Kochia, one of Western Canada’s most abundant and economically devastating weed species, has won another major battle in overcoming herbicide.
This summer, the first known Group 4 herbicide-resistant kochia was identified in a durum wheat field in southwestern Saskatchewan. And samples currently being tested from a farm in southern Alberta are also suspected to be Group 4 resistant.
This new resistance follows just five years after Alberta’s first glyphosate (Group 9) resistant kochia was identified in 2011 in Warner County.
“There’s no question this will complicate kochia management. When you consider that all kochia is Group 2 resistant, and then you stack on Group 9 resistance as well, and now Group 4 resistance too, that three-way resistance starts to limit options,” said Hugh Beckie, a weed scientist with Agriculture and Agri-Food Canada. “It could be worse — there are still other herbicide options to control kochia — but it puts another nail in the coffin in terms of herbicide efficacy.”
The two cases of Group 4 resistance are likely only the tip of the iceberg.
“When new resistance comes up there are almost certainly many more cases out there that just haven’t yet been identified,” said Beckie. “We expect to see a number of cases across the Prairies in the next few years.”
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Kochia’s prolific seed production and tumbleweed nature mean new resistant strains have the potential to spread virulently.
By 2013, just two years after it was first identified in southern Alberta, glyphosate-resistant kochia had spread to at least 18 nearby fields. But, said Beckie, do not assume that only nearby fields are at risk: kochia can tumble long distances, spreading seeds all the while. And additional resistant strains are likely to spontaneously appear in other fields as kochia manages to repeatedly breach the herbicide barrier.
Though no official surveys have been conducted to map its spread since 2012, Beckie suspects glyphosate-resistant kochia now exists in many western Canadian fields. Surveys will be conducted in Alberta this year, in Manitoba in 2018 and in Saskatchewan in 2019 to determine how common and how widespread glyphosate-resistant kochia has become.
“Back in 2012, about five per cent of fields showed glyphosate-resistant kochia,” said Beckie. “The new surveys will give us a good indication of how quickly it is evolving and spreading. How much of an increase we should expect is hard to say. We’ll have to wait and see but I would expect (the per cent of fields impacted) to be in the double digits.”
Now that Group 4 resistance has also been identified, researchers will attempt to map its early spread as well.
“I expect the Group 4 resistance will repeat what we’ve seen with Group 2 resistance — fairly rapid and widely spread resistant population increases,” said Beckie. “However, an apparent fitness penalty associated with that type of resistance may slow its development and spread.”
Beckie is not surprised by the new Group 4 resistance. Group 4- and 5-resistant kochia biotypes appeared in the northern U.S. several years ago. Even if resistant strains do not blow up from our neighbours to the south, herbicide resistance is simply a numbers game. The more often a herbicide is applied, especially on a common weed and highly prolific seed producer like kochia, the more likely resistance will develop. Given producers’ heavy reliance on herbicides, scientists agree that resistance to all herbicide options is only a matter of time.
“I’m actually surprised how durable the Group 4 chemistry has been to date considering they are very old chemistries dating back to the Second World War,” said Beckie. “Today, a lot of herbicide products are prepackaged or tank mixed with a Group 4 so we are seeing increased Group 4 use Prairie-wide. And new crops like soybeans are coming with a Group 4 gene, which also puts more pressure on that kind of herbicide. Selection pressure is there in most fields now, so we knew this was coming.”
So be vigilant about field scouting, he said.
“The awareness is there, but when you add time pressures, sometimes things fall through the cracks. Producers need to really be watching for how their herbicides are working so that they can catch problems before they burgeon out of control.”
If you suspect Group 4 resistance, contact Beckie at [email protected] to arrange free testing.