Two studies involving the use of altrenogest (Regu-Mate) to control behavior of stallions appeared on the surface to have contradictory results. However, there was a key difference–age of the stallions involved–that appeared to eliminate the contradiction. The results of a study at Texas Tech, presented by Heidi A. Brady, PhD, indicated that altrenogest was effective in modifying stallion behavior, while a study at Texas A&M, reported by Corey D. Miller, DVM, indicated that it had little effect.

The key difference was that at Texas A&M, Regu-Mate was given to mature stallions, while at Texas Tech, it was administered to stallions which were two to four years of age. The young stallions, reported Brady, became virtual “puppy dogs” to handle. The mature stallions, said Miller, showed some subtle changes, but, basically, their behavior was unchanged.

The Regu-Mate had a definite effect on other parameters. Scrotal size diminished, sperm output was decreased, and testosterone levels were lowered. Just how much time is required for a return to normal in these areas needs more study, the speakers said