Using a multi-prong strategy against herbicide resistant weeds

There are more pre-seed control options available to suppress herbicide-resistance, but use cultural management as well

These thriving kochia plants are probably resistant to more than one herbicide mode of action.

More than half of western Canadian fields under annual crop production contain at least one herbicide-resistant biotype, according to surveys from 2014 to 2017. In Alberta, roughly 60 per cent of fields surveyed have resistant weeds; in Saskatchewan that figure is 57 per cent and in Manitoba it’s 68 per cent.

“The surveys have been repeated over the years since the early 2000s and have shown a large increase in herb­icide resistance,” says program head Charles Geddes, a research scientist with Agriculture and Agri-Food Canada in Lethbridge.

He says new biotypes of herbicide resistance are always cropping up and it’s important to find them while they can still be controlled because in some cases — like multiple-resistant kochia — there are few tools left in the toolbox.

In kochia, herbicide resistance is snowballing. In Alberta, all kochia populations are resistant to ALS inhibitors (Group 2), while at least half are also resistant to glyphosate (Group 9) and a growing number (18 per cent) are resistant to dicamba (Group 4). According to the results of a 2017 study led by Hugh Beckie, triple resistance was found in 10 per cent of surveyed Alberta fields.

Geddes is currently leading a project looking at options for managing herbicide-resistant kochia, including cultural weed management approaches such as rotation planning, higher seeding rates and optimal row spacing, as well as herbicide packages. Unpublished data from this work shows that including a range of methods does prolong the effectiveness of herbicides.

Row spacing

Geddes says the first two years of the ongoing study compared narrow (9″) versus wide (19″) row spacing as well as recommended versus twice the recommended seeding rates for a wheat-canola-wheat-lentil crop rotation. Results show that the same rotation and optimal pre/post-emergence herbicide packages result in half the kochia biomass for narrow versus wide row spacing.

“When we’re talking about herbicide resistance, it’s becoming abundantly clear that the answer to solving our problem with herbicides is not to apply more chemicals,” Geddes says. “So the real question is how to integrate the herbicides that remain effective with other tools to help prolong their efficacy down the road.”

That means integrating non-chemical tools, whether mechanical or cultural, to help improve herbicide efficacy. Any weed control method, including tillage, will put selection pressure on weeds, he explains — so the more methods in play, the better.

“When you have a resistant weed like kochia growing in a production region like Lethbridge to which that weed is very well suited, even if you’re using the best and most expensive herbicide package, it’s still quite difficult to control, so in addition to that you need to be adding in these other non-chemical tools,” Geddes says.

Pre-seed options

Short of in-crop herbicide options, pre-seed herbicides are becoming an important tool for some producers.

The number of choices is increasing, says Andrew Geerligs, a senior brand manager for BASF.

“The pre-seed market is very interesting. The canola pre-seed add-in market is pretty new relative to some of the others. The market started off with a very small number of products but it’s been growing.”

A good U.S.-based website for producers interested in integrated weed management options is growiwm.org, which contains IWM resources compiled by GROW (“Getting Rid of Weeds”), a publicly led outreach effort led by agronomists, weed scientists and economists. photo: GROW (website screencap)

The company just launched Certitude (topramezone), the first Group 27 herbicide registered in canola, which provides pre-seed weed control of herbicide-resistant kochia and volunteer canola.

The competitive landscape for pre-seed treatments in canola includes Bayer’s Pardner (Group 6), a glyphosate tank-mix partner for control of resistant volunteer canola; Corteva’s Prospect with Arylex Active, a Group 4 and Group 14 glyphosate tank-mix partner; FMC’s Aim (Group 14); and Nufarm’s Conquer II, a Group 14 and Group 6 pre-seed burndown product registered for control of volunteer canola, kochia and other herbicide-resistant weeds.

Last fall, BASF received PMRA registration for another pre-seed burndown product, Smoulder (Saflufenacil and Metsulfuron-methyl), a Group 14 and Group 2 herbicide registered against volunteer canola. Geerligs says the product should be available to producers for the 2022 growing season.

Certitude is intended to be mixed with glyphosate and the label includes a large number of weeds but the company’s trials have focused on hard-to-control weeds like volunteer canola and kochia.

Geerligs says it’s a contact and systemic product that does not provide residual control, and producers will see best results by applying as close to seeding as possible when most weeds have emerged. Producers with low weed pressure should use a water volume of five gallons per acre; volume should increase to 10 gallons per acre on fields with moderate to high weed pressure.

Hit Kochia early

Kochia is among the first weeds to emerge in the spring, which means most of the population is targeted by pre-seed herbicides, Geddes says. “This is why pre-seed herb­icides are so important for kochia control.”

He says resistance development isn’t a straightforward process and there isn’t always a clear demarcation showing which weeds have developed resistance to which chemicals. That means it isn’t always clear when producers should stop using particular herbicides.

Wild oat is another weed of huge concern to scientists in Western Canada, where resistance to Group 1 and Group 2 herbicides is found in over a quarter of fields sampled. Glyphosate-resistant wild oat was found in Australia last year, which means Canadian farmers should be on the lookout. But Geddes says there are often various types of cross-resistance within and among chemical groups, and some weeds don’t demonstrate dramatic “blanket resistance.”

“A certain mutation causing herbicide resistance will not necessarily confer resistance to all the chemical families in Group 1, for example. There are multiple mutations that can cause that type of resistance.”

The best way forward, he says, is to prolong the use of the herbicides that still work by adding cultural control methods and even mechanical control methods to reduce the weed seed bank.

About the author

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Julienne Isaacs

Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]

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