Sustainability of Perennial Grass Yields as Bioenergy Feedstock for the Southeast.

Warm-season perennial grasses will be part of the biomass production system in the Southeast for the emerging bioenergy industry.  Among the candidates for dedicated feedstocks are energy cane (Sacchurum sp.), Miscanthus x gigantius, switchgrass (Panicum virgatum), and napiergrass (Pennistem purpureum). Two studies have been conducted to assess yields, and assess soil carbon and nitrogen.  One study was initiated in fall 2005 at Tifton, GA, to assess the performance of perennial grasses under rainfed conditions with no fertilizer inputs.  The test consisted of four replications in a randomized complete block design, and included two energycane cultivars, two napiergrass genotypes, and two switchgrass genotypes.  A second study initiated in the fall of 2006 consisted of napiergrass grown under three rainfed fertilizer treatments (no additions, poultry liter and inorganic fertilizer at approximately equivalent N, P, and K rates). Total shoot biomass was harvested and weighed each year in winter, and was analyzed for total carbon and nitrogen content.  Soil samples were collected periodically to assess possible changes in soil carbon and nitrogen.  Under no inputs dry matter (DM) yield was highest in the second year.  Averaged over three years, DM yields of energycane, and napiergrass were significantly higher than switchgrass.  Switchgrass had higher nitrogen use efficiency, based on the nitrogen content of the harvested plants.  Poultry liter and inorganic fertilizer treatment of napiergrass resulted in similar yields each year and were 17% and 48% greater than the unfertilized control in the second and third year of growth, respectively.  Carbon/nitrogen cycling and water quality were monitored and will be discussed as well.