Canada
March 6, 2025
Herbicide resistance is a growing problem for farmers across Canada and soybean farmers in Ontario and Quebec face some of the highest rates of resistant weed populations. As you make plans for the season and analyze your current weed management strategies, we’re taking a deep dive into Canada fleabane and waterhemp—two of Ontario’s most widespread and aggressive weeds.
Key Challenges With Canada Fleabane
Canada fleabane is an aggressive winter and summer annual weed that can wreak havoc on farmers’ crop yields without proper management.
Canada fleabane presents its own unique set of challenges for growers as the windblown seeds can travel up to 500 kilometers in one dispersal event. The time of Canada fleabane emergence can affect soybean yield loss so let’s take a closer look at how to best manage this aggressive weed biotype in both IP and genetically modified (GM) soybean.
Managing Canada Fleabane in IP Soybean
To ensure acceptable control of multiple herbicide-resistant Canada fleabane it must be controlled prior to soybean emergence. The addition of a soil-applied herbicide such as TriActor® EZ, Bifecta® EZ, Fierce® EZ, and Valtera™ EZ can provide residual weed control to help fields stay clean through the critical weed-free period. For the control of fall-emerged Canada fleabane consider adding BlackHawk to the tank-mix for improved burndown activity, or do an early burndown pass before coming back and applying a residual pre-emergent product.
TriActor EZ can provide up to eight weeks of residual weed control on a wide range of broadleaf and grass weeds. Featuring three modes of action, including metribuzin, in an all-in-one liquid formulation, TriActor EZ is considered the best pre-emergent herbicide for IP soybeans.
It is important to ensure that the soil-applied herbicide you select is based on weed species composition in each field, the characteristics of your soil, and the crops you plan to use in your field’s rotation.
Managing Canada Fleabane in GM Soybean
When controlling Canada fleabane, significant advances have been observed in herbicide-resistant soybean and farmers still see good results from Group 4- herbicides like dicamba or 2,4-D in-crop with glyphosate. However, by repeatedly applying the same modes of action on the crop, farmers risk rapidly selecting for herbicide-resistant weeds.
Using a two-pass weed control program of a soil-applied herbicide followed by a post-emergence herbicide improves weed control, reduces weed interference and can maximize profitability.
Growers should scout their fields after soybean emergence and apply a post-emergence herbicide where necessary. The selected post-emergence herbicide should match the weed spectrum in each individual field.
Key Challenges With Waterhemp
Waterhemp is another competitive weed that can reduce your soybean yield and profitability. While it’s relatively new to Eastern Canada and the province of Ontario, it’s grown to be a major concern for several reasons. Waterhemp is a dioecious species that has a rapid growth rate and can produce a high number of seeds that emerge throughout the growing season, especially following rain events.
Farmers are also noticing an increase in waterhemp due to agricultural practices. Reduced tillage systems favour small-seeded annual broadleaf weeds, such as waterhemp, and can increase its presence in the field. As farms have increased in size and began using equipment over a larger number of acres, contaminated equipment has also increased the prevalence of waterhemp as it can spread through farm equipment such as tillage implements and combines.
Managing Waterhemp in IP and GM Soybean
Unlike Canada fleabane, the effective management of waterhemp is quite similar in both IP and GM soybeans.
Growers who have multiple herbicide-resistant waterhemp in fields where they plan to grow soybean are highly encouraged to use a two-pass weed control program, starting with an effective soil-applied herbicide such as Fierce EZ, TriActor EZ, Bifecta EZ and Valtera EZ. After soybean emergence, growers are encouraged to scout their fields and apply post-emergent herbicides specific to the weed escapes you find.
To learn more about Nufarm products and the value they can provide for you and your soybean crops, visit nufarm.ca/soybeans.
Check out our Q&A with Dr. Peter Sikkema, Professor emeritus, University of Guelph and a leader in the management of multiple herbicide-resistant weeds.
How much have herbicide-resistant Canada fleabane and waterhemp affected soybean growing in Ontario?
In trials conducted in Ontario, multiple herbicide resistant Canada fleabane interference has caused an average yield loss in soybean of 61%. I want to stress that the yield loss is field specific and in our trials it ranges anywhere from zero to 99% and the level of yield loss is dependent on the fleabane density in each individual field as well as the relative time of weed and crop improvements. Canada fleabane that emerges in the fall has a greater competitive effect on soybean than spring-emerging Canada fleabane.
In a survey that I did of agronomists in Ontario during the fall of 2023, 60% of Ontario soybean and corn growers have changed their weed management practices because of the presence of a multiple herbicide-resistant Canada fleabane. These growers have used additional herbicides to their weed management program to control this one herbicide-resistant biotype.
Waterhemp is increasing rapidly in the province of Ontario. It’s evolved resistance to five different modes of action. Waterhemp is a dioecious species, which is unique among common weed species in the province. Most weed species are monoecious. Waterhemp is an obligate cross-pollinating species which results in wide genetic diversity which provides the possibility of individual plants having resistance traits for the various herbicides used in the province of Ontario. And it has some adventitious genetic traits, such as a high growth rate, high seed production, prolonged seed dormancy, and extended germination and emergence throughout the growing season.
How can Canada fleabane be best controlled in identity-preserved (IP) soybean?
In IP soybean, the backbone for managing multiple herbicide-resistant Canada fleabane is glyphosate + a saflufenacil-based herbicide + Merge applied preplant in no-, reduced- or strip till; however control is variable. To improve the consistency of control, metribuzin should be added; this will improve the consistency of Canada fleabane control dramatically.
What can GM soybean growers use to control Canada fleabane?
In Xtend soybean, dicamba provides good control of multiple herbicide-resistant Canada fleabane; but control can be variable; I recommend that you add a saflufenacil-based herbicide + Merge to improve the consistency of control.
In E3 soybean, 2,4-D provides variable control of multiple herbicide-resistant Canada fleabane; the addition of a saflufenacil-based herbicide + Merge improves the consistency of control.
How can IP soybean growers best manage waterhemp?
You have to plan to use a two-pass weed control program. I think growers need to start with an effective soil-applied herbicide and in our research, the most effective soil-applied herbicides were Fierce EZ, TriActor EZ, Bifecta EZ, Valtera EZ, and Authority® Supreme. Fierce EZ is the most effective soil-applied herbicide in terms of a starting point for managing multiple herbicide-resistant waterhemp in soybean, providing approximately 90% control, whereas the other herbicides it was between 80 and 85% control.
I think once you’ve put down your soil-applied herbicide after the crop has emerged, you have to scout the field regularly. If you do have waterhemp escapes, the Group 14 herbicides are the products of choice for managing high waterhemp escapes in identity-preserved soybean; however, Group 14 resistant waterhemp has been confirmed in some Ontario fields.
How can waterhemp be controlled in GM soybeans?
In genetically modified soybean, you would manage waterhemp similar to the way you would manage it in identity-preserved soybean. I think it’s imperative that you start with an effective soil applied herbicide such as Fierce EZ, TriActor EZ, Bifecta EZ, Valtera EZ, and Authority Supreme and then scout the field.
If you’re growing Xtend soybean, if there are escapes, you can come back with a post-emergence application of dicamba. If you’re growing E3 soybean, you can come back with a post-emergence application of 2,4-D.
I would like to stress that if you are going to come back with a post-emergence herbicide in either E3 or Xtend soybean, it’s really important that if sensitive crops have emerged in neighbouring fields that you may want to switch from a 2,4-D or dicamba-based herbicide to Liberty to reduce the potential of crop injury due to off target movement.
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