Time for a suspicious-weed take down

Ready for some detective work? The first few weeks of the growing season is a great time to police fields for suspicious weeds. Look for weeds that lived through the pre-seed burnoff or post-emergent sprays while others of the same species have died. These few weeds could be resistant to one or more herbicide groups, and it would be good to nab them early.

“It takes 14-21 days after a pre-seed glyphosate application for glyphosate-resistant kochia to become obvious, so keep an eye out while crossing over the farm while seeding or during in-crop herbicide applications,” says Angela Brackenreed, agronomy specialist with the Canola Council of Canada.

Kochia is a hot topic. It is the first weed on the Prairies with known resistance to glyphosate. But kochia is far from the only herbicide-resistant weed to watch for. The website www.weedscience.org reports 21 different weeds with herbicide resistance in the Canadian Prairies, many of them with Group 1 or Group 2 resistance. Some have resistance to multiple groups. Wild oats with stacked resistance were detected 20 years ago, and some have resistance to Groups 1, 2, 8 and 25.

Hugh Beckie, research scientist with Agriculture and Agri-Food Canada in Saskatoon, estimates that herbicide-resistant weeds cost growers $1.1 billion to $1.5 billion per year. “This is based on a grower management questionnaire in which growers identify yield or crop quality losses and additional herbicide costs associated with these weeds,” he says.

“Without management, these weeds will continue to spread and the effect on grower profits will only get worse,” Beckie says. The number of weeds with resistance continues to expand, as does the territory with resistant weeds. Beckie estimates that the number of acres on the Prairies with at least one resistant weed has gone from 10.9 million in the early 2000s to 38.0 million in 2014. And his research shows that cleavers is one of the next most likely weeds to develop glyphosate resistance.

Fortunately, effective management strategies are available to prevent any type of resistance from occurring in the first place and to contain it once it does occur. “We don’t want growers to throw up their hands and give up. This is a manageable situation,” Brackenreed says. “It starts with paying attention to weed patches and any performance issues that raise suspicion, then marking those spots and using integrated management tools to keep them contained or, ideally, remove them entirely.”

How to prevent herbicide resistance

Diversification is at the root of prevention. “The key is to outsmart weeds by applying different products, using them at different times of the year, and using crop rotation — including winter crops that provide cover and competition in the fall and spring,” Brackenreed says.

The following tactics are most effective when used in combination:

Control weeds early. Herbicides are more effective on small weeds. Pre-seed burnoff and early in-crop applications also limit weed competition and improve crop yields.

Rotate herbicide groups. Use a variety of herbicide groups (modes of action) through the whole crop rotation. If weed management depends on glyphosate in the pre-seed and post-harvest windows as well as in crop in Roundup Ready crops, the selection pressure for glyphosate-resistant weeds is very high.

Use crop rotation to your advantage. Crop rotation allows for a wider range of herbicide groups. “While cleavers tend to be a bigger concern in canola crops, growers have more options to effectively control cleavers in cereal crops,” Brackenreed says, giving just one example. Growing crops with different seeding and harvest timing — such as winter cereals and perennial forage crops — also diversifies weed removal timing options and provides weed competition in fall and early spring.
Use tank mixes. Hitting weeds with two modes of action reduces the risk of selecting for resistance, or having herbicide resistant weeds escape and set seed. Tanking mixing carfentrazone with pre-seed glyphosate applications will prevent the development or escape of glyphosate-resistant kochia, for example.

Use the right herbicide at the right rate and apply at the right time. Follow labels for timing, ideal spraying conditions, controlled weeds and correct rates. Cutting rates, for example, may reduce herbicide efficacy and increase weed seed return to the soil seed bank.

Control weeds throughout the season. Spraying pre-harvest can reduce the weed seed bank for escaped patches, and spraying in the fall can be highly effective for perennials and winter annuals.

Use integrated weed management (IWM). IWM includes seeding practices to improve crop competitiveness — such as higher seeding rates, shallow seeding depth, narrow row spacing and high quality seed. Placing fertilizer closer to the seed and using insect and disease protection when necessary will also improve crop competitiveness. Later seeding dates allow control of early weeds in the pre-seed window. Tillage and spot mowing to prevent seed set are mechanical alternatives.

“These techniques reduce the reliance on herbicides, which will actually improve results from herbicides and keep more herbicides working on the farm,” Brackenreed says.

For more on weed management, see the weeds section in the Canola Encyclopedia at www.canolacouncil.org/canola-encyclopedia. For other things to look for while scouting early in the season, go to www.canolawatch.org and search for the article, “The critical first 21 days.” While at the site, consider signing up to receive the free weekly Canola Watch agronomy email.


For more information, media can contact Canola Council of Canada agronomy specialist Angela Brackenreed or a CCC agronomy specialist in your region:

Angela Brackenreed, Manitoba


Warren Ward, Southeast Saskatchewan

Shawn Senko, Northeast Saskatchewan

Nicole Philp, Southwest Saskatchewan

Clint Jurke, Northwest Saskatchewan

Justine Cornelsen, Alberta South

Keith Gabert, Central Alberta South

Dan Orchard, Central Alberta North

Greg Sekulic, Peace Region

This media release is supported regionally by:
Alberta Canola Producers Commission; SaskCanola; Manitoba Canola Growers Association; Canola Council of Canada; B.C. Grain Producers Association.

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