Switchgrass for Forage and Bioenergy: 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 9408 to 10528 kg/ha across harvest systems when no N was applied. With application of 179 kg N/ha, biomass yields averaged 10,715, 13,912, and 16,516 kg/ha when harvested during seed production (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, 69, and 120 kg/ha when cut twice, cut once at seed production, and cut once after frost, respectively. Phosphorus removal was 22, 11, and 12 kg/ha among these systems, respectively. Corresponding K removal was 204, 51, 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.