Are There Advantages to Altering Planting Arrangement of Sweet Sorghum?

Sweet sorghum (Sorghum bicolor [L.] Moench) has been an attractive crop for the biofuel industry due to high yields of fermentable juice and fibrous biomass, with lower requirements than other crops. Previous work has shown that stalks planted closer together in regular rows are thinner, weigh less, and yield less juice than ones planted further apart, but are more susceptible to lodging, which is a substantial barrier to harvesting. Some crops grown in hills or clumps benefit from conservation of soil moisture and mutual shading, especially in marginal or arid climates. This study examined the effects of planting sweet sorghum in clumps compared to regular rows.

Four varieties (Cowley, Hodo, M81E, and Mer74-2) were sown on May 26, 2015 in a split-plot design, with arrangement (hill or normal) as main plot and variety as sub-plot, with four replications. Normal rows were planted at 18 seeds m^-1. Hills were hand-planted as a cluster of three to five seed every 0.5 m. Each variety was harvested 30 days after half of the plants were flowering. A 3.05 m section from one harvest row (out of four-row plots) was cut at the soil line and weighed. A subsample was weighed with and without leaves and panicles. Stem diameters were measured before the stalks were passed through a roller mill and juice collected and weighed.  Juice samples were analyzed by HPLC for sugars.

Stems in the hills were significantly heavier than those in the normal rows. Weights of the subsamples and juice were also significantly higher in the hills. However, field weight showed no significant difference between treatments. Plants in normal rows were smaller but there were more of them per area. There were no differences in number of leaves, but the overall weight was significantly different, due to leaf width and area, but not length. Overall rate of lodging was significantly lower in the hills (11.2% versus 59.4%), indicating that stalks in clumps might offer each other support, or that thicker ones are less susceptible to lodging. No difference was found in fructose, glucose, or sucrose between treatments.

For sweet sorghum grown in Arizona, greater water use efficiency and leaf area appear to be two advantages of planting in clumps, yielding thicker, sturdier stalks with more juice. Possible drawbacks include an increase in weeds between groupings, difficulty pressing large stalks, and adapting the current commercial planting machinery.