Switchgrass for Forage and Bioenergy: I. Effects of Nitrogen Rate and Harvest System

Switchgrass (Panicum virgatum L.) has been targeted for cellulosic ethanol production. Our objective was to evaluate effects of location, harvest system, and N fertilizer rates on switchgrass biomass yield and N, P, and K removal. Randomized complete block experiments with four replications were established on one-year old stands of ‘Alamo’ switchgrass at two Oklahoma locations in 2008. Harvest system and N rate interactions affected total annual yield. Biomass yields ranged from 9020 to 10530 kg/ha across harvest systems when no N was applied. With application of 179 kg N/ha, biomass yields averaged 10715, 13912, and 16516 kg/ha when harvested at seed maturity (October), after a killing frost (December), and twice per year at boot stage (July) and after a killing frost, respectively. Nutrient removal tended to increase with N fertilization and was generally twice as great for each nutrient within the two-cut system relative to the one-cut systems. When 179 kg N/ha was applied, N removal was 198, 82, and 122 kg N/ha when cut twice, cut once at seed maturity, and cut once after frost, respectively. Phosphorus removal was 22, 12, and 11 kg/ha among these systems, respectively. Corresponding K removal was 203, 62, and 25 kg/ha. Applying N and harvesting once after frost ensures both high biomass production and reduces soil nutrient mining. Total biomass harvest, however, was greatest under the two-cut system, enabling a potential use of switchgrass early in the season for forage and availability of regrowth for bioenergy purposes