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Get back to basics to fight kochia

Using multiple and rotating modes of action can reduce selection pressure

In recent years, kochia has become a real issue across the Prairies. Kochia loves hot, dry weather, and as a prolific seed producer, the tumbleweed-shaped weed can spread quickly. What’s worse, kochia has a growing history of resistance, which can make it a tricky weed to manage. Agriculture and Agri-Food Canada research scientist Charles Geddes explains.

Kochia is a unique weed in a number of ways. There are certain things about its biology that allow it to thrive, especially in the drier regions of Western Canada. First of all, said Geddes, it’s the first weed to emerge in the spring. “It emerges after 50 growing degree days,” he said. “So most of the kochia population has emerged before almost all other weeds.”

For this reason, a pre-emergence or pre-seeding herbicide application is very important for managing early seedlings. “But with that said, it also has what we call an elongated emergence periodicity, which means that it can emerge throughout the growing season up to the point that you do have kochia plants emerging after your in-crop or post-emergence herbicide application,” he said.

To complicate matters further, kochia is saline and drought tolerant, which makes it perfect for those low-lying areas where crops struggle. In the absence of competition, each plant can produce up to 30,000 seeds. Its dispersal mechanisms mean that the weed can easily disperse to more productive areas of the field.

Kochia also has the potential for outcrossing, which means it can select for resistance quite quickly. The seed bank, however, is very short lived, which is unique in the world of weeds. Most seeds — about 90 per cent — lose viability within the first year. “But when you think about 10 per cent of 30,000 seeds per plant, you’re still talking about 3,000 seeds per plant, which is still a lot that end up surviving that first year,” said Geddes.

The way that the weed emerges makes it very difficult to find a management “sweet spot” in terms of timing. Most, however, emerge before seeding. Geddes recommends herbicide layering, so using pre- and post-emergent applications to best manage the weed. Post-emergent solutions are becoming quite limited due to resistance.

Herbicide resistance a growing concern

Herbicide-resistant kochia first came on the radar in Canada in the late 1980s when Group 2 herbicides were starting to see failure. Today, every kochia population is considered resistant to Group 2 herbicides.

In 2011, the first case of glyphosate-resistant kochia was found in Warner County, south of Lethbridge, Alta. A baseline survey in Alberta in 2012 revealed that about five per cent of kochia plants in the province were glyphosate resistant. The same survey was repeated in the fall of 2017, revealing that glyphosate-resistant kochia populations had increased from five to 50 per cent.

kochia

The green plants in this low-lying saline area are kochia. In Alberta, 10 per cent of the kochia population is triple resistant, resistant to chemicals in Groups 2, 4 and 9.
photo: Courtesy Charles Geddes, AAFC

“So we’re seeing very, very, very quick selection pressure for glyphosate resistance,” said Geddes. “I was pretty surprised when I saw that. I didn’t think it was going to happen that quickly.”

A 2013 baseline survey in Manitoba uncovered two glyphosate-resistant populations. Fields were surveyed again in 2018, but results won’t be in until sometime in 2019.

“I’m almost certain that it has grown,” said Geddes.

A 2013 survey in Saskatchewan revealed glyphosate-resistant populations in the southwest of the province. Geddes doesn’t know what those numbers look like now. A new survey will be conducted this year.

“I’ll suspect that we’ll likely see something similar to Alberta, though,” he said.

The resistance story doesn’t stop there. The 2017 survey in Alberta confirmed a triple-resistance case in kochia. The weed showed resistance to Dicamba (Group 4, 18 per cent in total), glyphosate (Group 4, 50 per cent in total), and Group 2 (all populations). Triple-resistant population (Group 2, 4 and 9) levels are at 10 per cent in Alberta.

“What I’m really concerned about is we’re seeing a shift in herbicide use to try to manage these glyphosate-resistant populations, and much of that as far as a pre-seeding option, we’ve shifted to use of Group 14s,” said Geddes. “We haven’t found Group 14 resistance in kochia yet, but that being said, it isn’t something we’ve been focused on screening for.” (Group 14 chemicals use aryl triazone, and include brand names like Aim, Blackhawk, Heat and Authority.)

“I think that’s something we’re going to have to do in the future because it’s likely only a matter of time until we start seeing resistance to Group 14,” he added.

Now that there’s also resistance to Group 4, there will be more reliance on Group 6 for in-crop use, which also creates cause for concern.

“It’s become obvious that we are not going to be able to spray our way out of managing kochia, meaning that herbicides are definitely not the only solution,” said Geddes. “Using alternative herbicides definitely can help reduce selection pressure if used properly, but that’s only part of the solution.”

Solutions, said Geddes, might lie in the weak points of kochia’s biological makeup. For instance, the relatively short-lived nature of kochia seeds could mean that better management could choke out populations, thereby lowering the number of populations that can return in subsequent years.

Kochia is also responsive to competition, so growing a competitive crop could help reduce viable seed production significantly. Using narrower row spacing will increase the speed at which the crop canopy closes. Higher seeding rates can aid in this as well.

Researchers are also looking into how to manage the low-lying, high-saline areas in the field where kochia seems to thrive. “Trying to decrease salinization of the soil could be an effective option in the long run because kochia really thrives in those saline areas and your crop really does not,” said Geddes.

While this could be a good long-term solution, it will take several years to address. “I think a more immediate solution is seeding those unproductive areas down to a saline-tolerant forage mix, and having something there that also thrives in those saline areas that can compete well with the kochia population,” he concluded.

About the author

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Melanie Epp is a freelance farm writer.

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