- Nitrogen management recommendations for yellow mustard (Sinapis alba L.) production in the high rainfall environment of Oregon's Willamette Valley (WV) are not available. The objectives of this investigation were: (i) to determine the effect of applied nitrogen (N) on seed yield and yield components in yellow mustard, and (ii) to ascertain the effect of applied N on dry matter partitioning and oil production characteristics in yellow mustard. Field trials were conducted over a two-year period at Corvallis, Oregon, USA on 'IdaGold' yellow mustard with five N application rates: 0, 56, 112, 168, and 224 kg N ha⁻¹.
Although rainfall differed greatly between the two years, the effect of applied N on seed yield and yield components were similar. Seed yield was increased by 15%-85% with applied N, primarily as a result of N-induced increases in seed weight and number. Yield ranged from 1,080 kg ha⁻¹ with 0 kg N ha⁻¹ to 2,580 kg ha⁻¹ with 224 kg N ha⁻¹. Primary racemes plant⁻¹ was positively associated with increased N; however, secondary racemes plant⁻¹ was not consistently related to N. Siliques plant-1 increased in proportion to N rate and was the result of greater numbers of siliques main stem⁻¹ and siliques primary raceme⁻¹. The relationship of applied N and siliques plant⁻¹ was manifested in seed yield, and as an example, the 224 kg N ha⁻¹ rate yielded 95% and 172% more seeds than 0 kg N ha⁻¹ in 2013 and 2014, respectively. Contributions to yield among main stem and branches varied: the main stem raceme contributed 63-66%, primary branch racemes contributed 33-36%, and secondary branch racemes accounted for the remainder. Seeds on main stems were on average 2.4% heavier than seeds on primary branch racemes within N rates. Nevertheless, N rate differentially affected seed weight as main stem seed weight was increased by 11% with 168 and by 5% with 56 kg N ha⁻¹ in 2013 and 2014, respectively. No effect of N fertilizer on primary branch seed weight was evident in 2013, but 168 and 224 kg N ha⁻¹ increased seed weight in 2014.
Stand density, plant height, and above-ground biomass were determined at three developmental stages (stem elongation - BBCH 30, inflorescence emergence - BBCH 50, and harvest - BBCH 87) while leaf area index (LAI) and tissue N content were measured at BBCH 30 and BBCH 50. Applied N affected most dry matter partitioning and oil production characteristics with the exception of stand density, harvest index (HI), and seed protein concentration. Plant height, biomass, LAI, and tissue N concentration at BBCH 30 and 50 as well as crop growth rate (CGR) from BBCH 30 to 87 were related to the rate of applied N. Applied N increased plant height by 24 to 105%. Although lodging occurred in both years at the highest N rates, lodging did not negatively influence plant growth and seed yield development, nor total oil yield. As a measure of stand photosynthetic capacity, LAI was lowest across developmental stages with 0 kg N ha⁻¹ in 2013, and in the 0 and 56 kg N ha⁻¹ treatments in 2014. Increases in CGR by applied N ranged from 8 to 44% in 2013 and from 27 to 109% in 2014. Biomass N content at BBCH 30 was increased by all rates of applied N, but by BBCH 50, biomass N was only elevated over the control by rates ≥168 and ≥112 kg N ha-1 in 2013 and 2014, respectively. The rate of applied N incrementally decreased N use efficiency (NUE) with reductions ranging from 29% to 71% with only 56 kg N ha⁻¹. Seed oil concentration was generally inversely related to applied N rate; but highest oil yields were observed with 224 kg N ha⁻¹. Optimum yellow mustard plant growth, seed yield, and oil yield were found with 168-224 kg N ha⁻¹ under WV conditions. The results of this study improves our understanding of yellow mustard production in response to applied N in a high rainfall environment.