Response of Ratooning Grain Sorghum to Nitrogen Fertilizer and Insecticides

Interest in production of two grain crops on the same land during a single warm season in the subtropical regions has increased rapidly during the past few years. Presently, grain sorghum is being ratooned for a second grain crop but production practices for the ratooned crop are not well defiled. To evaluate the effects of fertilizer N and insecticides on the production of ratooning grain sorghum (Sorghum bicolor (L.) Moench), field studies were conducted for 2 years on a Greenville soil (Rhodic Paleudult). Nitrogen (0 to 157 kg/ha) was applied immediately after the first crop was harvested and a combination of insecticides was applied just prior to heading and during the early bloom stage. Nitrogen uptake, grain size, grain yield, and insect population counts were used to determine treatment effects. Leaf N increased with increasing N application rates in 1977. Grain yield did not increase when applied N exceeded 112 kg/ha. Average weight per hundred seed for all treatments was 1.6 g compared with an average of 2.7 g for the planted seed. Insect infestation was high in 1977 and yields of the ratooned crops (averaged over N rates) were 2,210 and 1,383 kg/ha for the insecticidetreated and untreated areas, respectively. In 1978, leaf K ranged from 1.4 to 2.0y0 and increased with increasing N rates up to 67 kg/ha. Weight per hundred seed averaged 2.22 and 2.91 g for the insecticidetreated and untreated areas, respectively, but did not respond to applied N. Although insect monitoring revealed no evidence of high densities of phytophagous insects, grain yield averaged 67% higher when insecticides were applied. Yield increased with increasing N rates up to 67 kg/ha in the insecticide-treated area but there was no response to applied N when insecticides were not applied. When averaged over N rates, yield from the insecticide-treated and untreated area in 1978 was 3,136 and 1,882 kg/ha, respectively. Total grain yield from the planted and ratooned crop (with optimum N and insecticides) in 1978 was 9,847 kg/ha. Data from this study suggest that 67 kg/ha of N fertilizer is sufficient for ratooned grain sorghum; the applied N will be economical if a well managed insecticide program is followed.