Heeding their instincts, bison graze on new spring grasses along U.S. Highway 191 north of West Yellowstone after leaving Yellowstone National Park in late April. In less than two weeks, after moving across the highway onto Horse Butte, cows and weeks-old calves will be chased back into the park by helicopters, ATVs and riders on horseback — or, worse, rounded up for slaughter.