BILLINGS, Mont. (AP)--Birth control for the Pryor Mountain herd of wild horse is being sidelined because of objections from two groups based in Colorado.

The Cloud Foundation and the Colorado Wild Horse and Burro Coalition petitioned to block the program. It is on hold, pending a hearing before the federal Interior Board of Land Appeals.

Plans called for 20 mares to receive injected contraceptives this month. Critics say they want the program blocked because results of the shots are unpredictable.

The fertility control effort in the Pryor Mountain herd began in 2001 and is overseen by the U.S. Bureau of Land Management. If the protest is not resolved within six weeks or so, the agency said, then wintry weather will leave the project on hold regardless of the case filed by The Cloud Foundation and the coalition.

The BLM uses the contraceptive PZP, porcine zona pellucida, to control the number of wild horses in the Pryor Mountains.

"What I'd like to see is natural selection," said Ginger Katherns of The Cloud Foundation.

Twenty mares were to receive shots this fall. The practice has been to give mares of birthing age shots of PZP with a dart gun twice a year. Some of the mares that are darted reproduce nonetheless.

At last count, the Pryor Mountain herd had 165 horses. The BLM has said its target population, based on the size and quality of the existing rangeland, is about 95 horses. Some researchers have recommended a herd of 140-150, for genetic reasons. Kathrens said she'd like to see the herd at about 200 animals.

The BLM's wild-horse range covers 38,000 acres, but the animals roam onto adjacent Forest Service and National Park Service lands in the East Pryor Mountains, about 90 miles south of Billings.

Kathrens said she is striving to legally expand the wild-horse range onto private, Forest Service and Park Service lands. "That's what we need to be doing, not running around with dart guns," she said.

The horses' impact on the East Pryor Mountains concerns Clayton McCracken of the Sierra Club's Yellowstone Basin chapter. McCracken said the herds damage Forest Service land, and erosion eventually could be serious.

"The BLM is battered by the wild-horse advocates," McCracken said. "But they know in their hearts that if the range isn't preserved, in 20 years they won't have any range."

McCracken said PZP has proven the most cost-effective way to limit herd size.

If the Interior Board of Land Appeals rules in favor of The Cloud Foundation and the Colorado Wild Horse and Burro Coalition, then the BLM will have to use roundups to control the population, said Linda Coates-Markle, horse and burro specialist for the agency.

"Natural controls aren't enough to impact the herd," she said.

The last roundup, in 2003, netted seven animals.