Evaluating Sweet Sorghum Germplasm for Maturity in Arizona

Sweet sorghum (Sorghum biocolor L. Moench) is being evaluated as a potential feedstock for ethanol production in Arizona.  Because it can be grown on poor soil and irrigated with reclaimed water, sweet sorghum production in the arid southwestern United States would not compete directly with food crops.  Because of the long growing season, there is the possibility of two harvests of a short-season variety.  One aim of this study was to determine if any available germplasm would fit in a double-crop model.  This germplasm was also evaluated for earlier maturity and other desirable characteristics. In 2012, nine lines with variable maturities were planted on May 8 in a split-plot design with four replications.  Plants were harvested by hand at physiological maturity (30 days after half the plants were observed to be flowering) and the weight of two 3.05-m sections of the middle two rows of the four-row plots was recorded.  A subsample of 15 plants was weighed, stripped of leaves and panicles, weighed again, then pressed through a roller mill.  Juice collected was weighed and then analyzed in the laboratory by High Performance Liquid Chromatography (HPLC) with a Refractive Index Detector for fructose, glucose, and sucrose.  Theoretical biomass, juice, sugar, and ethanol yields per hectare were calculated for each variety and the data analyzed using JMP software. For the four calculated yield components (biomass, juice, sugar, and ethanol yields per hectare), Mer 74-4 and TX09055 (maturing at 178 and 162 days, respectively) were significantly higher than the short-season types that were harvested after 113 days in the field. In most cases, the longer the plants were in the field, the higher their yields.  The predicted ethanol yield of the short-season lines was about half of that of Mer 74-4 and TX09055.  This indicates that as planted, a second crop would bring the total for the short-season types up to approximately the same as the top performers.  However, if planted at a tighter spacing than this study, there would be more biomass per area, which is one of the components contributing to ethanol yield.  The growth habit is very tall and these varieties do not produce tillers, so another strategy to increase yields would be to breed for early maturity and tillering.