For some breeders, lowering a breeding stallion’s testosterone concentrations in the off-season is, among other uses, an attractive solution for undesirable behaviors. They can do this by administering a substance that suppresses the horse’s hypothalamic-pituitary-gonadal (HPG) axis, which regulates testosterone concentrations.

Gabriel M. Davolli, DVM, MS, a theriogenology resident at Louisiana State University’s School of Veterinary Medicine, studied one gonadotropin-releasing hormone antagonist’s effects on suppressing the HPG axis and presented his results at the 2015 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas. He conducted his research while completing his master’s degree at the University of Kentucky’s Gluck Equine Research Center, in Lexington.

Gonadotropin-releasing hormones (GnRH) essentially tell the stallion’s reproductive system to produce testosterone. The appropriate antagonist would block this function.

Davolli described several reasons for testosterone suppression:

  • Stallions infected with the respiratory and reproductive disease-causing equine arteritis virus (EAV) can become carriers, constantly shedding the virus in their semen. Because the infection is testosterone-dependent, suppressing their testosterone might help prevent disease spread, he said.
  • Lower testosterone levels could help curb sexual and aggressive behavior during training and competition.
  • It provides an alternative to castration and its potential complications.

Historically, some veterinarians have used the progestin altrenogest (