Managing weed shifts -- Canada fleabane and waterhemp

Ontario corn and soybean growers have seen a tremendous shift in the weed spectrum over the past decade. Canada fleabane, a weed that wasn’t on the radar in 2007, is now the number one problem weed in the province. Waterhemp is another weed that’s quickly joining the hard-to-control ranks. In both cases, glyphosate (Group 9) resistance is at the root of this shifting weed control problem.

Syngenta Agronomic Services Rep Marijke Vanderlaan says agronomists can play a key role in helping growers manage these challenging weeds.

Managing multiple resistance

Marijke notes agronomists should be aware that any glyphosate-resistant weed also has a high probability of having resistance to other modes of action or becoming resistant to multiple modes of action in the immediate future. For example, glyphosate-resistant waterhemp was first identified in Ontario in 2014. Four years later, the first four-way resistant weed biotype in Canada was confirmed when waterhemp was found to be resistant to Group 2, 5, 9 and 14 herbicides at locations in Essex County. Canada fleabane is resistant to Group 2 and 9 herbicides.

When it comes to working with growers, Marijke says it’s important that agronomists help their customers understand the biology of these two weeds, especially germination patterns.

“Canada fleabane germinates the fall before, and can germinate basically any time of year, so you could be dealing with some pretty big weeds right off the hop in the spring. Waterhemp can germinate early and germinate all year long,” Marijke says. “That makes these weeds much tougher to control than a lot of other annual or winter annual weeds.”

Controlling these weeds in-season is critical. “Treat early using a pre-emerge residual herbicide so that you're starting clean and staying clean,” Marijke says. This is especially important in soybeans, which have limited or no post-emerge options for both weeds.

Record keeping is crucial

Scouting regimes also play a key role. For Canada fleabane, this activity starts in the fall.

“If weeds are detected in fall, they can be controlled and you don’t have to worry about controlling big weeds in the spring,” Marijke explains. “Then, when you scout in early spring, it’s important to know which weeds or weed families are susceptible to developing glyphosate resistance. When you see those weeds in the field, you need to manage them as though they are resistant to glyphosate.” She notes that glyphosate-resistant Canada fleabane has been found in 30 counties, stretching from Essex County in the southwest, all the way to Glengarry County in the east.

Marijke says good record keeping is a must. “Records become crucial. You need to record what weed species are in the field and what herbicide programs are being used so you can track modes of action. This helps ensure you target those weeds every year with multiple modes of action.”

It’s important for agronomists to help growers diversify both crop and herbicide rotations. In Ontario, that starts with expanding rotations to include corn, soybeans and wheat. Forages and other crops can add further diversity. Other recommendations include: rotating herbicide modes of action; using multiple modes of action; ensuring full rates are used to kill weeds; and shooting for excellent weed control to ensure potential resistant weeds do not set seed in your field.

Multiple modes of action

“Whenever we use a burndown herbicide, we need to add a second effective mode of action to the tank,” Marijke says. “The same is true for post-emerge applications. We don’t want to rely solely on glyphosate. We need to add another herbicide with a different mode of action to address the weeds on that farm.”

When it comes to tank-mix considerations, Marijke notes that Syngenta offers several strong options for controlling herbicide-resistant weeds in corn and soybean crops. When tackling waterhemp in corn, Acuron® offers 95% control when applied pre-emerge; Callisto® GT and Halex® GT also provide post-emerge control options. In soybeans, Boundary® LQD delivers 86% control of glyphosate-resistant waterhemp when applied pre-emerge. Reflex® also provides good control on waterhemp.

For control of Canada fleabane, Boundary LQD can be used in combination with Eragon® and glyphosate for burndown. Tavium™ Plus VaporGrip® Technology offers pre-plant/pre-emerge burndown control for Roundup Ready 2 Xtend® soybeans.

Testing for resistance

Seeding rates, row widths and cover crops can also play a role in waging war against herbicide resistance. Marijke says it’s important for growers to keep an eye out for the signs of resistance. These include: patches of uncontrolled weeds after spraying; live weeds beside dead weeds of the same species after spraying; a noticeable decline in weed control levels in recent years; or confirmed herbicide resistance in nearby fields, farms or ditches.

If resistance is suspected, growers can test for the presence of herbicide-resistant weeds. Ontario Ministry of Agriculture, Food and Rural Affairs Weed Specialist Mike Cowbrough says the key to collecting samples is to get as much seed as possible of the suspected resistant weed and to be sure to grab a representative sample from across the field. Cowbrough offers tips in this video.


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Always read and follow label directions. Roundup Ready 2 Xtend® soybeans contain genes that confer tolerance to glyphosate, an active ingredient in Roundup® brand agricultural herbicides, and dicamba, the active ingredient in XtendiMax™ herbicide with VaporGrip® Technology. Agricultural herbicides containing glyphosate will kill crops that are not tolerant to glyphosate, and those containing dicamba will kill crops that are not tolerant to dicamba. Genuity®, Genuity and Design®, Roundup Ready 2 Xtend®, Roundup®, VaporGrip® and XtendiMax™ are trademarks of Monsanto Technology LLC, Monsanto Canada, Inc. licensee. © 2019 Monsanto Canada Inc. Acuron®, Boundary®, Callisto®, Halex®, Reflex® and Tavium™ are trademarks of a Syngenta Group Company. Other trademarks are property of their respective owners. © 2019 Syngenta.