BILLINGS, Mont. (AP) — The annual effort to haze migrating wild bison back into Yellowstone National Park to make way for cattle is under way in southwest Montana.

Christian Mackay with the Montana Department of Livestock says workers began pushing about 270 bison into the on Wednesday. The animals are being driven from public and private land near Hebgen Lake using a helicopter and other means.

Bison migrate to lower elevations in Montana in the winter. But a federal-state agreement requires their return to the park in the spring, when cattle are brought in for grazing.

Officials say they are guarding against animal disease transmissions to cattle.

But critics of bison hazing say the practice is unnecessary and the risk of infection is low if cattle are properly managed.