Establishing a Fall Harvest Window for Switchgrass Biomass Based on Optimum Decline of Phosphorus and Potassium Levels in Shoots

Switchgrass (Panicum virgatum) is a perennial grass that remobilizes nutrients during senescence and is being used as biomass for cellulosic ethanol production. Phosphorus (P) and potassium (K) are removed in harvested biomass and replenished through additions of fertilizer. Identifying the appropriate harvest window in a one-cut system based on the remobilization of nutrients can be economically beneficial for biomass producers. The primary objective of this research was to determine if a one-cut harvest can be executed earlier in the fall based on the remobilization of P and K from stems and leaves to crown and roots of plants. Better harvesting conditions and reduced nutrient removal rates are potential benefits of earlier harvest. This project consisted of three parts, evaluating: (1) P and K levels in shoots and whole plants of Alamo and Kanlow cultivars, (2) P and K levels in shoots of upland and lowland switchgrass varieties, and (3) Effects of earlier harvest on yield. Twelve varieties, including ‘Alamo’ and ‘Kanlow’ cultivars, were planted in Knoxville, TN in 2007. Eight of these varieties were planted in Springfield, TN. Above and belowground samples were collected throughout the fall and analyzed for P and K concentrations. No significant declines of P and K were observed in stems and leaves from early October through November. Levels of P and K in leaves, stems, and panicles fluctuated during the fall season; however, final levels were similar in all tissues. Based on these changes in aboveground biomass, the harvest window could begin as early as mid-September. Data suggested that P and K in Alamo and Kanlow followed similar patterns through the fall, without significant declines in shoots. This is confirmed by data from whole plants, which showed no significant increases in P and K in crowns and roots. Levels of P and K in varieties of upland and lowland switchgrass did not differ and followed patterns observed in Alamo and Kanlow. Yields observed in different varieties did not decline when harvested as early as mid-September. Based solely on this study, it is not necessary to delay harvest and could take place as early as September.