Seasonal changes in chemical composition and leaf proportion of elephantgrass and energycane biomass

Abstract Changes in chemical composition of warm-season perennial grasses during the growing season affect conversion of biomass to biofuels, thus influencing choice of harvest date. The objective was to quantify these changes for three candidate bioenergy grasses in the USA Gulf Coast region during two growing seasons and relate them to optimal harvest management. Grasses included two elephantgrass [ Pennisetum purpureum Schum.; synonym Cenchrus purpureus (Schumach.) Morrone] entries, ‘Merkeron’ and breeding line UF1, and the energycane ( Saccharum spp. hybrid) cultivar ‘L79-1002’. Quantification of cell wall constituents and mineral composition of above-ground biomass occurred monthly throughout the growing season. With the exception of hemicellulose, elephantgrass cell wall constituents (cellulose, lignin, neutral detergent fiber and acid detergent fiber) increased from early in the growing season until late summer and either remained relatively constant (UF1) or increased slightly (Merkeron) during the remainder of the season. In contrast, concentrations of energycane cell wall constituents peaked in late summer and decreased during the remainder of the growing season. Nitrogen, P, and ash concentrations decreased with increasing maturity for all grass entries, and they were much greater in leaf than in stem. Elephantgrass leaf, particularly of UF1, contributed less to total biomass harvested than energycane leaf. Likewise, the proportion of total ash harvested that was in the leaf fraction was greater for energycane than for elephantgrass when harvest occurred late in the growing season. Thus, delayed harvest until late in the season was generally a superior management strategy for elephantgass because it resulted in biomass with greater cell wall constituent concentrations, lesser leaf percentage, and lesser concentrations of N and ash, all of which may provide advantages in some conversion processes. In contrast, greater accumulation of extractives by energycane and greater lignin concentration in elephantgrasses late in the growing season may reduce efficiency of some conversion methods.