NITROGEN-SOURCE, TIMING OF APPLICATION, AND PLACEMENT - EFFECTS ON WINTER-WHEAT PRODUCTION

Studies to increase profitability and N use efficiency in winter wheat (Triticum aestivum L.) production are needed to develop more sustaina ble agricultural systems in the 480- to 650-mm precipitation zone of n orthern Idaho and eastern Washington. Field experiments were conducted on Latahco silt loam (fine-silty, mixed, frigid Argiaquic Xeric Argia lboll) soils east of Moscow, ID, during the 1982-1983, 1983-1984, 1985 -1986 and 1986-1987 growing seasons. Fifteen different N placement-sou rce-application timing treatments were arranged in a randomized comple te block design with five replications. Fertilizer placements were (i) surface broadcast, (ii) band 50 mm below the seed, and (iii) combinat ions of surface broadcast and banded below the seed placements. Times of application treatments were (i) fall, (ii) spring, and (iii) variou s 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 earl y season biomass production. Both winter wheat grain yield and apparen t NUE were greatest when N applications were split between fall and sp ring. Splitting time of N application resulted in apparent NUE of 58 t o 61%, compared with 52 to 55% and 51 to 53% for fall only and spring only N applications, respectively. Grain yield and apparent NUE differ ences attributable to N source and N placement were not significant. B ased on this study, ideal N management in the 480- to 650-mm precipita tion zone would utilize AN, U, or comparable N sources and split N app lications 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 wil l improve both profitability and water quality by increasing both grai n yield and N use efficiency when compared with systems currently empl oyed.