Glacier FarmMedia COVID-19 & the Farm

Guenther: Decisions to be made on spring weed control

As Prairie farmers gear up for seeding, yield-robbing weeds will be on their minds. Here are six tips for effective weed control.


“It’s really important to think about what crops are safe to plant this spring,” said Kristen Phillips, agronomy specialist with the Canola Council of Canada, during a webinar Monday.

Some residuals can last up to 24 months, Phillips says, and farmers and agronomists need to think about a field’s history before seeding.

Farmers and agronomists need to think about whether previously sprayed herbicides have carry-over issues, what rate was sprayed, soil moisture conditions, and whether the product has had enough time to break down.


Early weed control is crucial to protecting canola yield. Young canola plants cover the ground slowly, and weeds will exploit that advantage.

But once the crop is further along, the canola is much more competitive, and little sunlight hits the ground.

The Canola Council’s grower’s manual outlines research carried out in Western Canada. Usually spraying for weeds earlier meant higher yields at the end of the year. Waiting for all the weeds to germinate didn’t pay in most cases; in the Canola Council’s own trials, spraying when the crop was at the one- to three-leaf stage bumped yield in 24 out of 27 cases.

Leaf stages of weeds

“It’s crucial to know the weeds that are present in the field, what their densities are, what their life cycles are, and what growth stage they are in,” says Phillips.

Broadleaf weeds may have whorled, opposite, or alternate leaf stages, which affects how leaf stages are counted.

Wild mustard has alternate leaves, meaning one leaf emerges at a time. Farmers can figure out the leaf stage simply by counting each leaf.

Weeds with opposite leaves, such as hemp nettle, have leaves that emerge two at a time, on opposite sides of the stem. There is always an even number of leaves. These weeds generally look shorter than plants with alternate leaves at a similar stage. To determine the leaf stage, each pair beyond the cotyledon should be counted as the two leaf stage, four leaf stage, and so on.

Weeds such as cleavers have at least three leaves at each node on the stem. Each node, or whorl, counts as a stage. For example, at the one whorl stage, one node will have emerged above the cotyledon. But labels may also go by individual leaf counts. If the label goes by leaf count, Manitoba Agriculture, Food and Rural Initiatives’ website suggests counting all the leaves, as each whorl might have a different number of leaves.

Identify the right weeds

Is that nasty little weed popping out of the soil foxtail barley or quack grass? It can be tricky to identify weeds at the seedling stage, but the Canola Council, for one, has weed seedling pictures online.

“I also recommend choosing your rate based on your hardest-to-control or your worst weed in the field because there’s a really big difference between going with a half litre of glyphosate versus a full litre of glyphosate when you’re going after an over-wintered dandelion,” says Phillips.

Know your glyphosate

Glyphosate types and formulations can be confusing, says Phillips.

“When it comes to formulation, there are different salts that are used, and you don’t want to mix the different salt products,” says Phillips. For example, farmers finishing off a Credit jug shouldn’t mix it with a fresh jug of Roundup WeatherMax.

Farmers and agronomists should know what the registered uses are, as well as the concentration (in grams per litre) and equivalent rate per acre.

“Also, different products have different rain fastnesses. Some products are serviced by companies. Some aren’t. Some have residual if they have top-up products already co-packaged. So lots of things to know when it comes to glyphosate,” says Phillips.

Check top-up products

Spiking glyphosate with another product helps delay glyphosate resistance, but farmers need to check labels on the second product.

“When it comes to top-up products, it’s extremely important to know what you can spray ahead of cereals, oilseeds, pulses. For example, PrePass ahead of wheat is acceptable, but PrePass would not be acceptable ahead of canola,” says Phillips.

Phillips says Crop Protection Guides are great tools, and she encourages farmers and agronomists to check them. Agricultural departments publish crop protection guides online in Alberta, Saskatchewan and Manitoba.

Contact information for Phillips and other Canola Council agronomists is also available online .

— Lisa Guenther is a field editor for Grainews at Livelong, Sask. Follow her @LtoG on Twitter.

COPA Medallion COPA finalist in 2012, 2014 and 2015.
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