BILLINGS, Mont. -- More bison would be allowed to roam outside Yellowstone National Park without being captured and slaughtered to protect cattle against disease, under an agreement drafted by state and federal officials.

The proposal was spurred in part by a recent Government Accountability Office investigation that sharply criticized the periodic slaughter of Yellowstone's bison.

It comes as state livestock officials push to carve out portions of Idaho, Wyoming and Montana to create a special management zone for brucellosis. That's a disease carried by bison and feared by ranchers because it causes cattle to abort their calves.

Last year, more than 1,600 bison leaving the park were killed to guard against the disease. While most of those bison would have died even if the new proposal had been in place, supporters touted it as moving beyond the rigid approach to bison management seen in the past.

Yellowstone's estimated 3,000 wild bison comprise one of the largest wild herds in the world.

"What we're working toward is increased tolerance for bison, but we're still focusing on (brucellosis) risk management," said Al Nash with Yellowstone National Park.

Final adoption of the proposal to let bison roam more freely is expected next month in Bozeman. On Dec. 17-18, five federal and state agencies will wrap up a series of meetings on the issue held over the last several months.

The agencies' proposal would allow an unlimited number of bison to stay during winter in an area west of Yellowstone known as Horse Butte. The animals would not have to be tested for brucellosis.

Some bison already have been allowed onto Horse Butte in recent years. The new proposal would formalize that practice and lift a 100 bison cap for the area.

Bison still would be subject to capture and slaughter, or hazing back into the park, if they pushed beyond Horse Butte.

"Instead of focusing on the numbers, we're going to focus on some triggers," said Pat Flowers with Montanan Fish, Wildlife and Parks. "We don't care if there's 500 there. If they're staying where they're supposed to be, so be it."

To the north of the park, a smaller number of bison -- 25 initially and possibly 100 in subsequent years -- would be allowed to roam through the privately held Royal Teton Ranch. Those animals would be tested for brucellosis and equipped with radio collars to track their location.

The areas on the west and north sides of Yellowstone no longer have active cattle operations. That lowers the chance of a disease transmission, livestock and wildlife officials said.

Still, the move to expand where bison can go is being resisted by livestock groups.

Some ranchers and the Montana Stockgrowers Association see it as a slippery slope toward loosening rules that to date have prevented a bison-to-cattle transmission. The Stockgrowers filed a lawsuit in state district court in May, seeking to stop the Montana Department of Livestock from allowing bison to linger outside the park.

If the litigation prevails, it could potentially block the latest proposal from going forward.

With brucellosis now eliminated everywhere in the country except around Yellowstone, livestock officials outside the region also remain wary.

"It is a de facto expansion of Yellowstone National Park and doing nothing to address the disease," said Texas state veterinarian Bob Hillman. "If the disease moved to another state, we're perpetuating and spreading brucellosis, which we've spent a lot of money on to eradicate."

Texas regained its brucellosis-free status in February following a lengthy testing and vaccination program. Montana lost its status this year after two infections in cattle herds. Both of those transmissions were linked by livestock officials to Yellowstone-area elk, which also carry brucellosis.

Elk are not subject to the same intensive oversight as Yellowstone's bison herds. Elk often travel in small groups that are much more difficult to track and manage.