The agreement carves out two areas next to Yellowstone where bison can winter, ending a long-standing stalemate on the issue. That means at least some bison leaving the park could avoid a widely criticized slaughter program meant to guard against transmissions of a disease, brucellosis, to cattle.
"This is a huge step in legally recognizing the bison's right to be outside the park," said Yellowstone Superintendent Suzanne Lewis.
More than 3,000 migrating bison have been slaughtered or shot by hunters in recent years, including 1,601 last winter. That practice will continue for bison that migrate beyond the two newly designated areas.
Wednesday's action was spurred by a recent Government Accountability Office investigation that sharply criticized the federal government's role in the bison slaughter.
Livestock interests had resisted any changes to the slaughter program, while conservationists and bison advocacy groups criticized authorities for being inflexible. Several groups, including the Natural Resources Defense Council and Greater Yellowstone Coalition, praised Wednesday's agreement, but others said it did not go far enough.
About half the park's bison carry brucellosis, which can cause pregnant animals to prematurely abort their young. After two infections in Montana cattle herds, federal officials earlier this year mandated that cattle ranchers statewide participate in a costly testing and vaccination program.
No wild bison-to-cattle brucellosis transmissions have been recorded, however. State and federal animal-health officials have said elk were the most likely source of the disease in seven infected cattle herds in the last several years - including the two in Montana and others in Idaho and Wyoming.
The plan adopted Wednesday allows an unlimited number of bison to stay, during the winter, on about 10,000 acres west of Yellowstone, an area known as Horse Butte. The animals would not have to be tested for brucellosis.
To the north of the park, a smaller number of bison - 25 initially and possibly 100 in subsequent years - would be allowed to pass through the privately held Royal Teton Ranch. A fenced corridor through the ranch will lead to about 2,500 acres within the Gallatin National Forest. Those animals would be tested for brucellosis and equipped with radio collars to track their location.
Neither area has active cattle operations within it.
The ranch corridor was established in a separate agreement approved by Montana officials earlier this week. Under that deal, the ranch's owners, the Church Universal and Triumphant, relinquished their cattle grazing rights for 30 years in exchange for $3.3 million.
