Bison are herbivores and eat simple foods. The bison's main foodstuff is grass, though they will eat any available low-lying shrubbery as well as sedges. In the winter, bison forage for grass under the snow. If there is little grass available, they will eat the twigs of shrubs.
Bison are notably better browsers than cattle, since cattle are more obligate grazers, even though wood bison have also been described as "obligate grazers". Wisent tend to browse on shrubs and low- hanging trees more often than do their cousins, the American bison. American bison prefer grass to shrubbery and trees.
Over the course of the year, bison shift which plants they select in their diet based on which plants have the highest protein or energy concentrations at a given time and will reliably consume the same species of plants across years. Protein concentrations of the plants they eat tend to be highest in the spring and decline thereafter, reaching their lowest in the winter.
Due to their size, bison have few predators. Five notable exceptions are the grey wolf, human, brown bear, coyote, and grizzly bear. The grey wolf generally takes down a bison while in a pack, but there have been cases of a single wolf killing bison.
Brown bear also prey on bison calves, often by driving off the pack and consuming the wolves' kill.