Curbing a Mustang Baby Boom

Fertility-control options for mares could help cut down on mustang overpopulation.
The Bureau of Land Management is searching for options to manage the population of wild horses, which has more than doubled since 1971. | © Arnd Bronkhorst

How can America’s wild horses be kept from overpopulating western rangelands? Activists, ranchers and politicians have been batting that question around for years. So have officials at the Bureau of Land Management, which oversees the free-roaming herds on public lands. Now the BLM is launching a research push to find better tools for managing the animals, including new fertility-control options.

Numbers show the need: Since the Wild Free-Roaming Horses and Burros Act became law in 1971, the equine population on public lands has more than doubled, topping 58,000, the BLM says. Overpopulation leads to overgrazing and that leads to deterioration of the range and of the animals’ health. Prolonged drought is adding to the stress. 

Fertility control would seem to be the answer—if a practical method can be found. The BLM has given thousands of wild mares contraceptive vaccines, derived from porcine zona pellucida, which promote an immune response that prevents fertilization. But the effects last just one to two years, so mares have to be repeatedly captured and treated. The program continues, but so far it hasn’t really dented population growth. Now the bureau is partnering with university researchers in the hunt for better methods. Several studies will evaluate surgical sterilization methods for use on the range and others will explore longer-lasting or even permanent contraceptive vaccines. 

With few natural predators or other controls, wild-horse herds can double in size about every four years, the BLM says. To take pressure off the range, some 47,000 horses and burros that have been removed from public lands are being cared for by the BLM in off-range pastures and corrals. Adoptions of these animals have dropped from nearly 8,000 to about 2,500 a year.

Elaine Pascoe

This article originally appeared in the October 2015 issue of Practical Horseman.

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