Long term effects of tillage and nitrogen source, rate, and placement on grain sorghum production

The use of conservation tillage practices has increased substantially in recent years. Few studies have directly compared N management practices and nutrient removal under no-till (NT) and conventional tillage (CT) for grain sorghum [Sorghum bicolor (L.) Moench] over long periods of time. The objective of this study was to determine long-term effects of tillage and N source/placement, and rate on grain sorghum production and composition. A field study was conducted from 1982 to 1992 comparing broadcast ammonium nitrate (BR-AN), broadcast urea ammonium nitrate (BR-UAN), and knifed UAN (KN-UAN), each at 30, 60, and 120 lb N/acre, under NT and CT (chisel-disk) practices. When significant differences in grain yields were observed, they were generally lower for NT than for CT and with BR-UAN than BR-AN. Tillage and N source/placement effects were more evident at the lower N rates. Nitrogen availability, as indicated by flag leaf and grain N concentrations, was higher for CT than for NT and for BR-AN than BR-UAN. The KN-UAN treatments did not have higher grain yields than BR-UAN treatments but did have higher flag leaf and grain N concentrations. There was greater uptake of N in grain under CT than NT. A similar effect was found for P and K uptake by grain but only at the lower N rates. Residue cover, which was measured in 1990 and 1991, was higher for NT than CT but did not increase with increasing N rate under either tillage system. In general, nutrient availability was higher for CT than for NT and N availability was higher for BR-AN and KN-UAN. Grain yields were similar across tillage and N source/placement at 120 lb N/acre.