Does Nutrient Mining Affect the Choice of Harvest System and Nutrient Management Strategy for Producing Switchgrass Biomass?.

Conventional analytical methods used to determine the most economical harvest system and corresponding nutrient management strategy for producing switchgrass (Panicum virgatum L.) biomass do not consider agronomic problems associated with soil nutrient mining. Data collected from a three-year, two-location agronomic study was used to determine the economically sustainable harvest system and corresponding rates of N, P₂O₅ and K₂O for producing switchgrass biomass. Three harvest systems were evaluated: (1) a single cut prior to senescence (October), (2) a single cut after senescence (December), and (3) a summer cut (July) followed by a second cut after senescence (December). Each system received 0, 45, 90, 135, 180, and 225 kg of N ha^-1 yr^-1, and received 67 and 135 kg ha^-1 yr^-1 of P₂O₅ and K₂O, respectively. The content of N, P and K in biomass was estimated for each plot and converted to N, P₂O₅ and K₂O equivalents on a kg ha¹ basis.  Random-effects mixed models and enterprise budgeting techniques were used to estimate and compare the effects of harvest system and nutrient levels on yield and economic net return. Model 1 represents the conventional economic approach that uses the fertilizer treatments applied in the experiment.  Model 2 uses the biomass nutrient equivalents to determine the economically sustainable harvest system and nutrient management strategy. Results show for a biomass price of $84 Mg^-1 and nutrients priced at 2011 rates, it is economically sustainable to harvest once after senescence (December) and apply 100, 35 and 67 kg ha^-1 yr^-1 of N, P₂O₅, and K₂O, respectively. Long-term economic sustainability requires producers to forgo $119 ha^-1 yr^-1 of additional profit that could be earned using the two-cut system and applying 177 kg N ha^-1 that was found to be optimal using the conventional economic approach. Results are most sensitive to biomass and nutrient prices.