Yellowstone’s largest elk herd winters along and north of the park boundary. After decades of debate over whether this range was overgrazed by too many elk, some concern has shifted to the herd’s small size. NPS/Diane Renkin. Site Index. Grazing and Migration. Grasses are an important part of the diet of most ungulates (hoofed animals) in Yellowstone. Bighorn sheep, bison, and elk rely on grasses for 50–80% of their food. Ungulates in this temperate mountain environment migrate seasonally and move selectively across the landscape following the new growth of grasses, when forage is at its most nutritious.