What Is The Goal With a Late Herbicide Spray?
Herbicide labels say that glyphosate and Odyssey cannot be applied to canola after the 6-leaf stage and Liberty cannot be applied after early bolting. These limits are in place to ensure effective product performance and crop safety. Later applications can lead to reduced weed control due to advanced weed staging and reduced herbicide contact with the weeds through the increased canola canopy closure. Late applications can also sometimes cause canola buds to abort, increasing the potential for permanent yield loss.
Yield loss from late herbicide applications vary on a field by field basis depending on herbicide rate, overall health of the field, and growing conditions during and after the application. Canola under any type of existing stress will likely have greater losses from a late herbicide application. It simply won't be able to recover as quickly. If canola is healthy at the time of spraying but conditions are hot and dry soon afterward, canola may not be able to compensate by adding more flowers later in the flowering period. Losses cannot be predicted, but they could be 30% - and even more in overlap areas.
Growers who haven't sprayed second (or in some cases first) applications because of excess moisture or wind or cool conditions should understand the risks before spraying after the label window has closed.
What is the goal with a late spray?
- Yield benefit. If canola is bolting and the canopy is filled in, then any yield benefit of a second spray is probably not enough to cover the cost of the spray. Weeds poking through the canopy have probably already done their damage, and late flushing weeds are unlikely to have much impact - and probably wouldn't be hit by the herbicide anyway if the canopy is closed in.
- Tough weed control. If growers use canola as a clean up crop for narrow-leaved hawk's beard, round-leaved mallow and other weeds tough to control in cereals, then they may consider a herbicide application after the spray window to be worth the risk of a significant loss in canola yield. Some growers with very poor looking canola crops are already looking toward next year and don't want the weeds to get away on them. Note however that these large weeds may require a higher rate for control - a rate that may exceed the in-crop maximum for the herbicide.
- Final touch up. If the second pass is to give the field a final clean up to knock out a few remaining weeds, it may not be worth the risk.
Source: Canola Council of Canada