Richard Taylor, Extension Agronomist; rtaylor@udel.edu
Although few Delaware growers still plant grain sorghum, there are a few growers still growing the crop in dryland situations or around poultry houses, where the shorter sorghum crop allows air movement through the house to keep the birds cooler during hot weather. Grain sorghum is an annual warm-season grass and therefore similar to the other annual warm-season grasses with respect to the need to hold off planting until the soil temperature at a two or three inch depth remains consistently above 65° F and actually is expected to rise soon into the 70°F range. In Delaware, this generally occurs in the third or fourth week of May. Recent rains and cool weather have dropped soil temperatures. This frequently happens throughout early and mid-May. Planting too early will delay germination and often results in poor weedy stands. Sorghum can be planted throughout June, although the yield potential begins to decline rapidly by late June. Although July plantings have occasionally produced excellent yields, this is the exception rather than the norm, so July planting should be avoided whenever possible.
Planting rate for sorghum varies based on row spacing. For 30-inch row spacing the final population should be between 5 and 6 plants per foot of row, while drilled sorghum should be planted to obtain 2 to 3 plants per foot of row. Adjust your seeding rate based on the germination and seed purity percentages to plant pure live seed per acre. Do not plant using the older suggestions of pounds per acre since grain sorghum seed size has varied from almost 8,000 seed per pound up to 18,000 seed per pound.
For choice of row spacing, research conducted at the University of Delaware Research and Education Center showed that grain yield can be as much as 35 percent greater when grain sorghum is planted on narrow (7 to 10-inch) row spacing as compared with wide (30 to 36 inch) row spacing. The narrower row spacing and density within the row that results as plants per foot of row is adjusted means that each plant is more evenly spaced than on wide rows and these factors result in more efficient use of the available water in the soil profile.
One of the advantages of grain sorghum over corn in dryland situations is the low requirement for fertilizer nitrogen (N). In studies conducted in Delaware and Maryland, it’s been found that sorghum will not respond to more than about 70 lbs N/acre, even under irrigation or in years with excellent rainfall. Generally if the crop is following another non-legume crop, 50 to 70 lbs N/acre is all that will be needed for maximum yields. If the crop is planted following a legume cover crop or even a previous soybean crop, application of 25 to 30 lbs N/acre is all that will be needed. Soil test values for phosphorous and potassium should be in the medium to optimum range for best yields.