Sugar and Ethanol Yield Response of Sweet Sorghum to Variable In-Row Spacing

Previous studies have shown that sweet sorghum (Sorghum bicolor (L.) Moench) can be grown in the arid southwest US with fewer inputs than other biofuels crops.  The need for specific information for growers on optimal stand density prompted the current research. In 2009 and 2010, 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. In 2009, the more plants per area the lighter the weight of the stalks; and they also contained less juice, and had smaller stem diameters.  Stems of M81E had significantly smaller diameters than the other two varieties, but these stalks had higher sugar concentrations, averaging 652.4 gL^-1 and with the highest at 775.7 gL^-1.  Sugar values for Cowley were generally in the middle, while Topper was consistently on the bottom.  Plants in the densest treatment were tallest with smaller stem diameters. In 2010, the field weights and stem weights of plants from the widest two spacings were significantly heavier and contained more juice than the two densest treatments.  The only significant differences in stem diameters existed between the two extreme treatments for M81E and Topper, with the smallest diameter stems in the 20 ppm treatments. In both years, all three varieties displayed similar trends in sugar accumulation.  Very little glucose and fructose are present up until late July (around 80 days after planting); less than 10 gL^-1 of each.  Sucrose concentration is below detection limits (5 gL^-1) until the end of August (about 120 days after planting), when a rapid increase up to and in excess of 100gL^-1 occurs at the final harvest in November (at physiological maturity, almost 200 days after planting). The two years of data show that sweet sorghum grown for sugar in the semi-arid Southwest have higher yields when grown at a stand density between 2.5 and 4 plants per meter.  When plants are too close together, competition for resources produces thinner stalks with less juice.