Petr David Josek, Associated Press
In this photo taken on Monday, March 16, 2015, a herd of 14 wild mares from Britain’s Exmoor National Park rest in an enclosure near the village of Milovice, Czech Republic.

SALT LAKE CITY — Some wild horse advocates are praising the Bureau of Land Management for implementing Utah's first fertility control program that does not require roundups. Other groups, however, oppose the practice.

The BLM West Desert District's Salt Lake Field Office announced Friday the decision to administer contraceptive drugs to up to 50 mares in the Onaqui Mountain Herd Management Area using dart guns. The program will start at the end of May and continue for the next five years, said BLM spokeswoman Lisa Reid.

The Onaqui Mountain herd roams freely on roughly 207,000 acres of land in Tooele County, about 60 miles southwest of Tooele. While the BLM has set the herd's appropriate management level at 121 to 210, its current population sits at almost 320.

"This is a very important program. The only tool we've had in the past to manage herds is through removal," Reid said. "We prefer not to round them up, so administering birth control through darting is a great tool because it's less invasive and less stressful to the herds, and it allows us to hopefully reduce reproduction effectively."

The dart-gun administration of the contraceptive drug porcine zona pellucida, which is most effective for one year, is the Utah BLM's latest effort to address burgeoning wild horse populations that have fueled a national firestorm of controversy. Years of drought, lack of federal funding, teeming holding pens, meager adoption rates, and animal advocate outrage have been a spiraling source of frustration for local ranchers and land managers.

Roughly 4,900 wild horses and burros freely roam Utah rangelands, Reid said, but the state's management level rests just below 2,000. More than 1,000 horses and burros currently reside in the state's short-term holding pens.

While roundups are not only controversial, they're also costly, said Gus Warr, Utah director of the BLM's Wild Horse and Burro program. He said helicopter roundups cost roughly $400-$500 per horse while fertility drugs cost roughly $100 per horse.

The American Wild Horse Preservation Campaign, which consists of more than 60 groups including the Humane Society of the United States, backs fertility control as a more humane method than removing wild horses from the ranges.

"This is the best case scenario," campaign spokeswoman Deniz Bolbol said. "We really applaud Utah BLM for doing this for the Onaqui herd and letting these horses stay with their families, remain wild and free, and at the same time manage the number of horses born so they don't have to do roundups into the future. We hope BLM management in Washington look at programs like this as a best practice and start applying this across the board."

But other groups still oppose using porcine zona pellucida in mares, pointing to studies that have found three to five doses can lead to sterilization, which Anne Novak, Protect Mustangs executive director, said can "wreak havoc on natural selection" by decreasing mares' chances of having healthier offspring after their first foal.

"This is an essential part of survival of the fittest. Nature knows best," Novak said. "By switching to an on-the-range breeding program the BLM and PZP advocates are taking the wild out of wild horses, … (and) no one should be shooting wild horses with dart guns. It's harassment, plain and simple."

Warr said measures will be taken to try to prevent sterilization in the Onaqui herd by working with photographers to create an "intensive catalogue" of mares that receive the drug, so that each mare will get no more than three doses.

Reid said that while terrain and skittish horses make it an unrealistic goal to start the dart-gun birth control in every herd, the BLM is exploring all options to maximize the use of fertility control in Utah, including finding a drug that is more effective for longer than just one year.