WINTER-WHEAT YIELD AND GRAIN PROTEIN ACROSS VARIED LANDSCAPE POSITIONS

Winter wheat (Trilicum aestivum L.) yield varies greatly among landscape positions in the Palouse region of eastern Washington, yet N fertilizer is typically applied uniformly. Varying N fertilizer rates within fields to match site-specific N requirements can increase fertilizer use effciency; however, spatially variable N management programs are limited by their ability to predict site-specific yield potentials and the resultant N requirements. The objective of this study was to ascertain the role of yield components and soil properties in determining soft white winter wheat grain yield and protein when N application rates are varied among landscape positions. Nitrogen fertilizer (0 to 140 kg N ha(-1)) was fall-applied on footslope, south-backslope, shoulder, and north-backslope landscape positions at each of two farms in 1989 and in 1990. Grain yield among landscapes varied by up to 55% in 1990 and by up to 33% in 1991. Landscape position grain yields increased by 199 kg ha(-1)/(cm precipitation + soil water reduction) (r(2) = 0.51) and by 706 kg ha(-1) per 100 spikes m(-2) (r(2) = 0.76). Grain protein concentration among landscapes increased by 2.7 g kg(-1) per each increase of 10 kg residual soil NO3-N ha(-1) (r(2) = 0.82). The large differences in grain yield among landscape positions may justify spatially variable N application. Improved N management should favorably reduce soft white winter wheat protein concentrations by minimizing high residual N levels as well as improve net returns and reduce environmental degradation. The basis for this improved N management may be site-specific field estimates calculated from soil water availability and spike density