If the moisture situation and yield outlook have increased since a canola field was seeded, and the field did not receive enough fertilizer to reach this potential, then a top dress application of nitrogen could provide an economic benefit.
When to apply: Ideally, a top dress of nitrogen should occur before the 4- to 6-leaf stage of the crop. That gets the fertilizer in place before canola plants hit their peak nitrogen uptake — which occurs from about the 5-leaf stage to full bloom. (See graph above.) Nitrogen applied later may not get to the roots in time to contribute fully to yield or profit. It could still provide some benefit, but not as much, meaning a poorer return on the investment.
The ideal timing for nitrogen fertilizer application is at seeding, provided this can be done without risking seed or seedling injury. This saves an extra pass over the field and ensures that the expected nitrogen requirement is in place when the crop needs it.
What product to use: Nitrogen options for in crop application are urea (dry), UAN (liquid) or ammonium sulphate (if insufficient sulphur for crop uptake is also a concern).
If using granular, apply when leaves are dry so prills roll off onto the ground without sticking to plants. If using liquid, dribble onto morning dew to dilute the product further and promote more runoff. Liquid nitrogen applied when hot and dry can increase leaf burn significantly. Avoid flat fan nozzles.
Full resource article: Top up tips for N and S