Ratoon Cropping of Sorghum. III. Effect of Nitrogen and Cutting Height on Ratoon Performance

The perennial habit of sorghum, Sorghum bicolor (L.) Moench, in a tropical climate permits the production of successive harvest of seed crops from an initial planting. Cultural treatments involving tillage and fertilization to favor such practice are termed ratooning. The objective of this study was to determine the effects of the various N rates and cutting heights on ratoon crops of sorghum at different seasons in relation to tillering behavior, yields, and other agronomic characteristics. Recognizing these effects would facilitate the timely application of the right amount of N fertilizer and proper cutting height to be done on ratoon crops. The performance of ratoon crops of grain sorghum, Sorghum bicolor (L.) Moench, ‘Pride 550 Br’, was studied as affected by four N rates (0, 100, 200, and 250 kg N/ha as urea) and three cutting heights (3, 8, and 13 cm) in the field in Hawaii. Performance characteristics studied were tillering behavior, yield and yield components, plant height, days to heading and maturity, and total plant N content. One half of the N was applied when the seedlings were about 3 weeks old; the other half side-dressed at early booting stage. Generally, in the plant crop and ratoon crops, more tillers, larger leaf area, larger stalks, larger heads with more and heavier grains, and taller plants, and therefore, increased grain and stover yields were produced with higher N treatments up to 250 kg N/ha. Application of 100 kg N/ha increased the total plant N (percent) and further application of N (200 and 250 kg N/ha) increased the N concentrations in the plant from the first to the third ratoons. In the fourth ratoon and in Kilauea planting, total plant N did not increase despite N applications. Although yields increased as the amount of N increased, a leveling off or decline in yields at high N rates, occurred, indicating that optimum N rates were reached in this study. During winter, highest yields were produced with 200 or 250 kg N/ha and when plants were at the 13cm cutting height. In summer, higher yields were produced with the same N rates but lower cutting heights (3 and 8 cm). Based on this study, it is recommended that to obtain high grain and stover yields during winter and summer, 200 to 250 kg N/ha be applied to ratoon crops harvested at 13-cm and 3- and 8-cm cutting height, respectively.