Bison are herbivores, meaning their diet is based entirely on plant matter. These massive mammals, which include both the American bison (Bison bison) and the European bison (Bison bonasus), are perfectly adapted to consume and process fibrous vegetation. As the largest land animals in North America and Europe, they play a defining role as large grazers within their respective prairie and woodland ecosystems.
Defining the Herbivore Classification
Within the broad classification of herbivores, bison are specifically identified as grazers, which are herbivores whose primary food source is grass (graminoids).
Grazers are distinct from browsers, which primarily feed on the leaves, soft shoots, or fruits of woody plants like shrubs and trees. The physical characteristics of bison, such as their broad muzzle, are adapted for cropping large mouthfuls of short prairie grass. While the American bison is a near-exclusive grazer, its European relative tends to incorporate a slightly higher percentage of browse into its diet due to its more wooded habitat.
Specifics of the Bison Diet
The bison diet is overwhelmingly composed of grasses and grass-like plants known as sedges. These plants, collectively called graminoids, can account for over 90% of a bison’s total food intake, especially during the spring and summer months.
A single adult bison typically ingests about 1.6% of its body mass in dry vegetation daily. For a large bull weighing 1,500 pounds, this translates to roughly 24 to 30 pounds of plant material consumed every 24 hours. This large intake volume is necessary to fuel their massive body size.
While their diet is grass-dominated, bison will supplement their intake with other plant types depending on availability and season. They consume forbs, which are broad-leaved herbaceous flowering plants, and occasionally woody plant leaves or lichens. This supplementary browsing is often a survival strategy during winter or periods of drought when the nutritional quality of grasses declines.
Biological Adaptations for Grazing
Bison possess a digestive system specifically designed to extract nutrients from a high-fiber, cellulose-rich diet. Like cattle and other hoofed mammals, bison are classified as ruminants, characterized by a complex four-chambered stomach. This system is necessary because the cell walls of grass are made of cellulose, which is difficult for most animals to break down.
The largest chamber, the rumen, functions as a massive fermentation vat where billions of specialized microbes, including bacteria and protozoa, reside. These tiny organisms produce enzymes that break down the tough cellulose into volatile fatty acids, which the bison then absorbs and uses as its main energy source. This process requires the animal to regurgitate and rechew its food, known as rumination or “chewing the cud,” to physically break down the fiber into smaller particles for the microbes to access.
A key difference in the bison’s digestive physiology is its longer retention time of feed compared to domestic cattle. Food material stays in the bison’s digestive tract for a longer period, around 78.8 hours, allowing the microbes more time to ferment and maximize nutrient extraction from lower-quality forages like sedges and mature grasses. The mechanical process begins with their dental structure, which features a dental pad instead of upper incisors and large, flat molars suited for the constant grinding of tough, abrasive grasses.