wild horses boise

Wild horses await adoption in a corral in Boise as part of the Adoption Incentive Program.

Faced with a population of wild horses that doubles every four years on Western ranges, the Bureau of Land Management is testing a new fertility control vaccine it hopes will curtail numbers.

The new vaccine tests began last week in Carson City, Nev., a state where most of the nation’s wild horse population exists. No plans are currently in the works to test the vaccine on Idaho’s wild horse population.

While wild horses are often treated with a fertility vaccine, the current treatment in use is only effective for about a year and horses must be gathered annually and retreated.

“For decades, the BLM has sought a long-term vaccine that could help effectively and humanely control the rapid growth of wild horse and burro populations on public lands,” said BLM Deputy Director for Policy and Programs William Perry Pendley in a news release. “Now more than ever, an all-of-the-above approach is needed as a rapidly growing overpopulation of wild horses and burros threatens the long-term health of our public lands. With the start of this trial, the BLM has taken a big and important step forward to developing better, more effective population management tools that can help solve this growing crisis.”

Idaho’s BLM wild horse specialist Heather Tiel-Nelson said a new, long-lasting fertility vaccine would help curb the population explosion.

“We generally apply the porcine zona pellucida (vaccine),” she said. “It’s pretty temporary. It might be effective for that first year, but it’s really not that effective after that. We’ve been applying that to all of our mares we return to the range for a lot of years now. What we’re finding, not just here in Idaho but nationwide is it simply isn’t effective to curb the population growth like we need it to be.”

The BLM estimates that there are 95,000 wild horses and burros in herds across the West compared to 27,000 in 1971 when Congress passed the Wild Free-Roaming Horses and Burros Act. Tiel-Nelson said Idaho’s wild horse population is easier to manage than Nevada’s.

“For instance, we have six herd management areas, whereas Nevada has more than 100,” she said. “Of the estimated 95,000 wild horses roaming the range, half of those are in Nevada.”

The BLM said having an ever-increasing number of herds on the rangeland “can stress critical ecosystems to the brink, causing severe damage to riparian and rangeland resources that, if capable of recovery, can take decades to do so. Moreover, overpopulation leads to the inhumane death of the horses and burros from thirst or starvation.”

The new vaccine was first tested on domestic horses and is now being tested on wild horses.

As part of the project, 16 previously gathered wild mares were treated with the fertility control vaccine and placed in a pen with a stallion once the vaccine takes effect. Researchers will monitor the mares’ response to the vaccine and compare the results to a control group.

“A single-dose vaccine that can last multiple years ... would provide a number of benefits for BLM, including requiring fewer instances of gathering animals for retreatment or permanent removal,” the BLM said.

The BLM has also in recent years taken more animals off the range and placed them up for adoption with financial incentives.

“The BLM hit a 15-year high in 2019 for adoptions and sales of excess animals,” the BLM said.

The Adoption Incentive Program has proved successful in Idaho. More than 50 wild horses were adopted from the Challis Wild Horse Herd in February.