By Scott Sonner, AP

United in their belief wild horses should remain free to roam public rangeland across the West, groups working to protect the mustangs are increasingly at odds over whether contraception should play a role in the decades-old dispute over efforts to reign in the natural size of the herds.

The American Wild Horse Preservation Campaign — made up of more than 60 groups, including the Humane Society of the United States, Animal Welfare Institute and In Defense of Animals — has been willing to accept treating mares with the anti-fertility drug PZP as a more humane alternative to gathering and shipping mustangs to costly holding facilities.

“The use of birth control, in the form of the PZP vaccine, was recommended by the National Academy of Sciences and is in line with public opinion and taxpayer interests,” campaign spokeswoman Deniz Bolbol said.

Leaders of two dissenting groups who recently won a court order blocking a roundup in Nevada are harshly critical of the national coalition, accusing some members of abandoning the mustangs’ best interests.

The New York-based Friends of Animals and San Francisco-based Protect Mustangs say recent studies show use of the contraceptive, which keeps the horses from reproducing for two years, is having an unnatural impact on the herds’ social structure.

U.S. District Judge Larry Hicks in Reno agreed in February when he blocked a roundup based partly on BLM’s reliance on a 5-year-old study that largely ignores newer data suggesting contraception prompts some mares unable to become pregnant to leave in search of stallions in other bands.

“We champion the herd’s freedom and will prevent special interest groups from using them as pharmaceutical lab rats for drug research,” Protect Mustangs executive director Anne Novak said.

Friends of Animals President Pricilla Feral stepped up the criticism last month.

“They may want to stick their head in the sand and use this as a fundraising opportunity, but the harsh reality for wild horses is that research shows PZP has long-term detrimental effects on wild horses,” she said.

The Humane Society has the patent on PZP, and Feral and others argue they benefit financially from its use.

Holly Hazard, senior vice president of the Humane Society of the United States, said internal dissent is nothing new but acknowledged the “tone and tenor is ramping up.” She said the claim her group pushes PZP to raise money is “ludicrous.”

“We have been working with PZP for 20 years,” she said. “We believe it’s the best hope for getting the wild horse management challenges under control.”

The BLM estimates there are 49,209 wild horses and burros roaming free in 10 western states — half in Nevada. The agency maintains that’s nearly twice as many as the range can sustain. It reported in February there were 48,335 animals in holding facilities with a total capacity of 50,209.

Hazard said the groups share the same “pure vision of what we’d like to see — which would be horses remaining on the range, untouched by man.”

“But if the only argument you can make is they should be left free on the range, I say that they are not now and will not ever be — at least in the reasonable future,” she said. “We want a solution. We don’t want to rattle our saber toward a victory that will never come.”