Now the herd is again at the centre of controversy because it harbours brucellosis, a disease that can cause stillbirths in cows. Nearly one in five -- 947 -- of the bison herd were sent to slaughter this winter for fleeing Yellowstone's snow-covered high country for food outside the park in Montana where cattle graze.

Bison advocates are outraged by the slaughter, which is authorised under an agreement between Yellowstone National Park and Montana's Department of Livestock. But Montana ranchers say the buffalo endanger the state's prized brucellosis-free status, which allows producers to ship their cattle across state lines without testing.

Fresh on the heels of Montana's first bison hunt in 15 years, Gov. Brian Schweitzer has charged into the debate, aiming to broker a deal between ranchers and buffalo admirers.

Schweitzer wants to expand the range the herd may roam outside Yellowstone National Park and would pay ranchers not to graze their cattle on the same land. He also has proposed increasing hunting permits for buffalo to up to 500 annually from 50 to help cull a Yellowstone herd that last year numbered 4,900, a record high since it was established in the early 1900s.

"I'm trying to come up with a solution that actually makes sense," Schweitzer said.

The rancher-turned-governor is promoting his plan even as a government operation last week to push buffalo back into the park using a helicopter and ATVs stirred fresh discord.

COWBOYS VS NATURALISTS

There is no documented case of brucellosis transmission in the wild from bison to cattle, and most of the animals killed this winter were not tested for the disease. Elk in the Yellowstone area also carry brucellosis, but the popular game animal has not evoked the same ire in Montana's US$1 billion livestock industry.

Ranchers want to see bison confined within Yellowstone's unfenced boundaries whatever the cost.

"We need to do whatever it takes in the state of Montana to make sure those (brucellosis) transmissions don't occur," said Jay Bodner of the Montana Stockgrowers Association.

By contrast, the activist Buffalo Field Campaign says Yellowstone's bison herd has already suffered "carnage."

"People should be very disturbed when the government comes in and starts executing wildlife," said Buffalo Field Campaign board member Scott Frazier.

The debate in America is mirrored in Canada, where officials are toying with killing off the 4,500 free-ranging bison at Wood Buffalo National Park because of the beef industry's fears about brucellosis.

Underlying the dispute about the Yellowstone herd is a perennial debate in the American West about public lands, pitting its cowboy culture against nature lovers.

The conflict is heightening even as tourism is eclipsing ranching as a leading economic engine and newcomers are flocking to the Big Sky State for its scenic beauty and abundant wildlife.

Cattle graze near Yellowstone mostly on federal acreage and Montana's ranchers fear the push to protect the park's bison is a thinly veiled attempt to drive them from that land.

"Some advocacy groups would like to see cattle off public lands and have it only used for wildlife," said Montana Department of Livestock Director Marc Bridges.

Bison advocates say the country's last wild herd cannot afford to be lost.