Forage Sorghum Response to Nitrogen Fertilization and Estimation of Production Cost

Forage sorghum [Sorghum bicolor (L.) Mocnch] possesses several agronomic characteristics that make it a potential candidate for biofuel feedstock production. While agronomic characteristics are necessary, other factors must be considered to determine whether forage sorghum can be an economically viable biorefinery feedstock. The objective of the research was to determine forage sorghum yield response to N, the optimal level of N fertilizer, and forage sorghum's farm gate production costs when produced for use as a biomass feedstock crop. Data were produced in two Oklahoma field experiments: one conducted at Chickasha in 2008 and 2009, and one conducted at Chickasha and Stillwater in 2010 and 2011. Treatments included four N rates and different cultivars. A linear response plateau functional form was found to provide a better fit to the data than the quadratic model for the 2008 to 2009 study. The estimated optimal N rate was 99.6 kg ha(-1) and the average plateau yield was 17.4 Mg ha(-1). For an N price range of US$0.56 to USS2.22 kg(-1), the expected farm gate production cost ranged from US$55 to USS65 Mg-1. Biomass yield did not respond to N fertilization at either location in the drought year of 2011 when April through September precipitation was 45% of the long-term average. Year-to-year biomass yield variability would be problematic for a regional biorefinery designed to use forage sorghum exclusively. In addition, storage, transportation, and surface residue issues remain to be resolved before entrepreneurs are likely to be willing to invest in biorefineries designed to process forage sorghum feedstock.