BILLINGS, Mont. -

State and federal agencies plan to start pushing bison into Yellowstone National Park on Tuesday in an annual event meant to keep the migrating animals away from cattle in southwestern Montana.
    
State Department of Livestock Executive Officer Christian Mackay said workers on horseback and ATVs plan to drive about 300 bison in the West Yellowstone are roughly 7 miles back into the park.
    
Unlike previous years, there is no plan to use a helicopter to assist in this year's efforts. That practice had drawn lawsuits from environmental groups but was upheld by a federal appeals court last year.
    
Bison migrate from the park to lower elevations in Montana in the winter. A federal-state agreement requires their return to the park in the spring to make way for cattle to graze.