Mitscherlich Modeled Canola Nitrogen Use Efficiency Influenced By Soil N and Water.

Despite early notions to manage canola similarly to wheat in the inland Pacific Northwest, our research has demonstrated that canola’s fertilizer requirement varies with yield potential: high unit nitrogen (N) requirement (UNR) for low yield potential and low UNR for high yield potential.  Since UNR is the inverse of nitrogen use efficiency (NUE) at optimal yield, the implication of this finding is that canola becomes more efficient at using N as moisture limitations lessen. Our research goal was to conduct an NUE component analysis in order to identify the components that contribute to differences in water limited yields.  In both years of the experiment, grain yield (Gw), grain N (Ng), and N supply (Ns) were significantly greater in the annual cropping zone than the transition region into grain-fallow. The grain N utilization efficiency (Gw/Ng) component had the largest impact on yield differences, which accounted for 43 to 62% of the variation.  The effect of the nitrogen retention efficiency (Nav/Ns), N harvest index (Ng/Nt), and available N uptake efficiency (Nav/Nt) components each explained less than 25% of yield differences.  Differences in grain N accumulation were mostly attributed to the available N uptake efficiency component (Ng/Nav), while plant N partitioning (Ng/Nt) contributed to only 1 to 24% of the variation. Therefore, as water became less limiting, spring canola was more efficient at accumulating and utilizing grain N. Our results highlight the need to select canola varieties with a high N utilization efficiency in low yielding areas.