Critical levels and nitrogen fertilizer equivalencies for grain protein in winter wheat

Abstract

AbstractPrecision nitrogen (N) application methods have been developed for dryland wheat that utilize site‐specific measurements of grain protein concentration (GPC) to determine N fertilizer recommendations for the next season. The objectives of this study were to determine the critical protein level and N equivalent to a unit change in GPC from relationships between GPC, and grain yield or plant‐available N in soft white winter wheat (SWW, Triticum aestivum L.), and assess the consistency of these relationships across SWW cultivars grown under a wide range of precipitation. A 3‐year study was undertaken near two sites: Lexington (225 mm of mean annual precipitation) and Adams (450 mm) in Oregon. Differences in precipitation and N fertilization rates between sites were used to induce variability in grain yield and GPC of four cultivars. A critical protein concentration of 117.5 g kg−1 was defined by Cate–Nelson analysis of scatter plots of relative yield versus GPC. Critical protein among cultivars ranged between 105 and 118 g kg−1 suggesting that 117.5 g kg−1 might be used as a general indicator of N sufficiency. Slopes of the regression of available N on GPC were consistent across cultivars and revealed that 4.2–8.4 kg N ha−1 is equivalent to a unit change protein (1 g kg−1) in lower precipitation areas of the region where SWW is under water stress during grain filling. This information is useful in calculating the N to apply from the GPC in the previous season to meet crop requirements in the next season.