INFLUENCE OF SWEET SORGHUM SPACING ON STALK PITH YIELD

Sweet sorghum was planted at two locations in 1987 and three locations in 1988 to test the hypothesis that higher populations producing smaller stalks are less desirable than lower populations of larger stalks, because the higher pith-to-rind ratio of the larger stalks should favor sugar recovery through juice expression. Total biomass yield, total sugar yield, and calculated pith (stalk core) volume were compared among achieved populations ranging from 50 000 to 170 000 plants ha-1 in each year and between conventional and strip-till planting in three 1988 trials. Measured parameters were largely unaffected by planting method, strip-till versus conventional. The expected tendency for higher populations to yield smaller stalks was not found for all location/years. In general, however, each cm increse in mean spacing in the range 13-21 cm increased per-stalk pith volume by 1.5-3.5% and decreased biomass yield by 0.5-4%. The sorghum was able to take only partial advantage of the additional growing space to increase in size; consequently, the total pith volume per hectare decreased 2-5.5% cm-1 increase in mean stalk spacing, or slightly more than the decrease in biomass yield. Results suggest planting a target population of 80 000 plants ha-1, recognizing that the achieved population will range from 68 000 to 56 000 plants ha-1, or lower. This intermediate target population represents a compromise to optimize per-stalk yield of the sugar-rich pith with a modest reduction in total biomass yield.