Prairie fields rife with resistant weeds

The number of weeds that have developed resistance to one or more herbicides is jumping in Western Canada.

Fifty-seven percent of Canada’s largest cropping province now has fields infested with herbicide tolerant weeds.

Random weed surveys conducted by researchers on 400 fields have found that more than 19 million acres in Saskatchewan likely carry one or more weed species that are tolerant to one or more herbicides.

The weed resistance survey of 2009 showed about 14 million acres with the problem, compared to 3.52 million acres in 2003.

Not every acre of a field has the issue, but the spread of the problem within fields is growing and along with it a reduction in the ability to easily control the growth as tools such as spot tillage and spraying become less practical, even where the issue is recognized.

“In Manitoba we now know it is over 60 percent … and when we do the survey in Alberta this year I suspect it too will be over 50 percent as well,” said agricultural weed researcher Hugh Beckie.

He said early issues with resistant weeds began largely with wild oats, where growers had been using the same products each June year in and year out.

“Our research has now pointed to mixing multiple modes of action in all application windows of application,” said Beckie about a change in herbicide planning.

Up until a few years ago, producers were encouraged to rotate herbicides and crops to avoid weed resistance. Often issues were caused by long-term cereal-on-cereal production. Group 1 and 2 resistance in Saskatchewan’s wild oat population has reached 10.7 and 1.7 million acres, respectively.

Group 1 and 2 multiple resistance is now more than 6.1 million acres of fields.

“So where growers have wild oats, they are probably resistant to one or more herbicides,” he said.

Graham Collier of Nufarm in Ponoka, Alta., said his company is now advising producers to “keep your weed populations off balance.”

“Mixing multiple modes of action with some varied application timing, where you can, will keep resistance down,” said Collier, the company’s technical service manager on the Prairies.

That is important when considering that the Prairies now have weeds such as green foxtail that are Groups 1 and 2 resistant and that there is Group 2 resistance in chickweed, cleavers, redroot pigweed, shepherd’s purse, stinkweed and wild mustard.

Some of the most recent weeds include: Group 1 resistant Persian darnel from Radville, Sask., Swift Current, Sask., and Brooks, Alta.; Group 2 chickweed from Spiritwood, Sask.; hemp-nettle from Three Hills, Alta.; Group 2 redroot pigweed from Moose Jaw, Sask., smartweed from Erskine, Stettler and Vegreville in Alberta and Group 9 kochia at Provost, Alta., Hilda, Alta., and Assiniboia, Sask.

Beckie said one of the biggest economic threats to the southern Prairies remains kochia. Any time the pest is found it is “now assumed that it is Group 2 resistant, but 4 and 9 is also possible.”

Resistant versions of that weed are estimated to now populate 5.7 million acres of annually cropped land in Saskatchewan alone.

Collier said tank-mixing herbicides with multiple modes of action has long been a tool that producers use in-crop.

Some have also needed to control canola volunteers during spring burnoffs that way, but now that has been extended to many growers’ strategies to start the year.

Producers are aware of problems in their fields, if the samples being submitted to Saskatchewan Agriculture’s Crop Protection Lab for testing are any measure.

Record numbers of weed samples were submitted for testing for resistance in 2013 and 2015: 54 percent from Saskatchewan farmers, 40 from Alberta and six from Manitoba. The lab began testing for resistance in 1996.

“It was once a slow and steady increase, but it has been growing more rapidly … more or less as predicted,” said Beckie.

By and large, growers are still managing their resistance issues.

“When we look at weed densities at late summer and fall, we are finding that growers are managing to keep weeds from maturing in later summer and fall,” he said at a Nufarm field day last week.

“They are keeping up mostly, with few large yield losses, but it takes more management all the time.”

Kochia is the biggest threat in the south with resistance to three or more modes of action becoming an issue.

“When producers get their backs pushed to the wall, growers will need to figure out how to adapt to the challenges, but they can put this off with a strong set of strategies.… It’s not rocket science.”

Management advice

  • 1. Maintain a farm database of weeds that are present and all applications, timings and crops that are used annually. This is useful to operators and might be an important part of rental agreements and future buyers’ production audits.
  • 2. Strategic tillage for control of problem pests. If HT weed resistance is not managed well, benefits from reduced tillage practices may be lost because extreme tillage might be required for control.
  • 3. Site specific weed management to deal with problems rather than “one size fits all” tools across an entire farm.
  • 4. Field border and entry controls to keep resistant genetics out of whole fields.
  • 5. In-crop wheat-selective herbicide rotations, ensuring cereal pests are controlled, such as clethodim, quizalofop, seth-oxydim and imazapyr.
  • 6. Avoid back to back use of ACCase and ALS inhibitor herbicide families.
  • 7. Use herbicide mode of action mixtures.
  • 8. Pre- and post-herbicide application scouting to ensure awareness of control issues.
  • 9. Plant competitive crops that aid in biological control of weeds with heavy canopies.
  • 10. Crop diversity, avoiding tight rotations.

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