Each winter some wild bison leave Yellowstone and migrate across Park boundaries to traditional winter range following the Yellowstone and Madison Rivers. They migrate in search of winter habitat and spring calving grounds.
Today, under the banner of "disease risk management" and the Interagency Bison Management Plan, Montana Department of Livestock inspectors and National Park Service Rangers intercept and harass buffalo off of their winter range and spring calving grounds and capture them in a slaughter program that has destroyed more than 6,000 wild bison since the year 2000.
Arbitrary boundaries are drawn and America's last buffalo are hazed, shot, or slaughtered for stepping across a line into Montana, Idaho, and Wyoming.
The undocumented claim by the state of Montana and Yellowstone National Park is grazing cattle in the buffalo's range are at risk of contracting brucellosis - a disease introduced by exotic cattle to native elk and buffalo before 1917. Buffalo calves captured from the wild were "mothered with domestic bovine cows" and pastured with cattle that were brought into Yellowstone to feed Park tourists.
Recent investigations of brucellosis transmission to cattle in the Yellowstone ecosystem indicate that elk, and not buffalo, are the source of infections in Montana, Wyoming and Idaho.
There has never been a documented case of wild buffalo transmitting brucellosis to livestock.
See also Buffalo Science > Yellowstone Bison and Brucellosis: Persistent Mythology