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Fall weed management in canola

If you have time for fall weed control in canola fields, make the most of it

Canola is one of the higher value crops in Western Canada, but returns can be negatively impacted by poor weed control. Luckily, there are options for glyphosate-tolerant canola. Controlling weeds in the fall ensures that the crop suffers next to no yield loss due to competition in the following year. If pre-harvest control isn’t possible, there are still spring options to consider.

Scouting for weeds

Knowing what you’re dealing with is the first step in any weed management plan, says Angela Brackenreed, agronomy specialist with the Canola Council of Canada. She suggests that farmers first assess weed populations carefully. In particular, look for perennials and winter annuals. Perennials and winter annuals, she notes, can be much harder to kill in the spring when they have less leaf area and larger root systems to target.

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If populations of perennials and/or winter annuals are high, a fall glyphosate application is ideal, says Brackenreed. However, if there are mostly annuals in the field, a spring application is most economical. “The only consideration here is whether those annuals have time to set seed and contribute large amounts to the seed bank,” she says.

Troublesome weeds in canola include dandelions, Canada thistle, quackgrass, foxtail barley and winter annual cleavers. The seeds of cleavers, in particular, are difficult to separate from canola seed at or after harvest. Also, their seeds are much harder than those of canola, which can result in damage to crushing machinery. High populations of Canada thistle are also problematic as they can negatively impact yields. Brackenreed says that 10 Canada thistle plants/m2 can reduce canola yields by 10 bu./ac.

Options for optimum control

Clark Brenzil, provincial weed specialist with Saskatchewan’s Ministry of Agriculture, recommends pre-harvest glyphosate as the most effective form of control. “There are many advantages to this timing when compared to post harvest treatment,” he says.

Glyphosate combinations such as Cleanstart, which contains carfentrazone, are also effective. Brenzil does warn, though, that rapid burn down products can limit the translocation of glyphosate in perennial weeds since leaves are killed very quickly. Since in perennial weeds the target is the root, this is less desirable than glyphosate on its own.

Application timing is key

Timing is key, as pre-harvest applications that go on too early can lead to high residues in grain. If residues exceed the Maximum Residue Limit (MRL), marketing the crop, particularly to foreign buyers, could prove difficult.

Generally, it is best to apply when seed moisture is less than 30 per cent, says Brenzil. At this stage the seed is mature and separated from the mother plant by a cork layer.

If applying after harvest, producers need to be careful not to apply companion herbicides with glyphosate that could leave a soil residue. Soil residue can injure canola plants.

Brackenreed reminds growers to check product registration to see when products are best used, spring or fall. “Some registered products have application cut-off dates, so even if they’re registered, one should check with their retail or company representative to get all of the details on the product,” she says.

Ideal conditions for glyphosate applications are warm, sunny days when plants are actively creating and moving sugars from the leaves to other areas of the plant, says Brenzil. Glyphosate, he reminds growers, must get to where it needs to work in the plant within 24 to 48 hours. Cool, cloudy conditions where translocation is slow can cause glyphosate to get hung up in the leaves, which means it does not move to the roots where it is needed to control perennial weeds.

If frost occurs, farmer should evaluate the degree of frost in the following day or two. “A lot of producers wonder if a frost in the fall will enhance the control of their post-harvest glyphosate,” says Brackenreed. “Cooler temperatures increase translocation of sugars to the roots of perennial weeds, but temperatures below zero won’t enhance that movement. Although frost itself may not improve control of perennials, one could still get good control after a frost if there is enough healthy tissue still there for uptake.”

Brenzil agrees, but warns growers to evaluate leaf tissue carefully. “If tissues turn black they are no longer suitable for herbicide uptake,” he says. “Monsanto suggests that a loss of more than 40 per cent of the original leaf tissue can reduce the effectiveness of glyphosate on perennial weeds.”

“Application after a light frost can sometimes improve activity in perennial weeds,” he continues, “but due to the risk of a killing frost, gambling on a light frost for a slight bump in activity is risky.”

If fall frosts have been severe and there isn’t enough leaf tissue to warrant a spray, farmers should definitely plan to clean up their fields pre-seed in the spring ahead of their canola. When in doubt, be sure to contact your local agronomist for more information.

About the author


Melanie Epp

Melanie Epp is a freelance farm writer.



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