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Developing resistance: Group 2 herbicides

Each herbicide group kills weeds differently. Weed resistance differs by herbicide group

When talking about herbicide resistance, Bryce Geisel likes to make sure people realize that spraying herbicides doesn’t cause resistance in a weed. Instead there are individual plants that, by chance, resist the herbicide. Those plants survive and pass on their resistance traits.

“And with Group 2s in particular, it’s just altering the target site,” says Geisel, BASF technical marketing specialist.

The target site, in this case, is the ALS enzyme, which the plant uses to build amino acids. Group 2 herbicides bind to the ALS enzyme, disrupting amino acid synthesis. The weed stops growing and eventually dies.

“When people spray Group 2s on a weed, it can take a long time for that weed to die,” says Geisel, adding it could be 14 to 21 days. The plant eventually starts to show symptoms such as purpling of the growing point and chlorosis before dying.

However, if the weed’s ALS enzyme has changed, the Group 2 might not bind to it. The plant will keep synthesizing amino acids and keep growing.

Geisel says Group 2 resistance is one of the most common types of resistance. Odds are higher that Group 2 resistance will show up than glyphosate resistance, for example. Kochia tops the Group 2 resistance list — Geisel says about 90 per cent of the kochia population is resistant. Cleavers and wild mustard are also problem weeds with Group 2 resistance.

While glyphosate and some Group 4 resistance come with a fitness penalty, this isn’t the case with Group 2 resistance, Geisel says. That means ceasing to use Group 2 herbicides for a few years isn’t going to bring that chemistry back once resistance develops. And while glyphosate resistance has a fitness penalty, Geisel says it’s used so frequently, it would be tough to remove it completely from the system.

How to manage resistance

Geisel emphasizes that having resistant weeds doesn’t make someone a bad farmer. Sometimes it’s a matter of awareness. There are also weed seeds that can travel quite a distance, such as kochia and Canada fleabane.

“Group 2s are still very important for western Canadian farmers, whether it’s in pre-seed or in-crop (applications),” says Geisel. Pulse growers rely on them heavily. Cereal growers also use Group 2 herbicides to control wild oats, he adds.

But farmers need to supplement Group 2 herbicides with other groups in the tank mix. Geisel advises farmers to think about multiple modes of effective action when dealing with resistant weeds. For example, when dealing with kochia, there’s little point in spraying a Group 2 and Group 9 tank mix, given the widespread resistance to Group 2 herbicides in kochia.

In fact, Geisel encourages farmers with resistant weeds to look at three modes of action. While three mode products aren’t often available in one package, Geisel says farmers can combine products.

For example, farmers growing Roundup Ready soybeans could use a tank mix of glyphosate (Group 9) and Viper (contains Group 2 and 6), he says.

There are two basic issues farmers run into when tank-mixing herbicides:

  • Antagonism. Geisel says this is typically caused by a contact herbicide, as it works too fast for the translocation of a product.
  • Different formulations aren’t compatible. This might, for example, turn the products into a gel in the tank.

Geisel suggests farmers check with retailers and manufacturer reps before tank mixing to avoid problems. “We do a lot of testing with different products to see which ones are compatible and which ones are not.”

Geisel advises farmers to think about how to rotate chemical groups within the year as well. For example, farmers might use a Group 14 or 15 before seeding, and then use other products in-crop. He adds farmers can also rotate tank mixes.

It’s also important to know which chemical groups are in a product. Geisel says that because there are different chemical families within chemical groups, people don’t always realize that two or three different products are all Group 2 herbicides. For example, Odyssey, Express, and Simplicity are all Group 2 chemistry, even though the active ingredients belong to different chemical families.

Geisel acknowledges that resistance management becomes complicated for farmers.

“It all becomes that complex system that we need to weave together. But herbicide resistance isn’t going away. It’s definitely something farmers are going to have to deal with for a number of years to come.”

Group 2 resistance in Western Canada

A report to the Saskatchewan Weed Committee sheds some light on Group 2 resistance in Western Canada. The report details the results of over 1,100 weed samples submitted to Saskatchewan’s Crop Protection Lab between 2012 and 2016 for herbicide resistance testing. The Crop Protection Lab has been screening for herbi- cide resistant weeds since 1996. Both Saskatchewan Agriculture and Agriculture and Agri-Food Canada conduct herbicide resistance tests. Hugh Beckie, Scott Shirriff, Faye Dokken-Bouchard, and Clark A. Brenzil authored the latest report.

Among those samples, staff confirmed the following cases of Group 2 resistance:

  • 54 cleaver cases (31 in Sask., 22 in Alta., and one in Man.);
  • nine wild mustard cases in Sask.;
  • seven stinkweed (six from Sask., one from Alta.);
  • five shepherd’s purse, all in Sask.;
  • one case of chickweed, in Sask.; and,
  • one case of redroot pigweed, also in Sask.

Wild oat is the weed Geisel is most concerned about, as there are few control options available. His concern is well-placed. Of the weed samples submitted to the Crop Protection Lab for resistance testing, 87 per cent were wild oats. Group 2-resistant wild oats comprised 108 of those samples. There were 135 cases of Group 1 and 2-resistant wild oats, and 550 were Group1-resistant.

photo: Alberta Agriculture and Forestry

About the author

Field Editor

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

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