Planting Method Affects Stalk Size in Sweet Sorghum

Current efforts to grow Sorghum bicolor (L.) Moench for biofuels in Arizona have focused on maximizing biomass, sugar yields, and harvest windows in order to supply optimum feedstock to processing facilities.  Varieties tested have been either bred for the southeastern United States (harvested at 140 to 160 days after planting) or bred in Texas to be harvested sooner, around 110 days after planting. Our previous work has shown that as intra-row plant density decreases, stalk diameter and weight increase.  Thicker stalks contain more juice and sugar and are sturdier, but the greater biomass can be a challenge to transport and process. This experiment was designed to further explore the relationship between planting density and stalk diameter. Seeds of eight varieties were sown on June 6, 2014, mechanically (‘normal’) and manually in ‘hills’.  Normal rows were planted with a tractor-driven grain drill at 18 seeds/m.  Hills were planted as a cluster of three to five seeds every 0.5 m.  In this split-plot design, planting arrangement was the main plot and variety the sub-plot, with five replications.  Each variety was harvested 30 days after half of the plants were flowering, which ranged from 110 to 169 days after planting.  A 3.05-m section from each of two harvest rows (four-row plots) was cut manually and field weight was recorded.  A subsample of 15 plants was weighed with and without leaves and panicles.  Stem diameters were recorded before the stalks were passed through a roller mill and juice collected and weighed.  Juice samples were analyzed by HPLC with a differential refractometer. Theoretical yields of biomass, sugar, juice, and ethanol were calculated. For all varieties, stem diameters in the hills were significantly greater than those in the normal rows.  Weights of the 15-stalk subsamples and juice were also significantly higher for the hill arrangements.  However, results of ANOVA for field weight show no significant difference (P = 0.4305) between planting treatments.  Plants in normal rows were smaller but there were more of them per area. Leaves and panicles of plants in the hills were also heavier, suggesting greater leaf area and potential seed yield.  While not used as human food in Arizona, it is the fifth leading cereal crop for subsistence farmers in arid environments in Asia and Africa. One crop that produces sugar, grain, and biomass would be an economical use of finite resources.