expected to handle a certain book size or mating frequency based strictly on physical examination and measurement of testicular size."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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