Managing Weed Shifts: Canada Fleabane and Waterhemp

Ontario 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 prior to 2010, is now the number one problem weed in the province, with waterhemp soon to take the lead as number one. In both cases, glyphosate (Group 9) resistance is at the root of this shifting weed control problem.

In 2010, glyphosate resistant (GR) Canada fleabane was found in Essex County, and it only took six seasons for this weed to spread across all of Ontario.  GR Canada fleabane is now confirmed in 30 counties as of 2020. While waterhemp seeds don’t travel by way of wind dispersal, GR waterhemp continues to spread quickly. From the first GR case discovered in 2014, as of 2020 it has been detected in 14 different Ontario counties with over half of these counties showing four-way resistance!

Fortunately, we do have tools in the toolbox to manage these challenging weeds through multiple effective modes of action, which may include herbicides, tillage, and other agronomic practices such as row width, cover crops and crop rotation. It is important to do your own research to understand which management practices suit your farm, or to seek help from your local agronomist.

Managing multiple resistances

Growers 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. We’ve seen this happen with both Canada fleabane and waterhemp. It’s helpful to understand the biology of these two weeds, especially germination patterns.

Canada fleabane is both an annual and a winter annual. Some seeds will germinate in the fall, so you could be dealing with some pretty big weeds right off the hop in the spring. Other seeds will germinate in the spring. Summer germination is uncommon if your crop closes canopy in good time. Waterhemp is much different in that it can and will germinate all season long. That makes these weeds much tougher to control than a lot of other annual or winter annual weeds.

Controlling these weeds and aiming to achieve the highest level of weed control is critical. Treat early using a pre-emerge residual herbicide like Boundary® LQD for soybeans so that you're starting clean and staying clean. This is especially important because soybeans have very limited 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.

Weeds are easier to control when they’re small. If there’s an opportunity to control Canada fleabane in the fall, you’ll have a higher rate of success controlling it then versus relying on tillage or your herbicide program in the spring, when weeds are large and well established. Then, when scouting 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. That includes using multiple modes of action to control them.

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 that you can track modes of action. This helps ensure you target those weeds every year with multiple modes of action.

It’s important for growers to 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.

Management options with herbicides

Whenever growers use a burndown herbicide, they need to add a second effective mode of action to the tank. The same is true for post-emerge applications. You don’t want to rely solely on glyphosate. It’s important to add another herbicide with a different mode of action to address the weeds on that farm.

When it comes to tank-mix considerations, Syngenta offers several strong options for controlling herbicide-resistant weeds. In soybeans, Boundary LQD is a great option for control of glyphosate-resistant waterhemp when applied pre-emerge to either traited or conventional varieties. Reflex® also provides good control post-emergence on small waterhemp plants where group 14 resistance has not yet been detected. If you are dealing with four-way resistant waterhemp, the only post options include glufosinate (Liberty®) and 2,4-D choline (Enlist Duo™ or Enlist™ 1) in Enlist E3™ soybeans, or Tavium® Plus VaporGrip® Technology up to the second trifoliate (or other dicamba-based products) on Roundup Ready 2 Xtend® soybeans.

For control of Canada fleabane, Boundary LQD can be used in combination with Eragon® and Roundup® to provide excellent burndown. A combination of Tavium Plus VaporGrip® Technology and Roundup® also offers flexibility with the ability to apply pre-plant/pre-emerge and post up to V2 to control Canada fleabane in Roundup Ready 2 Xtend® soybeans.

Testing for resistance

Seeding rates, row widths and cover crops can also play a role in combating herbicide resistance. 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.

For more information including articles, resources and videos on resistance management, check out Manage Resistance Now. This is a collaborative effort of industry, academia and government experts, brought together by CropLife Canada, to raise awareness and promote the adoption of strategies to manage weed, insect and disease resistance.

Always read and follow label directions. Roundup Ready® 2 Technology contains genes that confer tolerance to glyphosate. Roundup Ready 2 Xtend® soybeans contain genes that confer tolerance to glyphosate and dicamba. Glyphosate will kill crops that are not tolerant to glyphosate. Dicamba will kill crops that are not tolerant to dicamba. Roundup Ready 2 Xtend®, Roundup Ready® and VaporGrip® are registered trademarks of Bayer Group, Monsanto Canada ULC licensee. © 2021 Bayer Group. All rights reserved. Boundary®, Reflex® and Tavium® are trademarks of a Syngenta Group Company. Other trademarks are property of their respective owners. © 2021 Syngenta.

The transgenic soybean event in Enlist E3™ soybeans is jointly developed and owned by Dow AgroSciences LLC and M.S. Technologies L.L.C. Enlist® and Enlist E3™ are trademarks of Dow AgroSciences LLC.