TIMING AND RATE OF TOPDRESS NITROGEN FOR RAIN-FED WINTER-WHEAT

Much of the N applied to rainfed winter wheat (Triticum aestivum L.) is topdressed as granular fertilizer in late winter or early spring after growers evaluate over-winter survival and potential economic returns from N fertilization. Information is needed to determine the optimum timing for efficient use of topdressed N by winter wheat. This study evaluated the effect of three rates (0, 40, and 80 lb N/acre) of ammonium nitrate on winter wheat grain yield, N uptake, grain protein, and net income when topdressed at or near planting through heading (1 April) on silty clay and clay soils. Field experiments were conducted at five different sites during the 1985, 1987, 1988, 1991, and 1992 winter wheat growing seasons on Udorthentic Haplustoll and Udic Pellustert soils located in the North Texas Blackland. From 1985 through 1992, topdress applications were timed at planting, jointing (Feekes 6), booting (Feekes 10), and heading (Feekes 10.5) winter wheat growth stages. Maximum grain yields (56-61 bu/acre) resulted from single N topdressings of 40 lb/acre at jointing, 80 lb/acre applied at planting through jointing, or split applications of 40 lb/acre at planting and jointing. Grain protein concentrations were highest (15.3%) when N was topdressed at jointing and booting compared with planting and heading applications (14.9%). Highest grain N uptake (108 lb/acre) occurred with single N topdressings of 80 lb/acre from planting through booting stages, and with split N application. Optimum economic winter wheat grain yields were obtained with single, adequate N topdressings made at or near planting through prebooting stage (I March). Fertilizer-N topdressed following booting resulted in grain yields that were 13 to 19% lower than observed with N topdressed prebooting. This was attributable to low available soil moisture from March through April. Water stress after booting increased winter wheat vegetative growth at the expense of grain yield, and probably reduced ammonium nitrate movement into the soil-plant-root zone.