Good moisture will increase yield potential. Does available nutrient match this new yield potential? If not, does it make economic sense to top dress some nitrogen or sulphur?
In fields where applications rates have not kept pace with crop removal over the past few years, nutrient shortages could be a bigger concern. Growers and agronomists who have fields that fit that description can make in-crop nutrient applications to improve the balance.
If farmers decide to top dress, the ideal time for nitrogen is any time up to the six-leaf stage to make sure fertilizer in place before the crop needs it. Sulphur can be applied up to early flowering and still provide a yield benefit. Top-dressing phosphorus will not improve your yield potential in the year that you apply it, and if that is your most limiting nutrient then adding additional nitrogen may not provide the yield increase you are targeting.
Canola Encyclopedia section on top-dressing nitrogen
Canola Encyclopedia section on sulphur top dress
Top dress tips for nitrogen and sulphur