A second in-crop spray only makes sense…
…If growers use canola as a clean-up crop for Group-1 resistant wild oats, narrow-leaved hawk’s beard, round-leaved mallow and other tough weeds, a second herbicide application may be required to do the job.
…If weeds are plentiful, at the same stage or ahead of the crop and the canopy hasn’t closed, these weeds may have a large yield impact. Note however that you’re still required to follow label rate directions even if these large weeds might require a higher rate for control. A pre-harvest product is another option to consider.
…If weeds are at levels too low to influence yield, but are potential grade impacting weeds — such as cleavers — a second spray may pay off if it can do a job on these weeds.
Further considerations:
Herbicide-resistant weeds are becoming a bigger challenge every year as the area taken up by resistant weeds continues to expand and new resistances (weed species, herbicide groups) are discovered. Scout for suspect patches that should have been controlled but were not. Higher plant populations and narrower row spacing are two effective cultural practices that provide an alternative to herbicide or, at a minimum, reduce the need for multiple in-crop sprays.
For perennial weeds that have escaped but are delayed relative to the crop, growers may consider pre- or post-harvest glyphosate application as an alternative, especially if the weeds are delayed enough to avoid seed set prior to swathing.
Another option, if you still haven’t made the first pass, is to use a higher registered rate the first time around.