A common request of horse owners and breeders is for veterinary assistance in using a drug or drugs to modify the behavior of stallions and geldings at shows, and for stallions during the breeding season. Although there are some drugs that may be beneficial, the most important part of horse behavior is training and handling, not the use of drugs. Drugs may be used to help with training or when training has been ineffective.
The most commonly used drug to affect behavior in male horses is progesterone. It has been used to quiet aggression and sexual behavior in training and performance situations. Oral altrenogest is the most available form of the drug and is called Regumate, is approved for use in mares, and it has been used at the normal or double dose once daily to reduce aggressive and sexual behavior. It also reduces testicular size and sperm production, which is a concern during breeding season, especially if a specific stud has lowered fertility.
Equine behavior specialist Dr. Sue McDonnell indicates that the changes in behavior have been modest in semen collection protocols. Also, sexual and aggressive behaviors were not significantly suppressed or eliminated, so at this time a treatment regimen with Regumate has not been developed to quiet behavior without affecting semen quality. It is believed the semen effects are temporary but it is not certain how long the effects last before full sperm output is achieved. So if you are breeding a stallion, giving progesterone is probably not a good option.