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 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) (r2 = 0.51) and by 706 kg ha−1 per 100 spikes m‐2 (r2 = 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 (r2 = 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.

Spatially variable N fertilizer application may reduce environmental impacts and increase the economic return of N fertilization. To achieve these benefits, N recommendations must account for within‐field differences in the amount of N required to produce a unit of yield (unit N requirement, UNR). Component analysis was used to determine the sources of variation in the UNRs of winter wheat (Triticum aestivum L.) among landscape positions. The UNRs were divided into two components, N uptake efficiency (plant N/N supply) and N utilization efficiency (grain yield/plant N) observed in N rate trials (0–140 kg N ha−1 fall applications) established on footslope, south backslope, shoulder, and north backslope positions of two farms for 2 yr. Variation in the UNR among the 16 landscape positions studied was most associated with differences in N uptake efficiency (r = −0.80), although N utilization efficiency (r = −0.62) also contributed to the variation. Nitrogen uptake efficiency among landscape positions declined as more fertilizer was required to reach optimum yield (r = −0.56) due to low N fertilizer uptake efficiencies (Δplant N/ΔN fertilizer). Nitrogen fertilizer uptake efficiency was related to the degree of apparent N loss (r = −0.87), indicating that N availability limited N uptake efficiency among landscapes. Overall, low N fertilizer uptake efficiencies (<50%) and high N loss percentages (>50%) indicate the need to reduce N losses and lower UNRs, particularly on north‐facing backslopes susceptible to N leaching.

Spatially variable N application may improve N use efficiency, grain yield, and net returns of winter wheat (Triticum aestivum L.) in fields exhibiting wide ranges of soil characteristics. The objectives of this study were to (i) determine the variability in optimal economic grain yield and in the amount of N required to produce a unit of grain at optimum yield, the unit N requirement (UNR), among landscape positions; (ii) evaluate landscape position as a criterion for dividing fields into units of equal productivity; and (iii) assess the economic benefits of spatially variable N fertilizer application. Replicated N rate (0 to 125 lb N/acre) experiments were established on footslope, south‐backslope (S‐backslope), shoulder, and north‐backslope (N‐backslope) positions of the hillslope profile at two eastern Washington farms in 1990 and 1991. Yield potential among these landscape positions varied by up to 63% and UNRs varied by up to 70%. Landscape position, however, was not an adequate criterion for dividing fields into equal productivity units. There was little economic benefit from variable N applications in a hypothetical case analysis if N recommendations were calculated assuming a constant UNR, however, if experimentally determined UNRs were used, variable applications increased net returns by up to $14.80. The degree of economic benefit depended on the levels of misapplication that occurred when a single N rate was applied to a field and the yield responses to N which determined the results of misapplication. Spatially variable N fertilizer management will require accurate estimates of yield potential, UNR, mineralization and available soil N to be economically viable. Research Question Due to variations in soil characteristics and yield potential, N requirements can vary widely within fields. However, farmers typically apply a single N rate across a field resulting in areas of over‐ and under‐application, which may affect water quality and grower income. Varying N rates within fields to match the site‐specific needs can reduce fertilizer misapplication. This study was designed to (i) determine the variation in grain yield and N requirements per bushel of winter wheat among landscape positions, (ii) evaluate landscape position as an indicator of yield potential, and (iii) assess the benefits of varying N fertilizer rates within fields. Literature Summary Several approaches for varying fertilizer rates within fields have been proposed. Montana researchers have varied fertilizer rates within fields by soil type. Others have intensively grid sampled soils and used the soil test results to divide fields into uniform areas based on projected yields and residual N levels. In addition to yield potential, N fertilizer recommendations are based on the amount of N required to produce a unit of grain at optimum yield, the unit N requirement (UNR). Past studies of variable N management have been based on the assumption that UNR is constant. Yet, the UNR probably varies within fields due to differences in soil and enviro...

Increased rates of seeding and N fertilization have accounted for higher yields with new cultivars of several crops. Our objective was to evaluate the new meadowfoam cv. Floral (Linuumthesjloccosa HoweU subsp. grandijlora Arroyo) x Oregon Limnanthes ORL77-84 (L. alba Hartw. ex Benth. subsp. alba). Combined effects of seeding rate (1.5, 4.0, and 6.5 million seeds ha-'), February applied N topdress rate (0, 80, and 160 kg ha-'), and irrigation (none, pre-bloom, and pre-and post-bloom) were evaluated near Corvallis, Oregon in 1987-1989. Seeding rate, N rate, and irrigation effects on all measured variables were independent of each other. Seed yield, seed oil content, and oil yield increased 45, 5, and 50%, respectively, while lodging did not increase as seeding rate increased from 1.5 to 6.5 million seeds ha-•. Increased flower number was the main reason for seed yield improvement when seeding rate was increased. Seed yield, seed oil content, and oil yield decreased slightly when N rate increased from 0 to 80 kg ha-•, and dropped sharply at 160 kg N ha-•, with severe lodging. The highest average yields of seed (1684 kg ha-') and oil (542 kg ha-') were achieved with the combination of the highest seeding rate, lowest N fertilizer rate, and pre-bloom irrigation.

Future development of high‐yielding meadowfoam (Limnanthes R. Br. spp.) cultivars may be enhanced if relationships between oil yield, yield components, and agronomical, phenological, and morphological traits can be established. To identify such relationships, two half‐sib L. floccosa Howell ssp. grandiflora Arroyo × L. alba Hartw. ex Benth. ssp. alba lines, ORL85‐765 and ORL85‐729, and ‘Mermaid’ meadowfoam were grown and compared in solid stand in 1987–1988 and 1988–1989 at the Oregon State University Schmidt Farm. Line 85‐765 produced the greatest oil yield in 1987–1988. Seed weight of line 85‐765 was 18 and 13% greater (P < 0.05) than lines 85‐729 and Mermaid, respectively, in 1987–1988. In 1988–1989, both lines 85‐765 and Mermaid produced greater oil yields than line 85‐729. Line 85‐765 produced a 7% greater seed weight and Mermaid produced 55% more seeds per flower than line 85‐729 (P < 0.05). Oil yields of lines 85‐765 and Mermaid were not significantly different in 1988–1989. Seed weight of line 85‐765 was 9% greater than Mermaid in 1988–1989, but Mermaid produced 31% more seeds per flower than line 85‐765 (P < 0.05). Seed weight differences were apparently due to variation in seed growth rate and not seed growth duration. Differences in seeds per flower were not related to pollinator activity or flower phenology. Both seed weight and seed number per flower were important yield components for determining the relative oil‐yield performance of lines 85‐765, 85‐729, and Mermaid.