Nevada hosts more than half -- about 17,700 -- of the 33,700 wild horses that roam around federal lands. But Bureau of Land Management rangeland scientists estimate the state can support only 12,700 horses and burros. And if left alone, wild horse herds typically grow 20 percent annually, doubling in size every four years. "We know we have to manage the range,” says Alan Shepherd, state lead for the Nevada BLM horse program. “We have to do something population-wise." Current birth control measures last just one to two years, but longer-lasting birth control, now being studied in Nevada, could be one tool to help address the imbalance.
Wild horses are protected from slaughter under the 1971 Free-Roaming Wild Horse and Burro Act. But in order to protect habitat and wildlife, the BLM needs to get the horses off the range without hurting them. It then tries to find them homes. In 2010, the agency rounded up 10,255 wild horses and burros, but only 3,074 got adopted. The rest are held in corrals or sent to pastures in the Midwest. In 2010, just holding excess horses cost $36.9 million, nearly 60 percent of the agency's wild horse and burro budget.
"The fertility drug itself costs about $300 per dose to give to the mare." Shepherd says, "The way we look at it, every baby that we don't create and have to take off the range later saves us thousands of dollars for removal, adoption expenses, holding. That's money we can use for looking into other fertility control and doing management work."
Jay Kirkpatrick has been studying how birth control can be used to manage horses for more than two decades, starting with treatments to horses on Assateague Island off the coast of Maryland in 1988. Darting 65 percent of the adult mares on the island each year has, over time, reduced the herd from 175 to 112 horses. Without these measures, the herd's population would have increased significantly. It's harder for this approach to work for larger herds on uncontained ranges in the West, but some believe fertility control is still key. Kirkpatrick emphasizes that simply removing horses through gathers will never take care of the problems they cause on the range, because the remaining horses will just breed younger and faster. "Overpopulation, overgrazing, trampling vegetation, competing with livestock and other wildlife -- those are all symptoms of the problem," he says. "The problem is reproduction. The only way you can address any of those other symptoms is by doing something about reproduction."
This year, the BLM is preparing to step up its use of fertility treatments to manage wild horses. The agency plans to treat 2,000 mares, a four-fold increase over the 500 or so treated during the past several years, but still just a tiny fraction of the tens of thousands roaming the range. Meanwhile, researchers from the University of Toledo Medical School are partnering with the Nevada BLM Horse Program to do pen tests of a new version of fertility control that could keep mares from getting pregnant for four years, a treatment that would enable the agency to more effectively manage herds.
The new treatment builds on an existing one that lasts only about a year by adding an additional time-release dose of porcine zona pellucida or PZP, which blocks sperm from fertilizing eggs. The injection includes four pellets that look like quarter-inch-long pieces of colored pencil lead. Each contains a PZP booster wrapped in a special polymer that dissolves and releases the treatment after a set amount of time: 30 days, 90 days, 9 months and 20 months.
If the Nevada pen studies show that the new treatment effectively prevents pregnancy for three to four years without negative side effects, it will help overcome the main scientific hurdle for horse contraceptive treatments -- that is, the need for a single shot, multi-year treatment. But, Kirkpatrick concedes, other hurdles remain including advocacy group and agency opposition to treatments based on social, cultural, political and economic grounds.
Furthermore, while it has helped slow population growth in a few management areas, fertility treatment has yet to prove effective for large, free-roaming herds around the West. The treatments and application are advancing, but much work remains before the BLM will be able to reduce existing herds to appropriate levels and keep them there.
Emilene Ostlind holds an editorial fellowship at High Country News.
Photo of bony wild mare and foal in Nevada courtesy Flickr user James Marvin Phelps.