BILLINGS, Mont. - Montana and federal officials are negotiating a proposal to open a large area north of Yellowstone National Park to roaming bison during winter - a move aimed at relieving pressure to slaughter the disease-bearing animals when they leave the park.

Officials from several agencies told The Associated Press Wednesday that behind-the-scenes discussions on the proposal have been taking place for about a month.

Montana Department of Livestock Executive Officer Christian Mackay said details could be ironed out as soon as next week and officials are aiming to open the new area to bison this winter.

The concept of more room for Yellowstone's bison has been promoted for years by conservation groups and wildlife agencies. It has been resisted by livestock groups, ranchers and - until now - Mackay's agency. Many bison carry the disease brucellosis, and those that leave the park face potential slaughter to prevent transmissions to livestock.

More than 600 bison have been captured so far this year and are being held by the park to block their winter migration into Montana. That includes 62 captured Wednesday.

The park has pledged to release at least some of the brucellosis-free bison in the spring. No decision has been made on more than 200 of the animals that have tested positive for exposure to the disease, through blood samples taken after their capture.

Gov. Brian Schweitzer last month ordered a 90-day ban on slaughter shipments. He said this week that a better alternative would be to create "buffer zones" around the park, where bison could freely cross the Yellowstone-Montana border and be managed through more aggressive hunting.

For the past two decades park bison entering Montana have been captured and slaughtered, hazed back into the park or shot by livestock agents. Exceptions are allowed in limited areas where bison are allowed to linger during the winter.

"Why treat the bison in this way? When they cross an imaginary line - boom boom boom - shoot them like a couch," Schweitzer said in a telephone interview. "We're attempting to enlarge the area where bison are allowed to leave the park, within the constraints of keeping them away from livestock."

The governor and other officials described a north side buffer stretching roughly 13 miles north of the park, encompassing the Gardiner Basin and ending at Yankee Jim Canyon along the Yellowstone River.

None of the bison captured this winter would be released into that area, Mackay said. Rather, it would be used to accommodate additional animals that migrate, he said.

Fences and other barriers would be erected at the mouth of the canyon to prevent diseased bison from heading into the Paradise Valley, which has numerous cattle ranches. Mackay said similar measures were being considered for two small cattle operations in the Gardiner Basin that combined have only a few dozen animals.

The last effort to carve out new habitat for bison outside the park ended in failure. Federal and state agencies in January herded 25 of the animals onto a 2,500-acre patch of the Gallatin National Forest - the first time in decades bison had been allowed to stay in that area.

Within weeks the initiative had collapsed. The bison repeatedly left the Gallatin before government officials returned them to the park.

The plan now being negotiated would cover an area about 10 times larger - roughly 25,000 acres, said Mark Pearson with the Greater Yellowstone Coalition. Pearson said it was an "obvious, common sense solution to the bison conundrum" since few cattle remain in the Gardiner area.

But Paradise Valley cattle producer Druska Kinkie said Wednesday it would be impossible to keep the bison contained. And if bison got into the valley, she added, "you're just inviting a transmission" of brucellosis to cattle.

"They can't fence that country. It's huge and it's wild," Kinkie said. "The bison in the park are the reservoir for brucellosis. If you allow bison to come farther, you're just increasing the acreage in that reservoir."

Schweitzer also suggested a second buffer zone in the Madison River Valley, stretching west from the park border about 20 miles to Quake Lake. Other officials said that area has not factored into the recent discussions.

Schweitzer said hunting in the buffer zones could be used to keep the park's bison population in check - to prevent herds from spilling into parts of Montana where they could potentially infect livestock.

Yellowstone Superintendent Daniel Wenk rebuffed Schweitzer's suggestion to include parts of the park in the hunt area during a Tuesday meeting in Helena. Federal law prohibits hunting in the park.

Wenk and representatives of the U.S. Forest Service said they embraced the more immediate plan to allow bison more room to roam in the Gardiner Basin.

Forest Service spokeswoman Marna Daley said details on the barriers that would keep the bison from traveling into the Paradise Valley still need to be worked out. She added that the Forest Service for years has sought to give bison more room to roam.

"We're thrilled they're considering it," she said.