What’s for Dinner?

The question is: what does a 2000-lb adult male bison eat?  We may be tempted to reply, “Anything he wants.”  We would be both right and wrong.  In some sense he does eat what he wants.  His biology, however, dictates what he wants.  So what are the favorite entrees of bison?

Bison diet tends toward grasses and sedges—grass-like plants with solid stems—but are now known to consume a wide-variety of plants including woody plants and herbaceous eudicots [1]. The eudicots are a diverse and abundant group of angiosperm plants. They include over half of the species of all plants and are found in a huge variety of habitats. Being angiosperms they are seed-bearing, vascular plants that produce flowers, fruit and pollen. They are also often woody plants and can grow into giant, long-living trees [2].  Examples of eudicots include dandelions, buttercup, maple, and magnolia. 

Sources of Protein Intake for Bison—Whatbisoneat.com

Eating from eudicots as opposed to grasses and sedges defines a species as a browser. Browsers glean leaves, bark, and green stems from plants, while grazers clip vegetation at or near ground level.  Browsing has a distinct advantage when grasses and other ground-level vegetation are covered by deep snow. The disadvantage of browsing is that height may make vegetation inaccessible. Another disadvantage is often-times browsers eat parts of the vegetation that are low in nutrients, chemically defended, or both. Grazers, such as sheep and cattle, can, when grasses are accessible, feed on the much more nutrient-rich meristematic [3] regions of the grasses [4].

The American Bison is not just a grazer, but also a browser.  However, their preference of entrees is for the grasses and sedges, and instinctually they seek the more nutrient abundant grasses.  Efficient acquisition of nutrients, then, is a primary driving force in this selection. Apparently, the bison evolved to take advantage of the nutrient production of grasses.

In the production of nutrients, and specifically proteins, plants require carbon.  The perennial grasses of the Great Plains and prairies are of two types—C3 or C4—in regard to carbon fixation (the acquisition and preparation of carbon for the production of nutrients). These terms refer to the different pathways that plants use to capture carbon dioxide during photosynthesis. All species have the more primitive C3 pathway, but the additional C4 pathway evolved in species in the wet and dry tropics. The first product of carbon fixation in C3 plants involves a 3-carbon molecule, while C4 plants initially produce a 4-carbon molecule that subsequently enters the C3 cycle.

The two pathways are associated with different growth requirements.  C3 plants are adapted to cool season establishment and growth in either wet or dry environments.   On the other hand, C4 plants are more adapted to warm or hot seasonal conditions under moist or dry environments.  A feature of C3 grasses is their greater tolerance of frost compared to C4 grasses.  C3 species also tend to generate less bulk than C4 species; however, feed quality is often higher than C4 grasses [5].

C3 grasses are more efficient fixing CO2 at cooler temperatures. However, at higher temperatures, above 90 degrees, C3 grasses have difficulty differentiating between CO2 and O2. In hot weather C3 grass can also catalyze the fixation of O2 in equal or greater portions as CO2. This is significant. When O2 is fixed, the process is called photorespiration, and the result is lowered carbohydrate production. When photorespiration occurs, growth slows, and without water, the grass will often go dormant.

C4 grasses—warm season grasses—on the other hand, are more efficient at CO2 fixation in high temperatures. This occurs because C4 grasses use a different enzyme than C3 and attach the CO2 to a different compound making up the 4-carbon compound. While it takes more energy for C4 to produce carbohydrates than C3, because of the extra steps involved, the process proceeds without photorespiration with the end result a greater carbohydrate production [6]. 

The differences between the C3 and C4 grasses become significant when considering bison dietary habits and the warming trend occurring across the Great Plains. Under the warming conditions, the nutrient production of C3 grasses becomes less efficient, and with the increased frequency of associated droughts [7], may even go dormant. The C4 grasses may then increasingly become a larger portion of the bison diet, since these grasses do well in higher temperature.  However, studies reconstructing seasonal variation in the diet of North American bison in two grasslands found the bison in the warmer grassland consumed a lower proportion of C3 grasses, but not a greater proportion of C4 grasses.   Instead, bison turned more to the N2 fixing eudicots, i.e., shrubs and forbs, which comprised as much as 60% of their protein intake.  The N2 fixing eudicots contain a higher concentration of protein than either C3 or C4 grasses, which may explain why bison turn to eudicots as a nutritional source over C4 grasses, when C3 grasses are not as abundant.  As the warming trend continues, buffalo may have to compensate through the browsing of eudicots.  Minimizing the warming-imposed nutritional stress may require the promotion of high-protein eudicots or the facilitation of increased protein concentrations of grasses [8]. 

In the previous blog, “The Incredible Shrinking Bison,” the conclusion emphasized land and vegetation management development. That effort requires an understanding of why bison turn to shrubs and forbs as opposed to consuming a greater proportion of C4 grasses under warming conditions.  In addition, research into how to promote greater protein production and concentration in grasses and eudicots is critical.

End Notes:

[1] Flowering plants with two seed leaves upon germination as opposed to monocots such as grasses and grass-like flowering plants.  Eudicots. Wikipedia. Retrieved 19-March-2021.

[2] Eudicots | Basic Biology.  Basicbiology.net. Retrieved 19-March-2021.

[3] Meristem. Botany.  Embryonic tissue growing, actively dividing in plants.  Webster’s Unabridged Dictionary of the English Language. 2001. Random House.

[4] Browsing versus Grazing (animalbehavioronline.com).  Animalbehavior.com. Retrieved 19-March-2021.

[5] What are C3 and C4 Native Grass? (nsw.gov.au) . Department of Primary Industries. Retrieved 19-March-2021.  Also: Difference Between C3 and C4 Plants | Compare the Difference Between Similar Terms.  Differencebetween.com. Retrieved 19-March 2021.

[6] Understanding C3 & C4 Grasses | AgriGro®.  Agrigro.com.  Retrieved 19-March 2021.

[7] Craine, J. M., Nippert, J. B., Elmore, A. J., Skibbe, A. M., Hutchinson, S. L., & Brunsell, N. A. (2012). Timing of climate variability and grassland productivity. Proceedings of the National Academy of Sciences, 109(9), 3401– 3405. https://doi.org/10.1073/pnas.1118438109. Retrieved 19 March 2021. Also Martin, Jeff M., Perry S. Barboza. 06-Dec-2019.

[8] Craine, Joseph M., E. Gene Towne, Mary Miller,  & Noah Fierer. (16-Nov-2015). Climatic warming and the future of bison as grazers. Scientific Reports. Nature.  https//www.nature.com/scientific reports. Retrieved 19-March-2021.

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