Yield and Nutrient Concentration Response to Switchgrass Biomass Harvest Date

Timing of biomass removal from stands of switchgrass (Panicum virgatum L.) impacts the nutrient content of harvested material and fertilizer requirements for subsequent growing seasons. This study was conducted to determine the change in N, P, and K content of harvested switchgrass biomass as a function of the harvest date and to determine the economic consequences of an extended harvest window. Data were produced in a randomized complete block study conducted at the Oklahoma Agricultural Experiment Station, Stillwater, with six replications over three harvest seasons from November of 2007 to March of 2010. Treatments on the established stand of cultivar Kanlow consisted of five harvest dates separated by about 30 d beginning in late November. Regression equations were used to fit yield and N, P, and K concentration response to the harvest date. Delaying harvest beyond December resulted in an average 5.4% decline in harvested biomass per month. Delaying harvest beyond November did not result in a significant change in the N concentration in the harvested biomass. However, delaying harvest did result in a significant decrease in both P and K content in the harvested biomass. Point estimates from the response functions were used to estimate production cost for each of five harvest dates beginning with 30 November and ending with 30 March. The quantities of P2O5 and K2O fertilizer that would be required to replace the P and K removed with the biomass were used in the budgets. Biomass production cost was similar across harvest dates.