Timing and Rate of Topdress Nitrogen for Rainfed 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 (1 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. Research QuestionGranular N fertilizer topdressing is a safe, economical application method for rainfed winter wheat production. Research is lacking on the effectiveness of single preplant through late-spring and split N fertilizer topdressings at more than one application rate for rainfed winter wheat. The objective of this study was to evaluate the effect of three N rates (0, 40, and 80 lb/acre) topdressed from planting through grain ripening (Feekes growth stage 11.1) on winter wheat grain yield and protein content. Literature SummaryNitrogen applications should be timed when crop use of N is high. Winter wheat N uptake is most rapid from tillering (Feekes 5) through booting (Feekes 10) developmental growth stages with 80% of the total accumulation occurring before grain filling. Research has shown N applications after booting may not become positionally available to plant roots early enough to benefit vegetative growth or may contribute to excessive late-season vegetative growth at the expense of grain yield. Split N applications have the potential to provide an adequate supply of N to the plant throughout the growing season without risking large early-season N fertilizer losses via leaching or denitrification. Results of these studies have been inconsistent, however, and have varied with soil moisture availability, soil texture, winter vs. spring wheat, and N fertilizer form. Study DescriptionField experiments were conducted on five different sites from 1985 to 1992 at two locations in north-central Texas. Field sites had been previously cropped to winter wheat, then fallowed 5 mo prior to planting. Conventional tillage (shred, disc, and plow) was used for winter wheat seedbed preparation on Austin silty clay and Houston Black clay soils. Hard red winter wheat varieties, ‘Mit’, and ‘Collin’ were planted mid-November. Ammonium nitrate (34-0-0) was topdressed at 40 and 80 lb N/acre at or near planting, and at the jointing, booting, and heading stages plus a split application at planting and jointing stage. No supplemental irrigation was applied, and wheat grain yield, N uptake and grain protein, and net income were determined. Applied QuestionWhat was the optimum rate and time of topdressed N for rainfed winter wheat production? Optimum grain yield (56–61 bu/acre) resulted from single N topdressings of 40 lb/acre at jointing (Feekes 6), 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 between jointing and booting developmental stages compared with planting and heading N applications. Fertilizer N topdressed following booting resulted in grain yields that were 13 to 19% lower than observed with N topdressed prebooting attributable to low available soil moisture following booting, which reduced movement of N into the soil-plant-root zone.