Theoretical Ethanol Yield for Potential Bioenergy Sorghum Genotypes of Differing Compositions.

Sorghum bicolor (L.) Moench has the potential to be bred as specialized multipurpose varieties that may be attractive and beneficial for the production of ethanol.  The objective of this study was to evaluate theoretical ethanol yield (TEY) determined by biomass yield and composition of sorghum genotypes as influenced by environment and N application rate. Nine sorghum genotypes and a maize hybrid (Zea mays L.) control were evaluated for grain and stover yield and stover composition over four N application rates (0, 67, 135, and 202 kg N ha^-1) and over two years (2008 and 2010). Stover composition was analyzed for total non-structural sugars and fibers as determined by neutral and acid detergent fibers, and acid detergent lignin, hemicelluloses and cellulose concentrations. TEY was estimated by using standard, industry-based stoichiometric conversion. A two-way ANOVA was used to interpret the effects of genotype, N- application rate, and environment on TEY. Grain and stover yield significantly varied with genotype, N rate, and environment, but stover composition only significantly varied with genotype. Theoretical ethanol yield for all genotypes over all N-rates ranged between 3,000 – 20,000 L ha^-1. The highest TEY was from the photoperiod sensitive sorghum (IS7777), although it had no grain yield, it yielded the most biomass, where fiber was the greatest contributor to the TEY. The low lignin varieties had the lowest TEY. Market influence for specific ethanol sources calls for attention to plant composition. The grain TEY, currently expected to be the most economically beneficial ethanol, ranged up to 6,000 L ha^-1 with commercial maize and sorghum genotypes producing the highest TEY. The sweet sorghum variety and IS7777 produced the highest TEY from sugar ranging up to 4,000 L ha^-1. The highest grain and sugar ethanol yields for all the genotypes were not always associated with the highest N-rates.