Due to its high yield potential and adaptability to marginal lands, switchgrass (Panacum virgatum) has received considerable press for its potential to contribute to the growing cellulosic ethanol industry. Many producers have considered entering this new industry, but the limited information on nutrient management of switchgrass needs to be addressed. Therefore, Noble Research Institute established a multisite (Frederick and Burneyville, Okla.) experiment to evaluate the effects of harvest systems and nitrogen application rates on biomass production.
Following the establishment of test sites in 2008, forage was harvested after seed set (October), after frost (December) or twice per year after boot stage (July) and frost. In addition, nitrogen was applied at six rates (0, 40, 80, 120, 160 and 200 pounds of nitrogen per acre). As expected, application of 160 pounds of nitrogen per acre significantly increased forage yields for all three harvest periods. In fact, compared to application of no nitrogen, switchgrass biomass increased by 49 percent when harvested after seed set, by 27 percent when harvested after frost and by 87 percent in the two harvest system. We also found that application of nitrogen beyond 160 pounds per acre did not significantly increase forage yields. Our research showed that for each pound of nitrogen applied, per acre yield increased by 13 pounds for after seed set harvest, 15 pounds for after frost harvest and 35 pounds for twice per year harvest.
Nutrient concentrations and removal rates varied significantly by harvest system. Quantifying this is important because of the soil nutrient mining associated with switchgrass for bioenergy production. Due to high amounts of nitrogen being available to the plant, application of 160 pounds nitrogen per acre increased the forage nitrogen concentration by 68 percent compared to unfertilized switchgrass. In addition, when compared to application of no nitrogen, application of 160 pounds nitrogen per acre increased nitrogen uptake by approximately 170 percent for after seed set harvest, 122 percent for after frost harvest and 147 percent for twice per year harvest.
Phosphorus and potassium concentration and removal rates at the boot stage harvest are much greater than from post-frost harvests. When compared to adding no nitrogen, application of 160 pounds nitrogen per acre increased phosphorus uptake by approximately 55 percent for after seed set harvest, 71 percent for after frost harvest and 50 percent for twice per year harvest. When switchgrass was harvested after seed set, after frost and twice per year, nitrogen fertilization at a rate of 160 pounds per acre increased potassium uptake by 49 percent, 31 percent and 110 percent, respectively, compared to no nitrogen application.
Nutrient removal (soil mining) was generally twice as high when switchgrass was harvested twice a year. Precipitation strongly affected biomass yields across the two years of experiments. When late summer precipitation is available, it supports regrowth, resulting in greater forage yields. However, harvesting twice per year increases nutrient removal and potentially increases fertilization requirements and costs.
As in any hay production enterprise, baling and removal of forage from the land results in depleting soils of critical nutrients. However, nutrients that are mined from the soil can be recycled back if the harvested hay is fed to livestock on the same site. However, with switchgrass production for bioenergy systems, the livestock recycling is absent. Therefore, it is critical to time harvest of biomass to minimize effects on soil nutrient concentrations or to apply nutrients that were removed from the soil. In order to have a sustainable switchgrass production system, harvest at the appropriate time accompanied by regular soil tests and application of necessary nutrients are needed.