Impact of Harvest Frequency on Biomass Yield and Nutrient Removal of Elephantgrass, Giant Reed, and Switchgrass

Although perennial grasses are being evaluated as a renewable source of biomass for energy production in many countries, no information exits regarding the use of these grasses in Uruguay. In 2008, an 8-year field study was implemented in western Uruguay to determine harvest frequencies for optimal biomass yield and nutrient removal for selected grass species. Elephantgrass (Pennisetum purpureum Schum.), giant reed (Arundo donax L.), and switchgrass (Panicum virgatum L.) were compared using two harvest frequencies: a single harvest after freeze (August) or two harvests (January and August) per year. We evaluated biomass yield, moisture content, nutrient concentration, and nutrient removal of these grasses. This study demonstrated the ability of these grasses to produce high biomass yields. Across years, the double harvest system significantly reduced cumulative biomass yield (similar to 15%) compared to the single harvest of elephantgrass and giant reed; however, switchgrass had 18% more biomass yield (12.70 Mg ha(-1) year(-1)) than the other grasses at the summer harvest but no cumulative difference was detected. The single winter harvest of elephantgrass had the highest cumulative biomass yield (140.8 Mg ha(-1)) and total nutrient removal (563 k N ha(-1), 199 kg P ha(-1), 2704 kg K ha(-1)) across a 6-year period among the grasses. Switchgrass may be the grass best suited for dual use systems under Uruguayan conditions because a farmer may utilize initial growth as forage while biomass regrowth is a good direct combustion o biofuel feedstock due to lower moisture content and nutrient removal compared to the other species evaluated.