Nitrogen Rate and Placement for Grain Sorghum Production in No‐Tillage Systems

Current farm legislation has mandated conservation plans for farms, and maintaining substantial amounts of residue on the soil surface after planting will be a key component in many of those plans. A critical factor in high‐residue systems is N fertilizer management, yet few studies evaluating N management practices for grain sorghum [Sorghum bicolor (L.) Moench] production under conservation tillage systems have been reported. Field research was conducted from 1986 through 1989 at two sites to evaluate N rates [0, 50, and 100 lb/acre as urea‐ammonium nitrate (UAN) solution], placement methods (surface broadcast, dribble‐surface band on 20‐in. centers, and knifed‐injected 5‐6 in. deep on 20‐in. centers), and the use of ammonium thiosulfate (ATS) at 10% v/v in UAN as an N management tool in continuous, no‐till, grain sorghum production. Grain yields and N concentrations of leaf tissue and grain were consistently and significantly increased by N fertilization at both sites. When grain yields exceeded 90 bu/acre, 50 lb N/acre was not enough to produce optimum yields. In addition, the 100 lb N/acre rate consistently produced significantly higher grain N concentrations than the 50 lb N/acre rate. Knifed placement of UAN in these high‐residue, conservation‐tillage conditions was superior to broadcast or dribble applications, producing 15 and 10 bu/acre more grain per year than the broadcast and dribble placements, respectively. Knifed placement also consistently produced higher N concentrations in leaf tissue and grain, resulting in much higher apparent N use efficiency. The addition of ATS to UAN produced inconsistent results, but in 2 of 6 site‐years, appeared to favorably affect apparent N use efficiency.