Sweet sorghum ethanol yield component response to nitrogen fertilization

• Three-year study evaluating sweet sorghum yield response to a broad range of N treatments.
• Nitrogen fertilizer yield benefits increase to 168 kg ha−1 for optimum ethanol yields of 7488 L ha−1.
• Varying environmental conditions limited yields and N response in some years.

Increasing demand for high-yielding, alternative biofuel feedstocks elicits the need to fully understand sweet sorghum (Sorghum bicolor (L.) Moench) yield response to varying nitrogen (N) fertilization rates in the U.S. Midwest. The objective of this three-year study was to determine the optimum N fertilization rates for the production of two common sweet sorghum cultivars (Dale and Top 76-6) in central Missouri. Five N rates (0, 56, 112, 168, 224 kg N ha−1) were imposed and tested for their effects on dry matter yield, stem juice yield, Brix, fermentable sugar yield, theoretical juice ethanol yield, theoretical lignocellulosic ethanol yield, and total theoretical ethanol yield. Except for Brix, N treatment significantly influenced all yield parameters in all three years. The two varieties yielded similarly across most measured parameters. Total dry matter yields averaged 16.8 Mg ha−1, juice yields averaged 9113 L ha−1, and fermentable sugar yields averaged 1055 kg ha−1 across years and varieties. Total ethanol yields averaged 7488 L ha−1 and were highest at 168 kg N ha−1 across the three years, indicating that sweet sorghum in Missouri may reach maximum yields near that fertilization rate. Annual precipitation and temperature differences greatly influenced dry matter, stem juice, and sugar yields, thereby affecting theoretical ethanol yields, such that yields were lower in years with less rainfall and lower temperatures, which also limited the N response in these years.