Field productivities of Napier grass for production of sugars and ethanol.

Napier grass (Pennisetum purpureum (L) Schum) is being developed as a bioenergy crop for production in the southeastern United States.  An important criterion for selecting a crop is establishing a consistent and dependable source of feedstock.  In this study, we considered the effects of fertilizer application and cutting regimes on production yield, chemical composition, and process yields.  Napier grass was grown for 4 years in field plots (Shellman, GA) with three treatments, which were selected to maximize production yields, and replicated four sub-plots.  It was observed that multiple seasonal cuts negatively impacted production yield by 21% over the total 4 years.  Samples from year 2 were analyzed for soluble sugars, starch, structural carbohydrates, uronic acids, and Klason lignin.  Total carbohydrate yields were 20.1-23.3 Mg/ha.  Samples were pretreated with low moisture ammonia hydroxide (110°C, 2 days) and evaluated for conversion to sugars using commercial cellulases as well as to ethanol using separate hydrolysis and fermentation with Scheffersomyces stipitis.  Extracted sugar yields were 14.8 – 19.1 Mg/ha. Glucose and xylose enzymatic conversion efficiencies were 59– 93% for glucose and 67-77% for xylose.  Ethanol yields were 8.56 – 10.3 kilolitre/ha and metabolic conversion efficiencies were 73.7 – 84.8%.  Statistical differences were observed for yields among the three crop management strategies.  Samples harvested in year 4 are now being analyzed for comparison.  In general, these results suggest that Napier grass gives commercially attractive sugar and ethanol yields on a land basis.