Compositional changes during storage of large round switchgrass bales

'Alamo' switchgrass (Panicum virgatum L.) was harvested in two different years and stored as large round bales, unprotected outside, for 26 weeks. Fresh and scored switchgrass samples were analyzed for their structural and non-structural constituents. These summative analyses of the switchgrass showed differences in the composition of materials harvested at different rimes and indicated that the compositions changed during storage. On a dry weight, whole biomass basis, significant differences were observed in the extractives (+3%), ash (+1%), hemicelluloses (-1.4%), and cellulose (-4%) components of the switchgrass harvested in October 1991 relative to August 1992. The bales from the switchgrass harvested in October 1991 were exposed to high rainfall (65 cm) during storage and experienced much greater weathering than bales from the second harvest. The largest change in composition occurred in the ethanol-extractable component. The switchgrass showed a significant loss of extractives in both the weathered outer layer (11%) and the unweathered inner fraction (8%) of the bales prepared from the first harvest. Switchgrass harvested in August 1992 underwent much smaller compositional changes compared to the October 1991 harvest. The extractives in switchgrass and other grasses may contain substantial amounts of fermentable carbohydrates. If so, their loss during storage could have a significant impact on the conversion of switchgrass into ethanol. This is an example of the feedstock assessment research being conducted to determine the influence of harvest and storage conditions on the thermochemical and biochemical conversion of various biomass feedstocks into fuels and chemicals. The results of this research are expected to provide a guide to agricultural practices that optimize production of feedstocks for conversion into fuels. (C) 1996 Elsevier Science Ltd.