According to a study, most mares show dramatic improvements in behavior—and, thus, rideability—after having their ovaries removed, regardless of whether altrenogest (Regu-Mate, Altren, OvaMed) was effective in the past. And that can give owners wishing to enjoy their performance mares without dealing with periodic estrus-related behavioral issues hope, said Elizabeth M. Collar, DVM, PhD, Dipl. ACVS-LA, assistant professor of equine surgery at the University of Tennessee, in Knoxville.

“This is good news,” Collar said. “Just because a mare doesn’t improve with altrenogest doesn’t mean there are no options.”

Putting Numbers Behind Post-Surgical Observations

In her clinical experience, Collar said she’s understood that owners were generally happy with the results of an ovariectomy. However, veterinarians hadn’t recorded success rates on a large level, and debate in the equine community seemed to exist about whether ovariectomies reduced unwanted estrus-related behavior, such as stopping, peeing, and vulva winking during exercise or acting out aggressively with biting, kicking, and bucking.

So she and fellow researchers investigated how owners felt about the ovariectomies their mares and mollies (female mules) had undergone at Oregon State University at least six months earlier. They surveyed the owners of 51 equids, 10 of which had had surgery due to pathologies (diseases) such as ovarian tumors and the rest because the owners wanted the ovaries removed in hopes of resolving unwanted estrus-related behaviors.

Among the pathological cases—nine of which included severe behavioral problems—the success rate was 100%, Collar said. Half those equids had been treated with altrenogest in the past with no improvement.

As for the other 41 (elective) cases, 90% of the owners reported full satisfaction with the ovariectomy, including those who reported altrenogest had not worked on their mares. Even mares that had been severely aggressive showed complete improvement. A few mares still showed minor signs of estrus, Collar said, but they were not significant enough to cause problems for the owners.

“About a third of mares will show signs of winking or peeing after ovariectomy,” she said. “While these are ‘signs of estrus,’ they are not associated with pain or bad behavior, and the signs of estrus did not interfere with performance. Most owners commented that it was barely noticeable and that it didn’t bother them. In all cases where horses were ovariectomized for signs of excessive estrus (excessive winking/peeing and unable to go forward or perform under saddle), the behavior was completely resolved, which is why owners were pleased with the outcome.”

The study didn’t take into consideration where the mare was in her heat cycle at the time of surgery—a factor some veterinarians have cited as affecting the procedure’s success. But Collar said she hasn’t noted such a trend in her experience as an equine surgeon.

“I do prefer to ovariectomize mares when they are in anestrus, as the ovaries are smaller, making the procedure easier,” she said. “Mares in heat may have higher levels of hormones, like estrogen, in their system at the time of surgery, creating a slight delay in behavior improvement if the behavior is hormone-related. However, it is my impression that these horses still improve in the long term.

Altrenogest Failed? No Problem for Ovariectomies

These results overturn a popular belief—one that Collar herself held for many years—that altrenogest can indicate how mares might respond to an ovariectomy.

“I used to tell owners and referring veterinarians that the surgery would be less likely to help a horse if altrenogest did not help her,” she said. “However, I had some cases that had not improved with altrenogest but responded very positively to the surgery; therefore, I was very interested in looking into that in this study. Now I can tell owners that the surgery is likely to help mares, whether or not altrenogest improved them, which is great information to have.”

Altrenogest doesn’t fully stop a mare from cycling, she added, so it “makes sense that completely removing the source of hormones as well as the process of ovulation could improve a horse more completely than altrenogest.”

The Ovary-Free Mare: The New Gelding?

Because altrenogest is costly long-term and carries the risk of contaminating human handlers—potentially affecting their health—finding better solutions for managing estrus behavior is important, Collar said. As ovary removal has become simplified and less expensive thanks to modern and minimally invasive techniques, it’s a promising way to make more rideable “geldings” of mares.

“We regularly geld stallions so as to have more manageable male horses, yet ovariectomy to remove the female sex organs and hormones is far less common in horses,” she said. “There are many sales posts out there looking for ‘geldings’ only. My hope is that people will realize they can buy a mare and have a ‘gelding’ by pursuing ovariectomy, and that more horses will have a more pleasant life and better relationship with their people through this procedure.”

Even so, ovariectomies cannot improve behavior that has developed for reasons besides estrus alone, Collar added.

“While no procedure or medication can completely guarantee resolution of all bad behavior—as a range of other health problems, training, and personality traits obviously can also play a role—(an ovariectomy) is a cost-effective and successful procedure that can be considered to improve the behavior and comfort of many mares,” said Collar.

The study, “Outcome of bilateral equid laparoscopic ovariectomies,” was published in the July 2021 edition of Veterinary Surgery.