Bison, often called buffalo in North America, are large, hoofed mammals that once roamed the continent in vast numbers. These iconic animals are herbivores, primarily grazing on grasses and sedges. Understanding their diet is crucial to understanding their ecological role and conservation needs.
Bison Species and Characteristics
The genus Bison within the tribe Bovini includes two extant species: the American bison (Bison bison) and the European bison (Bison bonasus), also known as the wisent. The American bison is further divided into two subspecies: the plains bison (Bison bison bison) and the wood bison (Bison bison athabascae).
American bison are the largest terrestrial mammals in North America. Male bison, or bulls, are significantly larger than females, or cows. A mature bull can reach 5.5 to 6.5 feet high at the hump and 9 to 12.5 feet in length, while females typically measure 5 feet high at the hump and 7 to 10 feet in length.
Bison have a distinctive hump over their front shoulders and slimmer hindquarters. Both males and females have sharp, pointed horns that curve outward and up from their heads. They have cloven hoofs, enabling them to reach speeds up to 30 miles per hour. Their shaggy coat consists of a long, dark-brown winter coat and a lighter-weight, lighter-brown summer coat. The head, neck, forelegs, and front parts of the body are covered with a thicker coat of long, dark hair. Bulls have a black beard that can measure about one foot long. The rear part of the body is covered with shorter hair. The shaggy head is the most heavily insulated part of their body, helping them withstand blizzard conditions.
Grazing Habits and Nutritional Needs
Bison are herbivores, primarily grazing on the grasses and sedges of the North American prairies year-round. While they feed primarily on grasses, they also consume flowering plants, lichens, and woody plant leaves. They typically ingest 1.6% of their body mass per day of dry vegetation. Their daily schedule involves two-hour periods of grazing, resting, and cud chewing, before moving on to a new location to graze again. In the winter, they find grass by sweeping their head from side to side to clear snow.
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Bison are ruminants, able to ferment cellulose in a specialized stomach prior to digestion, much like cows and elk. Their four-chambered digestive system allows for the absorption of cellulose, a fibrous plant material that is hard to breakdown. This enables them to efficiently extract nutrients from grasses and other plant matter. It takes about 80 hours for grass to pass through their digestive systems, allowing ample time for nutrient absorption.
Seasonal Diet Variety
The diet of bison is surprisingly variable and dynamic throughout the growing and non-growing seasons. While often thought of as quintessential grazers, they also snack on non-grass species throughout the year. Bison consume a widely variable diet, with a greater proportion of cool-season grasses (C3) and non-legume forbs early in the growing season, shifting towards more warm-season grasses (C4) and leguminous forbs later on.
Research has shown that bison diet composition by volume averages nearly 91% grasses, even though the variety of grass species is less than 50% of the total diet. Despite the dominance of grasses, bison also consume a variety of forbs and browse. Forbs and browse have more lipids than grasses, and browse, forbs, and grasses have nearly the same levels of protein, with forbs leading the way, for bison.
Methods for Studying Bison Diets
Scientists use a combination of techniques to determine what bison eat. One powerful tool is eDNA (environmental DNA). By analyzing bison fecal materials, researchers can extract and sequence DNA fragments, identifying the specific plant species the bison consumed - even those that are quickly digested. This is valuable because some species are highly digestible and leave no physical remains behind for visual identification, especially early in the growing season. For that, researchers use techniques like microhistology (microscopic examination of plant fragments in processed fecal or rumen matter). This technique is labor-intensive and requires expert knowledge, but it allows for volume correction, which is necessary to fully understand how much biomass of each forage species was consumed.
Impact on Prairie Ecosystems
Bison are considered generalist foragers, meaning they eat a wide array of herbaceous grasses and sedges commonly found in mixed-grassed prairies. These types of plants include species such as Blue gramma, sand dropseed, and little bluestem. Although bison graze heavily on grass species, they will occasionally consume woody vegetation when food is limited.
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By feeding mostly on grass species and selectively avoiding other plants, bison influence the local prairie biodiversity. Bison create a spatial mosaic pattern of grazed and ungrazed areas. This "quilt" can often exemplify an area after a bison herd forages, leading to high plant biodiversity and an increase in gas exchange, biomass, and plant cover.
Bison are also attracted to recently burned areas, therefore, influencing plant diversity. After a disturbance, such as a wildfire, grasses establish before other plant species. Bison prefer these regrowth areas because they have a plethora of grasses available to them without having to graze selectively around woody plant species. By grazing in these new grass-dominated sites, bison help increase the local diversity.
Bison grazing patterns influence the prairie ecosystem. Their foraging is important for plant community structures, because woody vegetation can flourish in a grass-dominated landscape. Selective grazing by bison can result in a diverse, heterogeneous landscape of plant species.
Bison Meat and By-Products
There is a strong demand for bison meat and by-products, and prices are consistently higher than for cattle. Bison meat is generally considered to taste very similar to beef but is lower in fat and cholesterol, yet higher in protein than beef. Several pet food companies use bison as a red meat alternative in dog foods.
The delicious taste of bison begins on the native rangelands and pastures of North America. Mother Nature perfected bison over thousands of years to produce flavorful, healthy meat from the native grasses and other plants in our ecosystem. Bison spend the majority of their lives on these native pastures. In the last few months before the meat is harvested, many bison are finished on a diet of grains, forbs, and roughage. This provides a consistent product that many customers desire. An increasing number of bison today are kept on pasture throughout their lives. Producing grass-finished bison is challenging because of the variable nature of our seasons, grasses, and because of periodic droughts. All bison are raised without questionable drugs, chemicals, or growth hormones.
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Conservation and Current Status
Prior to 1800, bison were widespread and numerous throughout much of western North America. They were a major source of meat and hides in the United States, and formed the basis of the economy for a number of Native American tribes. Bison hunting was an activity fundamental to the Indigenous peoples of the Great Plains.
By the 1890s, there were fewer than 1,000 American bison left on the continent. The U.S. government slaughtered many bison in an organized effort to destroy the livelihood of Plains Indians during the American Indian Wars.
Since the species neared extinction in the late 1800s, American bison have made a comeback. But the species is still heavily dependent on conservation action for survival. Conservation threats to American bison include habitat loss, hybridization in managed populations, and low genetic diversity among individual herds.
Many conservation measures have been taken by Indigenous communities. The Intertribal Bison Council (ITBC) is one of the most significant efforts. It is a collection of 80 tribes and 20 states, which facilitate the management of over 20,000 buffalo.
Of the remaining American bison population, approximately 500,000 individuals are managed as livestock by private commercial ventures, while conservation herds are comprised of around 30,000 individuals. According to IUCN Red List guidelines, commercial herds are not eligible for inclusion in the IUCN's scope of wildlife conservation, so the IUCN status of bison reflects only those found in the 65 known free-ranging or partially-free-ranging herds.