Quality Vs Quantity in Napiergrass Harvest Alternatives.

Napiergrass (Pennisetum purpureum (L) Schum) has been shown to be a high yielding bioenergy feedstock southeastern United States.  It is also an excellent forage crop for much of the tropics. Multiple cuts or harvests produce more leaf matter and may be better in animal feed and for biochemical conversion to useable sugars.  In this study, two harvest intervals and two fertilization practices have been evaluated for the past four years at Shellman, GA.  Material from each replicated plot was tested for chemical composition, theoretical ethanol yields, and enzymatic sugar conversion efficiencies.  The sample set consisted of material from the second and fourth year of established plots with three treatments.  Low-moisture ammonium hydroxide (LMLA) was selected as the pretreatment strategy to prepare the biomass for enzymatic conversion.  Pretreatment conditions were optimized for enzymatic sugar conversion using a central composite design and a single selected sample; these were: 20% g ammonium per g biomass, 110°C for 2 days.  Biomass yields ranged from 39.4 to 34.5 Mg/ha dry matter, with no significant differences between treatments the first two years.  However, the yield of the two harvest system declined significantly in years 3 (24.7 Mg/ha) and 4 (19.5 Mg/ha) while the one harvest plots maintained high yields (40.1 and 44.3 Mg/ha).  In year two the two-harvest system producing 10% higher biomass and 16% higher glucose yields after ammonia pretreatment.  Estimated ethanol yields were 8,500 and 10,200 L/ha for one and two harvest systems, respectively, in year 2.