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Sweet sorghum (Sorghum bicolor Moench) is an attractive crop for the biofuel industry in the arid-southwestern United States due to its high yields of fermentable juice as well as fibrous biomass, with lower water requirements and high water use efficiency. Previous work has shown that when planted closer together the resulting stalks are thinner, weigh less, yield less juice, and are more susceptible to lodging, thus affecting yields. The studies examined the effects of planting sweet sorghum in clumps (three to five plants in a hill, spaced 0.5 m apart) compared to conventional rows (planted equidistant with a grain drill).

Four varieties (Cowley, Hodo, M81E, and Mer74-2) were sown on 6 June 2014 and 26 May 2015 in a split plot design, with arrangement (clump or conventional) as main plot and variety as sub-plot, with five replications in 2014 and four in 2015. Conventional rows were planted with a tractor-driven grain drill at 18 seeds m(-1). Clumps were hand-planted as a cluster of three to five seed in a single hole every 0.5 m. Harvest commenced 30 days after half of the plants in a variety were flowering. A 3.05 m section was harvested, and weighed. A subsample of plants was weighed with or without leaves and panicles, stem diameters measured, the stalks pressed, and juice collected and weighed. Sugars in the juice samples were analyzed by HPLC with a refractive index detector.

Individual stems in the clumps were significantly greater in diameter and weight, although field weight showed no significant difference between treatments. Plants in conventional rows were smaller but there were more of them per area than the clump treatments. There were no differences in number of leaves, but the overall leaf weight was significantly greater in the clump treatments, due to increased leaf width and area, but not length. Lodging was significantly lower in the clump treatments in 2015, indicating that the thicker stalks are less susceptible to lodging.

Planting sweet sorghum in clumps resulted in thicker, sturdier stalks with more juice, and did not decrease yields over conventionally planted plots. Planting in clumps has potential under irrigation in the arid southwestern United States to increase yields.