Unveiling Bison Summer Diet Selection at Northern Fringe of Historical Distribution Back »

Figure 1. Adult female bison on a cold, autumn morning in Manitoba, Canada. Photo by J. L. Leonard.

Written by Joshua L. Leonard under the direction and review of Jonathan Jenks.

Bison were historically distributed throughout North America with the Northern edge of the distribution occurring in North Central Manitoba and surrounding provinces. Despite occupying the boreal zone of North America, little is known of bison forage selection patterns when occupying a densely forested aspen ecosystem. In a global review of more than 60 ruminant species, bison were classified as grass-roughage feeders primarily consuming grasses, sedges and rushes. We were interested to determine if this was similar for bison inhabiting north central Manitoba.

Examining Bison Forage Selection Patterns

During June-August 2015, we initiated a study on Olson’s Conservation Bison Ranches, Pine River Ranch, Pine River, Manitoba, Canada, to examine forage selection patterns for bison. We thought that vegetative composition of bison diets would be consistent with availability, diets would shift along with forage availability, and bison diets would predominately consist of grass and sedge species.

We opportunistically collected adult female fecal samples and identified forage composition using the DNA barcoding method; which typically identifies plant fragments within feces to the genus or species level. We estimated percent cover of grasses, forbs, browse, sedges, and rushes from 0-100%, in 5% increments, to determine the amount of forage available to bison. We then compared bison use (% plant composition of diets) to availability (% plant cover on the ground) to determine if bison were actively seeking out forages or randomly consuming forages as they were encountered.

Bison Diet Makeup

Overall, bison diets were comprised of 44% grass, 38% forb, 16% browse, and less than 2% sedge and rush. Forage availability was comprised of 51% grass, 28% forb, 2% browse, 11% sedge, and 8% rush. Statistical analyses indicated that bison consumption differed from availability for each forage group throughout the summer, as predicted. However, bison diets and availability of grasses and forbs were inversely related as the summer progressed. Grasses became more available throughout summer, whereas grass composition in bison diets decreased over summer, while forbs were opposite of that.

Grasses & Forbs: Grasses and forbs were important dietary components for bison because combined, they comprised > 80% of bison diets. Bison actively selected grass during June, but avoided grass during July and August, whereas bison selected forbs during July and August, but avoided them in June.

Sedges: We predicted sedges would be an important food item during the summer, but they appeared to be unimportant, which we did not expect given bison’s classification. Although sedge may not be an important summer food item, previous research has shown that bison in similar habitats almost exclusively forage on sedge during the winter.

Browse: Browse comprised 16.3% of all bison summer diets, which is unusually high considering bison do not typically utilize woody vegetation, unlike cattle. Moreover, our browse selection indices (more than 6.00) were extraordinarily high in comparison to previously reported indices of 0.25 and 0.10 derived from research in Central South Dakota. These high indices may result from a lack of sampling forest vegetation, since in our study forests were not sampled since research has shown that bison spend 80% of their time in or within 75 ft. of meadows. Nevertheless, our results suggest that future sampling of forest vegetation may be advantageous to determine if browse is an essential forage class for bison or if it is being incidentally consumed.

In Summary

Our results indicated that bison consumed more browse and other low cellulose (forbs), high cell soluble forages to meet their dietary needs. Thus, suggesting that domestication of bison may have resulted in foraging behaviors more similar to elk and other intermediate feeders than that of cattle or sheep, especially at the Nnorthern edge of the historical distribution of the species.

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