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Boise, Idaho • Wild horse advocates have filed a lawsuit challenging a U.S. Bureau of Land Management plan to sterilize a herd of wild horses in southwestern Idaho.

The lawsuit filed Monday in U.S. District Court in Idaho contends the BLM is violating federal environmental law by failing to analyze consequences of the action.

The American Wild Horse Preservation Campaign and two other groups also said the federal agency is violating the Wild Free Roaming Horses and Burros Act by adopting a plan that fails to protect wild horses.

Nick Lawton, an attorney representing the groups, said in a statement that the BLM's plan "sets a terrible precedent that threatens the viability of wild horse herds across the West."

Heather Tiel-Nelson, a spokeswoman with the Bureau of Land Management, said Tuesday the agency hadn't yet received legal documents and couldn't comment.

The wild horse advocate groups are asking that the federal agency be prevented from implementing the plan and it be sent back so changes can be made the groups say are needed to make it align with environmental laws. They are also asking for court costs.

The BLM in September released its 72-page Resource Management Plan for the Jarbidge Field Office that includes the 150-square-mile Saylor Creek Herd Management Area.

Under the plan, the herd of wild horses in the management area would be sterilized either chemically or physically and kept to between 50 and 200 horses. Officials said when they released the plan that process is likely years away, though, as details had yet to be worked out.

The non-breeding herd would be replenished with wild horses from Idaho and sometimes other states. Wild horses are sometimes rounded up and offered for adoption when their numbers exceed the amount of food available to support them. But not all the horses get adopted and the BLM has some 47,000 wild horses and burros that are being held and fed in corrals and pastures.

Officials considered removing the herd in Saylor Creek Herd Management Area altogether but ultimately decided on a non-reproducing herd. The herd twice since 2006 has been rounded up and held in the agency's Boise Wild Horse Corrals after wildfires destroyed rangeland forage.

The Range Management Plan guides management of everything from wild horses to cattle grazing to recreation to sage-grouse habitat restoration on the 2,200-square-mile Jarbidge Field Office that contains desert canyons and remote rangelands.