Studies to Refine Nitrogen Requirements, Row Spacing, and Population Density for Sweet Sorghum Biofuel Cropping Systems.

Sweet sorghum [Sorghum bicolor (L.) Moench] is a unique C4 grass that is capable of high soluble sugar and lignocellulosic biomass yields, feedstocks for production of liquid and advanced biofuels. Work is still needed, however, to establish viable cropping systems for this emerging biofuel plant. Because of a regular fallow period in sugarcane production and harvesting equipment optimized for handling sugary stocks, sweet sorghum could perhaps most readily be integrated into sugarcane cropping systems. This will require plant population densities and row spacing configurations amenable to the harvester dimensions. There is also a need to refine the nitrogen requirements of the crop, because of a lack of resolution in past studies due to high soil nitrogen. In the 2012 and 2013 growing seasons, we therefore studied single- and multiple-row spacing configurations (61 and 76-cm single rows; 71 x 107-cm double rows; 35.5 x 35.5 x 107-cm triple rows) with a range in plant population density (74,100 to 148,100 plant ha^-1). On a marginal sandy soil with little nitrogen fertility, we also studied nitrogen requirements of the crop. We found no advantage of multiple-row spacing configurations in 2012 on biomass yield, Brix, or estimated sugar yield, and a definitive disadvantage in 2013 compared to the single row treatments. Narrow (61 cm) single rows yielded as well or better, depending on population density, than conventional (76 cm) single rows in 2013. The narrow spacing is amenable to sugarcane harvester dimensions and conventional rows are not. Considering native soil nitrogen and a range of fertilizer rate treatments, our analysis indicates a crop nitrogen requirement of 85 to 105 kg ha^-1 to optimize biomass and sugar yields in the conditions of the study. These data will help to improve productivity and energy return on investment in emerging sweet sorghum biofuel cropping systems.