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### Abstract

Conventional analytical methods used to determine the most economical farmer-based harvest system and corresponding nutrient management strategy for producing switchgrass (Panicum virgatum L.) feedstock do not consider agronomic and subsequent economic problems associated with soil nutrient mining. The objective of this study was to determine the long-term, economically sustainable harvest system and corresponding rates of N, P and K for producing switchgrass feedstock in the southern Great Plains. Data collected from a four-year, two-location agronomic field trial in south-central Oklahoma were used for analyses that included two harvest systems, including (1) a single cut after a hard freeze, after plant senescence (WNTR), and a summer cut at plant maturity in July followed by a second cut after a hard freeze, after plant senescence (SMWNTR). Each system received 0, 45, 90, 135, 179, and 224 kg of N ha-1 yr-1, and received 67 and 135 kg ha-1 yr-1 of P2O5, and K2O, respectively. A standard forage analysis was used to determine the concentrations of N, P and K nutrients in the feedstock harvested from each plot in each year and location and converted to N, P2O5 and K2O (kg ha-1) equivalents. These data were used to determine the extent of soil nutrient mining or nutrient remobilization for each harvest system. Two separate econometric models were estimated and used with enterprise budgeting techniques to 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 reflects the long-term, economic sustainability approach and uses the nutrient concentration levels calculated from the switchgrass forage samples collected in the study. For a farm-gate feedstock price of $83 Mg-1 and nutrients priced at 2012 market rates, the results showed that it was economically sustainable to harvest only once after a hard freeze (i.e., the WNTR system) and apply 84, 28 and 50 kg ha-1 yr-1 of N, P2O5 and K2O, respectively. For this base-case scenario, farmers earned$79 ha-1 more net return with the economic sustainability approach compared to the results generated from the conventional economic approach. However, the comparative results between the two economic approaches are quite sensitive to the assumptions about yield response to nutrient concentration levels and assumptions about the percentage of nutrients remobilized in the WNTR system that are actually available for reuse by switchgrass plants.