BOZEMAN, MONT. -- Yellowstone National Park rangers and Montana state game wardens have been authorized to shoot bison cows if the large animals follow traditional patterns and migrate beyond park boundaries this winter.

This would mark the first time in more than two decades that Yellowstone rangers have culled wildlife herds, but officials said they must keep the bison from mingling with cattle herds.

About half of Yellowstone's bison are infected with brucellosis, a disease that causes domestic cattle to abort their calves. Montana cattle herds are free of the disease, and state officials said economic losses caused by infection would be great.

Yellowstone has three bison herds totaling about 3,000 animals. For six years, some shaggy animals from the northernmost herd have been migrating across park boundaries, seeking forage at lower elevations along the Yellowstone River.

At first, state game wardens dispatched the animals, but Montana later allowed hunters to harvest them. National publicity attending a hunt last year, when 569 of the animals were shot to death, generated hundreds of complaints to newly elected Gov. Stan Stephens (R), who has not received as much comment on any other issue.

During a hunt earlier this year, animal-rights activists tried to hinder hunters, jabbing one with a ski pole and smearing blood on another. Police reported two arrests.

In a plan announced recently, park and state officials said they would try to minimize some of the outcry with a cooperative effort to have wardens and rangers shoot only cows that leave the park because cows most often transmit brucellosis.

Calves would be tranquilized, neutered and shipped for auction. Montana hunters could still shoot the bulls.

"No one involved in this plan will win a popularity contest -- no one," said K.L. Cool, director of Montana Fish, Wildlife and Parks. "I don't think any of us enjoys implementing this plan. We see it as a duty."

Bob Barbee, superintendent of Yellowstone, said that neither agency has been able to find a palatable solution to bison migration but that the park service will cooperate in the venture in an effort to be good neighbors.

"We cannot repopulate the Great Plains with bison, and we cannot repopulate the upper Yellowstone Valley with bison," Barbee said, adding that any harvest of the animals will not harm bison populations in the park.

Tony Povilitis, senior scientist with the national office of the Humane Society, said his organization objects to the proposal.

"This is hardly an improvement on what has been going on," he said. "I think they're way off the mark on what should be done."

The Humane Society and other conservation groups in the region have complained that, instead of focusing on managing bison to keep them away from cattle, officials should try harder to move cattle away from bison.

Povilitis said bison could be trapped along the boundary and those infected with brucellosis shot. Those without the disease could forage in a buffer zone on the boundary, and any bison that wander out of the zone could be shot, he said.

State and park service officials said they cannot risk infecting cattle herds and that the most effective way to deal with the bison is at the park boundary. Barbee said the park service cannot legally trap wildlife inside the park without an extensive environmental review.

State and federal officials said the plan will be in effect only this year while they study a more permanent solution.

Michael Scott of the Wilderness Society expressed concern that the temporary plan will become permanent, without a thorough examination of other solutions.

Scott said officials "should look at perhaps exchanging some grazing allotments and other cooperative arrangements with local ranchers so there can be some balance between the economic necessity of the local ranches {and} the very legitimate needs bison have to look for winter forage and warmth."