Nitrogen management of winter canola in the inland Pacific Northwest

Marissa Jeanne Porter

Integrating a new crop into a cropping system requires an understanding of nutrient use, specifically on the rate and timing of fertilizer applications in relation to soil type and climatic conditions. In the semi-arid dryland region of the inland Pacific Northwest (iPNW), winter canola (Brassica napus L.) offers an economically viable rotation to wheat, providing breaks in pest and disease cycles and soil health benefits. Production in Washington State has increased from 4,000 to 28,000 hectares in the recent decade, yet little regional fertility research has been conducted. Canola has an nitrogen (N) requirement higher than wheat (Triticum aestvum L.) but previous studies on spring canola demonstrated limited yield responses to N fertilizer and observed significant differences in unit N requirement (UNR) in different agroecological classes (AECs) of the iPNW. Unique N management strategies are necessary to maximize economic return on fertilizer while minimizing environmental losses. Nitrogen rate and timing studies were conducted over two years at seven locations spread across the AECs. Soil samples were collected in the fall and spring prior to fertilization and post-harvest, then analyzed for baseline and in-season nutrient and moisture content. Spring plant samples and harvest yield and biomass data was collected, with plant and seed components analyzed for total N content and seed quality. There was no yield response to fertilizer N application at six out of seven sites, suggesting that preplant soil residual N fulfilled N requirements. Seed quality was affected by N, with increasing rates and later application timings leading to higher protein and lower oil content. Maximum yields increased with available water, whereas unit N requirement, the inverse of NUE, decreased with increased available water. This research suggests that winter canola grown in different AECs has varying yield potentials and N management decisions should be made differently. The preplant soil residual N measurements to 180 cm soil depth and water availability are most important factors to consider when making N management decisions for winter canola. The end uses of canola should also be taken into consideration, specifically whether maximizing protein or oil content is more desirable.

Nitrogen management of winter canola in the inland Pacific Northwest

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