Your Guide to Equine Health Care

What Makes Some Stallion Semen Subpar?

Learn about 14 factors that can affect your stallion’s fertility.

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What Makes Some Stallion Semen Subpar?
Many factors can affect semen quality. If a stallion has a fertility problem, work with your veterinarian to determine and address the root issue. | Anne M. Eberhardt/The Horse

14 Factors that can affect your stallion’s fertility

Breeding stallions are sensitive guys. Anything from ambient temperature to body weight can affect their semen quality. In this article, we’ll describe those factors and 12 more that can impact fertility. Three veterinarians well-versed in stud health will help: Edward Squires, PhD, Dipl. ACT (Hon), adjunct professor in the University of Kentucky’s Department of Veterinary Sciences; Dickson Varner, DVM, MS, Dipl. ACT, professor and Pin Oak Stud Chair of Stallion Reproductive Studies at Texas A&M University’s College of Veterinary Medicine and Biomedical Sciences; and Peter Sheerin, DVM, Dipl. ACT, owner of Nandi Veterinary Associates, in New Freedom, Pennsylvania.

1. Breed Differences

“There is a lot of anecdotal evidence that certain breeds of horses have more fertility problems, in both the mares and stallions,” says Squires. Draft horses and Friesians, for instance, tend to have poorer quality semen than most other breeds. Friesian semen, in particular, doesn’t cool or freeze as well, he says.

“There’s likely a genetic component to that, which may include inbreeding in a breed with low numbers,” says Varner. “The lesser-known breeds with less population dynamics may have some subfertile sires that transfer their reduced fertility to their get. Some of these breed variations are simply due to limited numbers in the breed and the need to propagate the breed by use of subfertile individuals.”

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Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey’s Guide to Raising Horses and Storey’s Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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