Switchgrass (Panicum virgatum L.) has been chosen by the US Department of Energy (DOE) as a model species for biomass energy production. While numerous studies have demonstrated the effect of climate and management factors on biomass yield and chemical characteristics of switchgrass monocultures, information is scarce regarding to mixtures of switchgrass and other species harvested for biomass energy. Therefore, the objective of this quantitative review was to determine the effect of climate and management factors on the yield of switchgrass monocultures, as well as mixtures of switchgrass and other species. We examined all peer-reviewed articles describing productivity of switchgrass and extracted dry matter yields, stand age, nitrogen fertilization (N), temperature (growing degree days) and precipitation/irrigation.

Results/Conclusions

Monocultures (397 observations) yielded an average 10.6 Mg ha^-1 and mixtures (85 observations) yielded an average 4.4 Mg ha^-1 for switchgrass and 6.9 Mg ha^-1 for total biomass (90 observations). There were differences between monocultures and mixtures in their responses to these variables. Both monocultures and mixtures showed a significant response to temperature and water. Monocultures, but not mixtures, showed a significant response to nitrogen. Monocultures showed a stronger response to nitrogen when the stand age was one or two than mature stands (stand age ≥ 3). Monoculture yielded an average 9.8 Mg ha^-1 for younger stands (1 or 2) and 11.0 Mg ha^-1 for mature stands. Mixtures with legume species yielded an average 9.9 Mg ha^-1 and 5.7 Mg ha^-1 without legume species. Switchgrass in the mixture yielded 8.7 Mg ha^-1 when grown with legume species, but yielded 3.2 Mg ha^-1 when grown with non-legume species.

Since maximal production of biomass from minimal inputs is required for a biomass crop to be both economically and energetically viable, these results suggest that N should be applied only to switchgrass monocultures, but not to mixtures, and for monocultures, N should be applied differently based on stand age.