Nitrogen Source, Timing of Application, and Placement: Effects on Winter Wheat Production

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Abstract
Studies to increase profitability and N use efficiency in winter wheat (Triticum aestivum L.) production are needed to develop more sustainable agricultural systems in the 480‐ to 650‐mm precipitation zone of northern Idaho and eastern Washington. Field experiments were conducted on Latahco silt loam (fine‐silty, mixed, frigid Argiaquic Xeric Argialboll) soils east of Moscow, ID, during the 1982–1983, 1983–1984, 1985–1986 and 1986–1987 growing seasons. Fifteen different N placement‐source‐application timing treatments were arranged in a randomized complete block design with five replications. Fertilizer placements were (i) surface broadcast, (ii) band 50 mm below the seed, and (iii) combinations of surface broadcast and banded below the seed placements. Times of application treatments were (i) fall, (ii) spring, and (iii) various fall‐spring splits. All treatments were evaluated with two N sources: NH4NO3 (AN) and urea (U). Parameters evaluated were (i) winter wheat stand counts, (ii) early‐season plant biomass, (iii) grain yield, and (iv) apparent N use efficiency (NUE). Placement, N source and time of application had minimal impacts on winter wheat stand counts and early season biomass production. Both winter wheat grain yield and apparent NUE were greatest when N applications were split between fall and spring. Splitting time of N application resulted in apparent NUE of 58 to 61%, compared with 52 to 55% and 51 to 53% for fall only and spring only N applications, respectively. Grain yield and apparent NUE differences attributable to N source and N placement were not significant. Based on this study, ideal N management in the 480‐ to 650‐mm precipitation zone would utilize AN, U, or comparable N sources and split N applications where as little as 25% of the N is banded below the seed or surface broadcast in the fall, with the remainder applied as a spring topdress prior to Zadoks growth stage 24. This proposed management will improve both profitability and water quality by increasing both grain yield and N use efficiency when compared with systems currently employed.

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