Crusting: Rain is The Only Practical Solution
Wet conditions followed by warm sunny days may cause soil crusting, which stops seedlings from emerging. No research has been conducted to show the best ways to break up crusting and free the crop. If a few plants have emerged, it may be best to leave them be. One to 2 plants per square foot are better than none.
If nothing is coming through, a light harrowing might help -- if the crop isn't germinated. Set spring tooth harrows at a gentle angle. Do a couple passes or a lap and then get out and assess whether canola seedlings are being ripped out of the ground.
Harrowing too close to emergence can be harmful to a shallow seeded crop such as canola and may not be worth the risk.
Using a roller may be worse than harrowing when soils are wet below the crust. Instead of cracking up the soil surface, a roller could turn the whole topsoil zone to concrete. Again, there is very little research on how to manage crusting. It's trial and error.
Source: Canola Council of Canada