From Canola Council of Canada
Seedlings that curl up and start to brown off before emergence could be infected with seedling disease. But they could also be damaged by herbicide carryover.
Dry conditions or saturated conditions can extend the period required for herbicide breakdown. For this reason, carryover damage may be occurring when it wasn’t expected. In the Peace region, some canola seeded into pea stubble is showing signs of possible group-2 herbicide damage. The Peace was dry last year and herbicide breakdown may have been slower than expected.
Damage from herbicide residue can also be worse when it rains after a few weeks of dry weather. In this case, clay particles can release herbicide molecules they had taken up earlier.
Symptoms for herbicide carryover are generally the same as symptoms when the wrong product is accidentally sprayed over the crop. There are different symptoms for the different herbicide groups.
Hints that soil herbicide carryover may be the cause:
—Residual herbicide has been used on the field before.
—Symptoms appear worse on hilltops or valleys. When symptoms are worse at the edges of the field, that indicates drift. When symptoms are even over the field or change intensity in a clean line from one portion of the field to the next, the damage was probably caused by an in-crop application this year.
Click here for a table showing herbicide carryover.