SWITCHGRASS BIOMASS COMPOSITION DURING MORPHOLOGICAL DEVELOPMENT IN DIVERSE ENVIRONMENTS

Production of herbaceous biomass for biofuel seeks to maximize lignocellulose yields and minimize N and mineral (especially K) concentrations in the biomass. Our objective was to relate changes in biomass composition of switchgrass (Panicum virgatum L.) to thermal time [degree days (DD), base = 10 degrees C] in several environments. 'Alamo' switchgrass at Stephenville, TX, was sampled weekly or every 2 wk from April to June in 1989 and April to September in 1990, 1991, and 1992. Field plots of Alamo and 'Cave-in-Rock' at Stephenville were sampled 17 times during 1993, and at Blacksburg, VA, were sampled on 11 dates in 1992 and nine dates in 1993 from May to September. Biomass samples were analyzed for lignocellulose, crude protein, total ash, K, and Ca concentrations. Linear and nonlinear regressions were used to relate concentrations of constituents to cumulative DD after burning in spring. Concentrations of each constituent were closely related to cumulative DD and a single equation was able to describe the relationship across several yeats. At Stephenville, lignocellulose concentrations increased linearly up to 800 to 1000 DD. At Blacksburg, lignocellulose concentrations increased linearly up to 400 to 800 DD after burning. At both locations, the time at which the rapid increase in lignocellulose concentration ended coincided with internode elongation. Crude protein, ash, and K concentrations decreased curvilinearly with accumulated DD. There were no differences in composition between Alamo or Cave-in-Rock at Blacksburg. At Stephenville, Alamo had a greater concentration of lignocellulose than did Cave-in-Rock, but similar concentations of crude protein, ash, and K. The close relation between DD and composition of switchgrass could be used for predictive purposes.