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 (BRAN), 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 BRAN 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 than for BR-UAN. Grain yields were similar across tillage and N source/placement at 120 lb N/acre. Research QuestionExtensive research has been conducted on corn concerning the relationships of tillage, residue cover, and N use efficiency. Much less work has been done addressing these issues for grain sorghum production. Since grain sorghum is often fertilized and planted later than corn, there is a higher likelihood of losses of available N with surface applications via ammonia volatilization with some N sources and immobilization because of higher temperatures. This study sought to evaluate the effects of tillage and N rate, source, and placement on grain sorghum production over the long term. Literature SummaryMany studies have investigated the effects of N rates, sources and placement methods for no-till (NT) corn. Results have indicated that surface broadcast urea-containing materials generally perform poorly in NT systems compared with ammonium nitrate (UAN). Knifing UAN (KN-UAN) also performed much better than broadcast UAN (BR-UAN). Nearly all reported studies have looked at corn and few have directly compared tillage systems and N management with grain sorghum on a long-term basis. Study DescriptionA long-term study was initiated in 1982 on a Smolan silty clay loam soil in Kansas. Grain sorghum was grown continuously through 1992. Tillage systems (conventional till [CT] and NT) were established as main plots in a split-plot design. The CT system consisted of fall chiseling and two disking operations. Nitrogen rates of 30, 60, and 120 lb N/acre were applied as either broadcast ammonium nitrate (BR-AN), BR-UAN, or KN-UAN. A check plot (0 lb N/acre) was included in each tillage system. Grain yields and N concentrations, flag leaf N concentrations, and N, P, and K grain uptake were determined. Applied QuestionsWhat are the long-term tillage effects on grain sorghum production? When significant differences in grain yields were observed due to tillage, yields were lower for NT than for CT. This occurred in four out of 10 yr at 30 lb N/acre and 3 yr out often at 60 and 120 lb N/acre (Table 1). The yield differences were more frequent in the later years of the study. Nitrogen availability, as indicated by flag leaf and grain N concentrations and grain N uptake, was generally higher in CT. This could be due to the mixing of crop residue and fertilizer N with the soil. The NT system did produce significantly higher residue cover than the CT system. How was grain sorghum production affected by N management in the NT and CT systems? Grain sorghum yields, flag leaf and grain N concentrations, and grain N uptake were consistently and significantly increased by N fertilization, regardless of N source/placement or tillage system. Broadcast ammonium nitrate performed well, often producing significantly higher yields and flag leaf and grain N concentrations than BR-UAN regardless of tillage system (Table 2). The KN-UAN treatments did have higher flag leaf and grain N concentrations than BR-UAN treatments, indicating a higher N availability from the subsurface placement, but grain yields were similar. Table 1Tillage effect on grain sorghum yields at 120 lb N/acre (averaged across source/placement). Yields were significantly higher with NT compared to CT in 1985, 1987, and 1992. TillageYearNTCT---------------bu/acre---------------1982969919835460198492891985110123198612713219879510719881271071990126121199169691992148154Table 2Nitrogen source/placement effects on flag leaf N concentrations under NT. Flag leaf N concentrations were significantly higher for BR-AN than for BR-UAN over 6 yr. Source/placementYearBR-ANBR-UAN---------------%---------------19822.72.719832.02.019852.72.519862.11.919872.32.119882.31.819892.21.919902.42.319912.22.119921.81.7