Effects of Planting Date and Stand Density On Sugar and Ethanol Yields of Sweet Sorghum Grown In Arizona

Sweet sorghum (Sorghum bicolor (L.) Moench) is currently being investigated as a direct feedstock for biofuel production in the semi-arid Southwest US. Previous studies in Arizona have shown that the plants can be grown with fewer inputs, e.g. water and nitrogen, than other biofuels crops. Since the juice in the stems can be directly fermented, a further energy savings is realized. The need for specific information for growers on optimal date of planting and stand density prompted the current research.

Four commercial varieties (Dale, M81E, Theis, and Topper) were each sown on four dates (April, May, June, and July 2008) and harvested at physiological maturity. In 2009, three varieties (Cowley, M81E, and Topper) were each planted at five densities (2.5, 4, 6.7, 10, and 20 plants per meter). All treatments of each line were harvested at maturity; in addition, juice samples were analyzed for sugars on a semi-weekly basis.

To obtain field weights, two-6.1 meter sections from the middle two rows of each four-row plot were cut and weighed. A 15-stalk subsample was weighed, stripped of leaves and panicles, and re-weighed to calculate percent stalk. In 2009, stem diameters were also measured. Stems were passed through a roller mill, and the resulting liquid weighed. Juice samples were analyzed by HPLC with refractive index detection for fructose, glucose, and sucrose. Predicted ethanol was calculated using a formula developed previously.

In 2008, Dale and Theis matured faster, and all lines planted earlier took longer to reach harvest. Total sugar yield was higher in the May and June plantings, while the April and May plantings had the highest predicted ethanol.

Results from 2009 highlight interactions between line and density, suggesting that certain lines may be less susceptible to competition. Lower densities (2.5, 4, and 6.7 plants per meter) generally produced plants that were taller, thicker, and heavier, and contained more juice. As in previous years, more biomass and juice were better predictors of ethanol yield than other factors. Sugar accumulation over time demonstrates near constant levels of monosaccharides, while sucrose increases up to and after harvest.

Sweet sorghum may play a transitory role in the biofuels industry in Arizona. For the lines studied, planting in May is preferred, at a density of 4 to 6.7 plants per meter. Since many of the plots planted in June and July germinated poorly, these dates are not recommended.