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