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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 weedresistance

By Lisa Guenther | Published: February 16, 2018


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


Group 1 herbicides are commonly applied in-crop to wheat. Group 1herbicides 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 lipidsin 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, Martenssays. If a farmer continues using the Group 1 on these resistant weeds, theywill survive and reproduce. Eventually the Group 1-resistant weeds willdominate the population.


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

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

There are some tank mix options that work well for Group 1s, Martenssays, such as Assure and glyphosate. But Group 1 herbicides target grassyweeds, 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 1resistance. Not rotating crops puts pressure on the land, with both resistantweeds and disease, Martens says. Rotating crops also helps farmers switchchemical modes of action.


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


Group 1 contains three sub-groups:

Aryloxyphenoxy propionate. Known as fop, it includes active ingredientssuch as clodinafop-propargyl.

Cyclohexanediones. Known as dim, this sub-group includes activeingredients such as clethodim.

Phenylpyrazolin. Known as den, it includes the active ingredientpinoxaden.

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


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


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


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


Strategic tillage can work for weeds such as thistles, depending on thetiming, 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. Resistantpopulations 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 providessome insight on problem weeds. Between 2012 and 2016, Saskatchewan’s CropProtection Lab screened over 1,100 weed samples for herbicide resistance fromthe Prairie provinces.


Unsurprising, wild oats were the biggest culprit. There were 550 Group1-resistant wild oat samples and 135 cases of Group 1- and Group 2-resistantwild oats from the three provinces.


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


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


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


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

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