Moisture and Soil N Supply-Based N Fertilizer Recommendations for Canola in the Pacific Northwestern U.S.

Nitrogen (N) fertility recommendations for canola vary widely across the semi-arid inland Pacific Northwest, where significant proportions of plant N is derived from residual and mineralized soil sources.  Canola has a high N uptake efficiency (Nt/Ns, unit of total plant N per unit of supplied N) but low N utilization efficiency (Gw/Nt) unit of grain per unit of total plant N) leading to an overall low N use efficiency (NUE) (Gw/Ns, unit of grain produced per unit of N supplied) compared to wheat. An experiment was carried out in 2007-2013 in Pullman, WA, and Davenport, WA, with 5 rates of N (urea), replicated 4 times in a randomized complete block design.  Soil series were Palouse silt loam and Broadax silt loam, respectively.  Spring canola (cv Dekalb Genuity RR) was  reseeded after winter canola freezing  in 2008 and 2010, or planted as a spring crop from 2011-2013.  Grain yield responses to N supply over the 12 site years were each regressed using the Mitscherlich model for representing diminishing returns of increasing N supply. Optimized grain yields ranging from 700 to 2500 kg/ha were linearly correlated with site-year precipitation.  Economically optimal unit N supply (Ns/Gw, 1/NUE) varied between 9 and 26 kg Ns/kg Gw, but was linearly related to the economically optimal yield for each site year.  N fertilizer recommendations can be calculated based on precipitation based yield potentials, and an accounting of non-fertilizer soil N supply and vegetative N requirements.