Winter canola requires 5-7 lbs N/acre for every 100 lbs/acre seed yield (this is called unit N requirement) depending on N use efficiency. The high rainfall area where winter canola has higher yield potential, the unit N requirement is lower and vice versa. Winter canola is a deep rooted crop that can reach water and nutrients to 6 feet below the surface. Measuring available N in 6-foot soil depth is important to avoid excessive N applications. A recent study conducted in the Pacific Northwest found that there was no yield benefit when soil test N, including ammonium-N and nitrate-N, in the 6-foot soil depth was higher than 100 ppm at planting in fall regardless rainfall zone. However, seed quality could be affected by N applications. The higher the N application rate, the higher the seed protein and lower oil contents were. Farmers only need to apply N when soil test N in the 6-foot soil depth is lower than 100 ppm.
The timing of N application makes a difference in canola seed quality and nitrogen use efficiency. Spring application results in the greatest reduction in oil content (Figure 1). For example, the oil concentration reduced by 10% when N was applied where soil test N in 6-foot was higher than 100 ppm in spring in a field located in intermediate rainfall zone. This was because N was not a limiting factor for reaching its yield potential. The additional available N only increased protein concentration. Oil and protein concentrations are linearly and negatively related (Figure 2). This is due to competition for carbohydrate skeletons during carbohydrate metabolism, therefore, increased N supply enhances the synthesis of proteins at the expense of fatty acid synthesis.