Not all stallions are created equal and neither is the amount of sperm they produce. The most sought-after stud in the area, for instance, could produce minimal amounts of semen, while a less desirable horse might produce more than his breeders ever need. So what are the deciding factors when it comes to sperm production?

Ed Squires, MS, PhD, Dipl. ACT (hon.), reviewed some of those factors at the 2015 World Equine Veterinary Association Congress, held Oct. 8-10 in Guadalajara, Mexico. Squires is a professor at the University of Kentucky’s Gluck Equine Research Center, in Lexington.

“It is important for both veterinarians and stallion managers to know the factors that affect sperm output in stallions,” Squires said.

Those key factors include stallion age, season, testis size, ejaculation frequency, and sex drive.

Veterinarians, he said, can evaluate these factors during breeding soundness exams and recommend adjustments to help improve a stallion’s sperm output. Managers, on the other hand, can use these factors to determine how many mares the stallion can realistically breed in a season.

Squires reviewed each factor in detail, giving attendees considerations to make for each one:

Age—Most stallions enter stud when they’re 3 years or older, Squires said, and reach sexual maturity by the time they’re 5 or 6. At some point, the stallion’s testis begin to degenerate. But, like aging in general for horses, the point at which this occurs depends on the individual horse.

“The timing for testicular degeneration varies in stallions and may occur as ear