ADDITIONAL INDEX WORDS. bioenergy, biofuel, harvest date, lignocellulosic feedstock, nitrogen fertilization, carbon, cellulose, hemicelluloses, lignin, ash, and warm-season grasses
ABSTRACT. In the Midwest, native warm-season grasses have been used to fill summer deficiency in forage supply over past decades. Recently, warm-season grasses have been accepted as a potential biofuel feedstock in North America and elsewhere. However, information on interaction of harvesting and nitrogen fertilization to optimize quality in warm-season grasses grown as biofuel is limited. This study was conducted to determine biomass quality of four warm-season grasses as affected by varied nitrogen levels and cutting dates. A field study was conducted at Sorenson Farm (41°59' N; 93°55' W) near Ames, IA during 2006 and 2007. The experimental design was a split-split plot in time with four replications. The four warm-season grasses species, bigblue stem (Andropogon gerardii) eastern gamagrass (Tripsacum dactyloides L.), indiangrass (Sorghastrum nutrans), and switchgrass (Panicum virgatum) were main plots seeded in a randomized complete block. Three nitrogen levels (0, 65, and 140 kg/ha) were subplots and ten harvest dates were sub-sub plots, randomly assigned. The harvest dates were treated as repeated measures. The concentration of cellulose, hemicelluloses, carbon, lignin, and ash varied with grass species except for nitrogen when averaged over nitrogen rates and harvest dates were similar in 2006 and 2007. Nitrogen, carbon, cellulose, and lignin concentration in biomass of native warm-season grasses increased with greater rates of nitrogen fertilization whereas ash content in biomass of native warm-season grasses declined when nitrogen fertilizer was applied in both 2006 and 2007. Cellulose, lignin, and carbon concentrations in biomass of native warm-season grasses increased whereas ash and nitrogen concentrations decreased with harvest dates in both years. There were interaction effects between grass species and harvest dates on qualitative components indicating that each grass species had a different optimum harvest date for biomass quality. These results indicate that the warm-season grasses with nitrogen fertilizer applied at 140 kg N ha-1 and harvested at the end of season had the most potential quality for biomass production.