Long-Term Evaluation of Plant Species Diversity Effects on Rotationally Grazed Pastures.

Many short-term studies have addressed the effects of plant species diversity on forage yield and other ecosystem services but long-term studies are less common. This project compared species composition, forage yield and soil C sequestration between rotationally grazed pastures initially sown to two (orchardgrass-white clover) or five (orchardgrass-white clover-tall fescue-alfalfa-chicory) forage species. Pastures were typically grazed five times per year for nine years from mid-May to mid-October by beef cattle. Averaged across all nine years, the 5-species mixture had 31% greater annual yield than the 2-species mixture and had greater yield in all but the first year. By year 8 of the study, changes in species composition in both treatments resulted in both treatments becoming nearly identical orchardgrass-tall fescue-Kentucky bluegrass-white clover mixtures. Despite the lack of differences in species composition, the original 5-species mixture had 41% greater yield than the original 2-species mixture during the last two years of the study. Soil C sequestration rates were also significantly greater in the 5-species mixture, averaging 180 g C m^-2 yr^-1 compared with 60 g C m^-2 yr^-1 in the 2-species mixture. Initially beginning with a more complex mix of species had long lasting benefits that continued even after substantial differences in species composition had ceased to exist.