Timely Weed Control In Challenging Conditions

With challeng- ing spray conditions, including crop and weeds weakened by frost and cool weather, growers need to consider steps to improve herbicide performance and limit crop damage. These steps will be different for each herbicide, so pay attention to product label instructions and restrictions.

Here are key points from the recent webinar on spraying tips with Tom Wolf of Agriculture and Agri-Food Canada.

Timely spray is more important than nozzle choice. AAFC research showed that spraying seven days after crop emergence generated higher yields than spraying 17 days after emergence, no matter the droplet size. Using a low-drift nozzle early was better than waiting longer for a relatively calm day to use a finer spray.

Glyphosate is suited to low-drift (coarse droplets) sprays, but remember that at the low water volumes that favour glyphosate, coarse sprays may not provide enough droplets per square inch. A combination of coarse spray and low (but not ultra-low) water volume is best to make sure you get droplets on even the smallest weeds.

Liberty needs at least 10 gallons per acre. With coarse droplets, higher water volumes are needed to maintain efficacy.

Keep your boom low to reduce drift, but make sure to use a nozzle with a fan angle to provide 100 per cent overlap at low heights. Spray from one nozzle should reach to the middle of the spray pattern of the adjacent nozzle.

Calm, early mornings can actually increase spray drift damage. The spray can hang in the air, making it impossible to predict when and where it will settle. This dense cloud can do a lot of damage to a neighbour’s crop or yardsite. Bright, sunny days with some wind are ideal times to spray to minimize drift damage.


Aerial spraying may be the best option for controlling weeds on unseeded and seeded fields too wet for the ground sprayer.

Here are the options for aerial herbicide application in canola:

Roundup WeatherMax is the only glyphosate registered for aerial application at this crop stage, but use is subject to certain conditions which are outlined in detail on the label. (Many others are registered for pre-harvest.) WeatherMax is also the only glyphosate registered for aerial application on unseeded fields.

Liberty (glufosinate) is registered for aerial application at this stage. Remember, Liberty works best at higher water volumes.

None of the Clearfield herbicide system products are registered for aerial application on Clearfield canola.

Poast and Assure II have aerial labels.

An emergency use registration for Centurion application by aircraft has been submitted again for 2011, but confirm approval before use.

For more on these registrations and label requirements, read product labels and the provincial guides to crop protection.

Aerial spraying also avoids wear and tear on sprayers and on the field when conditions are wet. Under these very wet conditions, ground sprayers can leave deep ruts to contend with in subsequent spray applications and at harvest, while destroying the crop in those tracks. Besides, getting a sprayer unstuck can be a long, messy job.

There are downsides, however.

Buffer zone requirements for aerial application are substantially farther from sensitive habitats than buffers for ground application. For Liberty, for example, the buffer zone for aerial application is 30 metres from non-target plants and animals compared to a one-metre buffer for ground application.

Aerial application also represents another cost for a crop that does not have the profit potential it had before the delays and the moisture stress. On that note, in very wet conditions, growers should make sure the crop has recovered before stacking on the extra expense of aerial weed control.





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