The Bureau of Land Management (BLM) is studying the spaying of fertile wild mares in order to control the growth of the wild horse population. But the agency’s senior wild horse and burro program advisor said that use of the procedure is not imminent.

According to BLM estimates, there are approximately 37,300 wild horses and burros roaming BLM-managed rangelands in 10 western states. The agency estimates that another 50,000 animals currently reside in holding facilities. The 1971 Wild Free-Roaming Horses and Burros Act protects wild horses and burros and places them under BLM jurisdiction.

BLM spokesman Tom Gorey said that the wild horse populations can double every four years. To control wild horse herd sizes, the agency removes animals from their ranges to short-term corrals and long-term pastures. Some wild horse advocates oppose these gathers as a population control method. Also to control herd population, the agency has treated some mares with the fertility-control drug porcine zona pellucida (PZP) and released more stallions than mares back onto the ranges after gathers.

Dean Bolstad, senior advisor for the BLM Wild Horse and Burro Program, said that in 2012, the agency’s Wild Horse and Burro Advisory Board suggested removing the ovaries of some wild mares as a way to control herd growth. The procedure, known as an ovariectomy, is sometimes used to eliminate mares’ fertility cycles, treat disease, or control estrus-related behavior. The BLM is deliberating exercising ovariectomies as part of its overall herd population growth plan, Bolstad said.

“We are considering the Advisory Board’s recommendation and consulting with fertility experts and with veterinarians,” Bolsted said. “But we have not started spaying mares.”

Bolsted said that the BLM’s decision to use the spay procedure and how it will be applied will be contingent on the opinions of veterinarians and other specialists, and on the experiences of those who have used the procedure in both wild and domestic mares.

Wildlife fertility authority Jay F. Kirkpatrick, PhD, director of the Science and Conservation Center in Montana, said that wild horse fertility is a very complex issue. Kirkpatrick said that the form of PZP used by the BLM (PZP-22) has not “delivered on its expectations,” and he believes, from a pharmacologic standpoint, this could be because the agency has not apply PZP treatment to as many mares as it should in order to get “decent” results. He believes the BLM should carefully consider the factors surrounding ovariectomy and the agency’s ultimate population control goal before spaying mares.

The safety of the procedure should be the agency’s initial concern, Kirkpatrick said. While he said ovariectomy is generally safe and effective if performed under ideal circumstances, he believes that performing the procedure on wild mares is another matter.

“If done carefully and professionally, modern ovariectomy is, in a clinical setting, with experts and laparoscopic techniques, fairly safe and reasonably stress-free (beyond capture),” Kirkpatrick said. “But, of course, spaying (wild mares) in a corral after a stressful capture … is not the same thing.”

Kirkpatrick also said he believes it is critical that the BLM know which mares have undergone the procedure and which have not. He explained that if a mare has made a genetic contribution to the herd, rendering her unable to produce any more foals via an ovariectomy "isn’t bad. But if she has not made that genetic contribution to the herd, then it is bad.”

Beyond that, the use of ovariectomy could also influence the social structure of wild horse bands, Kirkpatrick said: “Ovariectomy will, of course, eliminate estrous behavior and that may or may not have an impact on social structure, depending upon whether or not (the BLM) allows some mares to remain intact in each band.”

Ultimately, Kirkpatrick believes that focusing on spaying itself does not address the central issue: wild herd population control. He does not believe spaying is the answer to the wild horse population issue; with ovariectomy, he noted, the BLM would have to spay more than 50% of adult mares—65% in some herds, depending upon factors such as mortality rate—to achieve the desired population control.

“If the BLM could not treat enough horses with a hand-injected contraceptive to make a difference, how are they going to treat enough mares with spaying to make a difference?” Kirkpatrick said.

Finally, Kirkpatrick believes that the agency should not exercise ovarectomy as population control until rigorous testing has taken place.

“Every contraceptive approach used or even discussed has been either carefully vetted and tested in controlled trials before application on a large scale … or various small trials with controlled populations have been (completed), and that has not been the case with spaying,” Kirkpatrick said.

Bolstad said that the BLM will research whether ovariectomy might achieve population control without endangering wild horse herds. A National Academy of Sciences report due to be released in June will also address wild horse population control issues, he noted.

“I think ovariectomy is one of many of the tools we have to address wild horse population control,” Bolstad said. “We will proceed carefully, cautiously, and respectfully.”