When a veterinarian is considering a stallion's suitability as a breeding animal, the horse's history, current physical condition, and libido are just as important as his semen quality. "It's not just the semen we're looking at, it's the whole package," said Steven Brinsko, DVM, MS, PhD, Dipl. ACT, associate professor at Texas A&M University. Brinsko presented a session on reproductive evaluation of the stallion at the Hagyard Bluegrass Equine Reproduction Symposium 2006, which was held Oct. 18-21 in Lexington, Ky.

"The approach of the breeding soundness examination of a stallion is similar to any other system you're examining," Brinsko said, and such an exam is often completed prior to the sale of the horse or prior to the breeding season.

"The history can be more important than anything else," said Brinsko. "We're evaluating the whole package from the age/breed, present usage, prior breeding soundness exams, illnesses or injuries, medications, or vaccinations."

He said that a variety of medications could affect semen quality--cimetidine, for example, has a dramatic effect on spermatogenesis.

When looking at the horse's breeding history, the veterinarian should consider the horse's previous breeding records. Was the horse breeding via natural cover or artificial insemination? What were his resulting pregnancy rates? Were the mares considered fertile? ("It's not always the stallion's fault," said Brinsko.) When was the horse's last breeding or collection? These all could help cast light on the stallion's current reproductive capabilities.

Physical Examination
The veterinarian visually examining the stallion should look for conditions such as back pain or any visible, heritable traits that are considered unsatisfactory. "We want to try to prevent the passage of any heritable traits to the breed," Brinsko said. It's also important to carefully watch the horse in a breeding situation and examine his external genitalia. "Tease the stallion and evaluate his libido, notice how long it takes for him to get an erection...and (at that point) look for any lesions on the shaft of the penis or in the urethra that could be pre-cancerous."

Brinsko said his decision on whether to culture a stallion for infection is based on the horse's history and intended use or if the horse has inflammatory cells in his semen.In these cases, prior to washing the stallion's penis, Brinsko cultures the fossa glands, shaft, and prepuce. Post-wash, he cultures the urethra after collection and the semen. It is important to remember that some bacteria live on the surface of the penis without  causing problems, so as long as the cultures aren't heavy with bacteria, then the horse likely isn't infected. The only exception is Taylorella equigenitalis, the causative organism of contagious equine metritis (CEM), which is considered a foreign animal disease and should always be addressed if it is found.

Measuring a stallion's testicular size can help veterinarians predict the horse's semen-producing capability. "Testicular size and volume gives us an idea of the size of that factory," Brinsko said. "These are tied together with potential fertility." He not only palpates the stallion's testicles, but he uses ultrasonography to examine the scrotum, spermatic cords, epididymides, and testicles for evidence of abnormalities such as excess fluid, dilated ducts, tumors, etc.

Evaluating Semen
Brinsko collects semen on one of two different schedules to evaluate and compare ejaculates. On the first schedule, he gives the stallion a week of sexual rest before obtaining two ejaculate samples, one hour apart. On the other schedule, he collects semen once a day for 10 days. He says the average sperm number from Days 8-10 is generally representative of the horse's average daily ejaculate in a breeding season.

It is important to find a situation that is encouraging for the stallion to breed. Brinsko described a stallion that wouldn't breed an outdoor phantom mare that had a wooden neck and head and leather ears because the ears had fallen back after a rainstorm and left the phantom appearing as if it were threatening. "The stallion wouldn't have anything to do with it," he said.

Here were some of the points he made about semen collection, handling, and evaluation:

Temperature and handing All equipment used in collecting and handling the semen must be warm to protect the living spermatozoa in the sample.

Volume Measure semen volume using a graduated cylinder. "Volume is an important part, but not the most important part (about a semen sample)," said Brinsko. "Volume alone and concentration alone are not important."

Contaminants There are a number of things that can contaminate the ejaculate, including immature sperm cells, urine, blood cells, cellular debris, and semen extender. Contaminants can "artifactually inflate your sperm count," he explained, since photometric counters will count anything that prevents light passing through the sample, even if it isn't sperm.

Morphology Count the number of defects on each sperm using a dry or wet mount. "Count all defects," he said. "Combinations of defects are probably more important than any one defect alone." Be cautious about abnormalities that occur in high frequencies.

Motility "I tend to put more emphasis on progressive motility than total motility," he said. "Is a straighter, faster sperm a better sperm? Probably. A lot of things can affect motility and velocity of sperm, especially with frozen-thawed semen because of the viscosity of extender."

Longevity Texas A&M veterinarians assess sperm longevity by protecting the semen sample from light, handling, and extreme temperatures for six hours. They also look at post-cooling longevity. "Not every stallion's semen cools or freezes well," he said. "But there are a number of things we can look at (if a stallion's sperm seem unsatisfactory), and a lot of times it's the handling of the semen (that damages it)."

DNA quality Using a sperm chromatin structure assay (SCSA) or scattergram, you can assess the sperm DNA quality and flag damaged sperm. "Despite the fact his sperm motility might look good, there might be something wrong with his DNA," said Brinsko. Researchers have found that sperm DNA damage can be increased with cooling.

Brinsko said a satisfactory breeding prospect has no significant abnormalities and no physical, heritable, or behavioral faults. The stallion should be able to produce a minimum of one billion progressively motile, morphologically normal sperm in the second of two ejaculates that are collected one hour apart. Sixty percent of his sperm are progressively motile, 60% have normal morphology, and he has a minimum of 8 cm of scrotal width. If the stallion has the requisite semen quality and a good libido, he should be able to attain a 75% seasonal pregnancy rate for a full book of mares (generally natural cover of 40 mares and artificial insemination of 120 mares).

Stallions can also be considered questionable, unsatisfactory, or specifically qualified (he has good qualities, but is not capable of handling a full book of mares), depending on their breeding soundness exam components.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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