Summary
A 6-year study was conducted on bluestem Andropogon-dominated tallgrass prairie in the Flint Hills of Kansas (central USA) to assess the effects of different cattle stocking densities and grazing systems on plant community composition and diversity.
Prairie sites were grazed during the growing season (May to October) at three cattle stocking densities (3.8, 2.8 or 1.8 ha/animal unit), under a late-season rest rotation grazing system at the same range of stocking densities, and with some sites ungrazed as controls for comparison.
Plant communities were sampled twice each season using a nearest-point method.
growth form diversity were significantly higher in grazed compared to ungrazed sites, and diversity was greatest at the highest cattle stocking density. There were no significant effects of grazing system, regardless of stocking density, on plant diversity.
Increasing cattle densities reduced dominance of perennial tall grasses (e.g. big bluestem Andropogon gerardi and little bluestem A.scoparius), whilst warm-season perennial mid-grasses increased. Grazing treatments had little effect on perennial forb frequency. Annual forb abundance varied from year to year regardless of grazing treatments (perhaps primarily due to variations in precipitation), although there was a tendency towards highest frequency under intermediate stocking density.
Some species appeared to respond principally to direct effects of being grazed, whilst others to indirect effects such as reduced competition from preferentially grazed species.