A large herd of bison crossed out of the west side of Yellowstone National Park on Monday putting them in danger of being killed.

The bison are headed to the Horse Butte Peninsula for spring calving, said Mike Mease, of the Buffalo Field Campaign, a group that objects to killing the bison. Horse Butte is in the Gallatin National Forest, just west of Yellowstone.

"It is a common migration route because pastures green up faster on the western side of the park," said Hope Sieck, associate program director for the Greater Yellowstone Coalition. "We've been expecting this movement."

The Montana Department of Livestock (DOL) sent 231 wild bison to slaughter earlier this month. Those bison were captured by the Park Service inside of Yellowstone National Park near its northern boundary, Mease said. Although the 231 bison were tested for brucellosis immediately before slaughter, test results had no bearing on what animals were slaughtered or spared, he said.

The Interagency Bison Management Plan calls for killing bison that wander outside the park if the size of the herd is over 3,000. Rick Wallen, team leader for the bison program in Yellowstone, estimated that the park has 3,600 bison right now.

That means any bison in the Horse Butte area may be killed. Horse Butte is a special area in which disease-free bison will be tolerated. If populations figures are less than 3,000, b bison there are generally either hazed back into the park or captured and tested for brucellosis, with disease-free animals sent back into Yellowstone, and diseased animals sent for slaughter.

The management plan was developed to reduce the risk of transmission of brucellosis from bison to cattle. The disease may cause cattle to abort fetuses.

Sieck noted that although there has never been a documented case of the brucellosis bacterium being transmitted in the wild from bison to cattle, the DOL believes that could happen. Mease said the Department of Livestock has spent nearly $3.5 million since 1996 on bison management operations that have killed 2,064 wild bison.

The expenditure of all that money, just to protect a few hundred Montana cattle, seems unreasonable, said Sieck. She noted that bison and cattle graze together freely in Wyoming, without problems.

To save tax dollars, the Greater Yellowstone Wildlife Alliance has another idea, Sieck said. She urged Montana to find alternative grazing allotments for the few hundred cattle near the park and open public land adjacent to Yellowstone to buffalo. The end result would be no risk of disease to cattle, the saving of tax dollars and tourists drawn to see the free roaming bison beyond Yellowstone's borders.

Sieck said DOL personnel had been in West Yellowstone recently for a meeting, but were not present on Monday when the bison left Yellowstone. "I imagine they'll be back pretty soon," she said.

Karen Cooper, DOL spokeswoman, was out of her office and unavailable for comment on Monday. All inquiries for comment or further information were referred to her office.