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Getting to know your herbicides: Group 1

A look at how Group 1 chemicals work and what you can do to manage weed resistance

Although wild oats are an annual, Group 1-resistant wild oats have become a perennial problem on the Prairies. And wild oats aren’t the only weeds developing resistance to this chemical group. Grainews takes a look at how Group 1 chemicals work and what farmers can do to manage resistance.

Group 1 herbicides are commonly applied in-crop to wheat. Group 1 herbicides such as Centurion and Assure are also used in Liberty Link canola.

These herbicides block the ACCase enzyme, says Tanner Martens, Turtleford and District Co-op agronomist. The ACCase enzymes help form lipids in plant roots, Alberta Agriculture and Forestry’s website further explains. Susceptible plants yellow and die.

Some weeds will naturally be resistant to Group 1 herbicides, Martens says. If a farmer continues using the Group 1 on these resistant weeds, they will survive and reproduce. Eventually the Group 1-resistant weeds will dominate the population.

Martens suggests farmers try to prevent Group 1 resistance in their fields, rather than wait until they have a problem. “Once you have it, it’s kind of trouble,” he says.

Tanner Martens, agronomist at Turtleford Co-op, recommends rotating crops and chemical groups to prevent Group 1 herbicide resistance.
photo: Lisa Guenther

There are some tank mix options that work well for Group 1s, Martens says, such as Assure and glyphosate. But Group 1 herbicides target grassy weeds, and so overall there aren’t many in-crop tank mix options, he adds.

Crop and chemical rotation are the first line of defence with Group 1 resistance. Not rotating crops puts pressure on the land, with both resistant weeds and disease, Martens says. Rotating crops also helps farmers switch chemical modes of action.

But even if farmers are rotating crops, it’s important not to rely on the same herbicides all the time in wheat. “They still need to be switching it up, and that’s where that sub-group of Group 1s can help.”

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Group 1 contains three sub-groups:

  • Aryloxyphenoxy propionate. Known as fop, it includes active ingredients such as clodinafop-propargyl.
  • Cyclohexanediones. Known as dim, this sub-group includes active ingredients such as clethodim.
  • Phenylpyrazolin. Known as den, it includes the active ingredient pinoxaden.

For example, if a farmer uses Horizon or Signal every year he grows wheat, he’s pulling from the fop sub-group, Martens says. To help prevent herbicide resistance, he could look at using Axial (a den) the next time he grows wheat, Martens explains.

“If it worked one year, try something different the next,” Martens says.

Pre-emergent herbicides from other chemical groups are also handy tools to prevent resistance. Avadex (Group 8) or Edge (Group 3) can be applied in the spring or fall. Focus, which contains a Groups 14 and 15, was recently registered for wheat.

“It’s tough to use that every year because it’s expensive,” Martens acknowledges. But a farmer doesn’t necessarily need to use a residual on every field each year. Martens suggests incorporating them into the herbicide rotation by using them every three or four years.

Strategic tillage can work for weeds such as thistles, depending on the timing, says Martens. But tillage is detrimental to quack grass control.

“It can really deplete your soil nutrients as well,” says Martens.

Problem weeds

Group 1-resistant wild oats are common, Martens says. Resistant populations have developed from over-reliance on Group 1s. But in wheat, farmers don’t have many other options, he adds.

A recent report submitted to the Saskatchewan Weed Committee provides some insight on problem weeds. Between 2012 and 2016, Saskatchewan’s Crop Protection Lab screened over 1,100 weed samples for herbicide resistance from the Prairie provinces.

Unsurprising, wild oats were the biggest culprit. There were 550 Group 1-resistant wild oat samples and 135 cases of Group 1- and Group 2-resistant wild oats from the three provinces.

The lab also confirmed 10 cases of Group 1-resisant green foxtail. Most cases were in Saskatchewan. Two were in Manitoba’s parkland, and one between Calgary and Red Deer.

Three samples of Group 1-resistant Persian darnel were also submitted from southern Saskatchewan and Alberta.

The report was compiled by staff at Agriculture and Agri-Food Canada and Saskatchewan Agriculture, including Hugh Beckie, Scott Shirriff, Faye Dokken-Bouchard and Clark Brenzil.

The Crop Protection Lab tends to get fewer samples from Manitoba, as Ag-Quest does much of the herbicide resistance screening in that province. As well, farmers don’t tend to submit weeds once herbicide resistance is established for that weed and chemical.

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.

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