Winter wheat yield and grain protein across varied landscape positions

Winter wheat (Triticum 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 efficiency; 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 northbackslope 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 NO 3 -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 yield estimates calculated from soil water availability and spike density