Competition between palatable and unpalatable prairie grasses under selective and non-selective herbivory in semi-arid grassland

In a small plot study we used a systematic additive design to evaluate at what severity and frequency of defoliation sideoats grama (Bouteloua curtipendula) would remain competitive and recover from defoliation while growing with meadow dropseed (Sporobolus compositus). In the 3-year study plants were transplanted from pots into the experimental design when equal in size. When B. curtipendula was selectively defoliated moderately or severely, at both 30- and 60day intervals, it still produced as much as when undefoliated growing with undefoliated S. compositus. Moderately defoliated B. curtipendula did not reduce biomass yield of S. compositus growing with it compared to when S. compositus was growing alone. When both plants were non-selectively defoliated severely, S. compositus responded very weakly and B. curtipendula recovered at the same rate as it had under the other defoliation treatments. The total biomass produced by both species growing together was highest when B. curtipendula was moderately defoliated. As soil moisture was not limited in this experiment we do not believe our results refute the hypothesis that selective grazing by livestock in these ecosystems has contributed to a reduction of B. curtipendula relative to S. compositus. Although S. compositus was reduced more than B. curtipendula by heavy nonselective defoliation this treatment is unlikely to produce desirable results in the long term as the highest total vegetation production occurred when B. curtipendula was moderately defoliated.