Management of warm-season grass mixtures for biomass production in South Dakota USA

Switchgrass (Panicum virgatum L.), big bluestem (Andropogon gerardii Vitman), and indiangrass (Sorghastrum nutans (L.) Nash) are native warm-season grasses commonly used for pasture, hay, and conservation. More recently switchgrass has also been identified as a potential biomass energy crop, but management of mixtures of these species for biomass is not well documented. Therefore, the objectives of our study were to: (1) determine the effects of harvest timing and N rate on yield and biomass characteristics of established warm-season grass stands containing a mixture of switchgrass, big bluestem, and indiangrass, and (2) evaluate the impact of harvest management on species composition. Five N rates (0, 56, 112, and 224 kg ha^-1 applied annually in spring and 224 kg ha^-1 evenly split between spring and fall) and two harvest timings (anthesis and killing frost) were applied to plots at two South Dakota USA locations from 2001 to 2003. Harvesting once a year shortly after a killing frost produced the greatest yields with high concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) along with lower concentrations of total nitrogen (TN) and ash. This harvest timing also allowed for the greatest percentage of desirable species while maintaining low grass weed percentages. While N rates of 56 and 112 kg ha^-1 tended to increase total biomass without promoting severe invasion of grass and broadleaf weed species, N application did not always result in significant increases in biomass production. Based on these results, mixtures of switchgrass and big bluestem were well suited for sustainable biomass energy production. Furthermore, N requirements of these mixtures were relatively low thus reducing production input costs.