Certain medicines that influence behavior can have harmful effects on a stallion’s ability to breed.

Stallion behavior can be problematic. In the arena, a stallion’s natural tendency to focus on mares rather than the job at hand can be costly. It impacts their ability to train and compete to the best of their ability, which drops their marketability when they do go to the breeding shed.

Both altrenogest (Regumate) and general sedatives have been used to alter poor behavior and redirect a stallion’s focus to training and competition. However, the use of these medications can create problems in the breeding shed.

Altrenogest (regumate) is a synthetic hormone used to suppress estrus and maintain pregnancy in high-risk mares. In stallions, it’s sometimes used off-label to control testosterone-driven behavior that may interfere with training or performance. Photo by Molly Montag

Here, Dr. Semira Mancill, DVM, DACT, explains why these medications should be used with caution in any potential breeding stallion. Mancill is a practitioner at Equine Partners Veterinary Services in Waller, Texas, and a board certified reproductive specialist.

Regumate

Altrenogest (Regumate) is a synthetic hormone used to suppress estrus and maintain pregnancy in high-risk mares. In stallions, it’s used off-label to control testosterone-driven behavior that may interfere with training or performance.

Unfortunately, behavioral-modifying dosage of altrenogest can come at a steep price to a stallion’s reproductive soundness. Photo by Molly Montag

“Altrenogest inhibits the luteinizing hormone, which in the male, stimulates the production of testosterone,” Mancill explained. “The suppression of testosterone shrinks the size of the testicles, therefore decreasing the total number of spermatozoa that the testicles produce. If testosterone levels are lowered or depleted, the stallion may act more like a gelding.”

While short-term, lower doses—30 days at the labeled dosage for mares by weight, as with one study—didn’t have adverse effects or desired affects as behavior was slightly altered, altrenogest still suppressed testosterone levels until the end of the study’s observation period, which was 60 days past the last dose.

Unfortunately, behavioral-modifying dosages of altrenogest can come at a steep price to a stallion’s reproductive soundness.

“High doses of altrenogest and prolonged use in stallions is associated with semen collection problems, complete ejaculatory failure, decreased testicular size and decreased sperm production, with a portion of those spermatozoa having flawed morphologic characteristics,” Mancill said. “In several studies the effects were long term, and stallion owners should be cautious with its use.”

Phenothiazines

Phenothiazines are commonly used tranquilizers in veterinary medicine. They’re routinely used during various minor medical exams and surgical procedures. At low doses, phenothiazines have been used as calming agents in performance horses.

Certain medicines that influence behavior, like regumate and phenothiazines can have harmful effects on a stallion’s ability to breed.
Phenothiazines are commonly used tranquilizers in veterinary medicine. They’re routinely used during various minor medical exams and surgical procedures. Photo by Abigail Boatwright.

They should be used with caution on all male horses, particularly stallions, as paralysis of the retractor penis muscle has been associated with the use of phenothiazine sedatives.

“Acepromazine and chlorpromazine are the most commonly used phenothiazines,” Mancill said. “They can cause penile prolapse, or paraphimosis. The pathway is not entirely understood but thought to be because the phenothiazines block the motor nerve pathways that allow the retractor penis muscle to return the penis to its protective sheath correctly.”

Paralysis of the retractor penis muscle can occur in geldings, but it’s more often with stallions. According to the literature accompanying a bottle of acepromazine, it’s postulated that the occurrence may be due to the presence of testosterone, either artificially administered or naturally occurring as with stallions.

The literature also notes that at the time of admiration it is not possible to tell the difference between the normal protrusion of the penis—the prolonged prolapse, or “dropping,” that occurs with any sedation—and the irreversible paralysis or priapism. Unfortunately, the consequences can be dire for a stallion.

“In severe cases, the penile tissues become edematous and proper blood circulation is lost,” Mancill said. “In extreme cases, penile amputation is the only treatment option.”

Certain medicines that influence behavior, like regumate and phenothiazines can have harmful effects on a stallion’s ability to breed.
At low doses, phenothiazines have been used as calming agents in performance horses. They should be used with caution on all male horses, particularly stallions, as paralysis of the retractor penis muscle has been associated with the use of phenothiazine sedatives. Photo by Abigail Boatwright
Certain medicines that influence behavior, like regumate and phenothiazines can have harmful effects on a stallion’s ability to breed.

Alternatives

Regarding the various nutritional supplements that may be used in favor of medicinal management, Mancill said it’s difficult to predict which one might negatively impact breeding soundness because in most cases, the products available on the market are not FDA-approved, therefore, there’s no research on their effects on fertility.

A good training program, she noted, is the safer alternative to gambling with medicinal management of a stallion’s undesirable behavior. She also advised working with a trusted veterinarian when it comes to medicating stallions for exams or procedures.


This article was originally published in the January 2022 issue of Barrel Horse News.

Author

Email comments or questions to [email protected]

Write A Comment