If you grow soybeans and have Canada fleabane, common ragweed or giant ragweed growing in your fields, the use of pre-plant herbicide tank-mixes will be your best chance at achieving good control of these weeds. Management options for these three weeds are either limited or do not provide acceptable control once the soybean crop has emerged.

Why are these weeds so difficult to manage? Populations of all three species exist in Ontario that are resistant to glyphosate. In addition there are also populations of Canada fleabane and giant ragweed resistant to FirstRate (cloransulam) and Classic (chlorimuron).

Canada fleabane (also known as Mare's Tail and Horseweed)

Herbicide control

  • Figures 1-3
  • 12 Counties in Ontario have glyphosate resistant populations (Figure 4)
  • 12% of confirmed glyphosate resistant sites in Ontario were also resistant to "group 2" herbicides (for example, FirstRate) and occur in six counties (Figure 4).
  • Tall plants (1.5 m (4'-6") or higher) can produce over 200,000 seeds
  • Majority of seeds (~80%) will germinate upon dispersal.
  • Seeds are air born and will move several hundred metres.
  • see Table 1. Herbicide control
A small seedling fleabane plant in mid-September.
Figure 1. A small seedling fleabane plant in mid-September.

I heard Sencor (metribuzin) was effective on fleabane. Why isn't it included in the Table 1?

The 600 g/acre rate of Sencor 75DF (metribuzin) provided exceptional control in Dr. Sikkema's trials. However, this is the highest label rate and can cause significant crop injury and yield loss if used on high risk soils (low organic matter, coarse textured). Efficacy of Canada fleabane with lower rates of metribuzin have not been evaluated in Ontario. However, Eubank et al., 2008 used the equivalent of 225 g/acre of Sencor 75DF and observed Canada fleabane control that ranged from 53–73%.

Photo of a Canadian fleabane rosette with a measuring tape beside it showing a size of 4 inches.
Figure 2. A Canada fleabane rosette in late-May. This is beyond the ideal stage to effectively manage with either herbicides or tillage
A 2 leaf giant ragweed seeding.
Figure 3. A 2 leaf giant ragweed seeding.

Giant ragweed

  • 7 Counties in Ontario have glyphosate resistant populations (Figure 5)
  • 6% of confirmed glyphosate resistant sites in Ontario were also resistant to "group 2" herbicides (for example, FirstRate) and occur in 3 counties. (Figure 5)
  • Considered one of the most competitive species in Ontario.
  • Female plants (seed producing) are wind pollinated by male plants.
  • Wind born pollen can travel several kilometers.
  • Figure 6
  • see Table 2. Herbicide control
Map of Ontario showing the six most southerly counties highlighted in yellow and five counties to the east adjacent to Lake Erie highlighted in red. The region of Niagara is also in red.
Figure 4. Herbicide resistant Canada fleabane occurrence in southern Ontario counties.
Map of Ontario showing the three most southerly counties highlighted in yellow and three counties to the east and north highlighted in red. The counties of Lennox and Addington ton is also in red.
Figure 5. Herbicide resistant giant ragweed occurrence in southern Ontario counties.
A giant ragweed plant past the ideal stage for control with a herbicide.
Figure 6. A giant ragweed plant past the ideal stage for control with a herbicide.

Pre-plant management options for Canada fleabane

Table 1. Visual control of glyphosate resistant Canada fleabane when herbicides with different modes of action were tank-mixed with glyphosate and applied prior to planting soybean.
Glysophate tank mix partnerRate% Control
Eragon + Merge14.4g + 0.5% v/v98
First Ratefootnote *17g/ac95
Amitrol 240footnote **3.3 L/ac93
2,4-D LV Ester 700footnote **0.32 L/ac86
Liberty1 L/ac75

Pre-plant management options for giant ragweed

Table 2. Visual control of glyphosate resistant giant ragweed when herbicides with different modes of action were tank-mixed with glyphosate and applied prior to planting soybean.
Glysophate tank-mix partnerRate% Control
2,4-D LV Ester 700footnote **0.32 L/ac96
Amitrol 240footnote **3.3 L/ac91
Eragon + Merge14.4g + 0.5% v/v87
FirstRatefootnote ***17 g/ac84

Acknowledgements

The research conducted by Dr. Sikkema's lab could not have been done without the assistance of Chris Kramer (research technician), and current and former graduate students Holly Byker, Laura Ford, Joanna Follings and Joe Vink. Funding for this project was provided by the Agricultural Adaptation Council, Monsanto Canada and the Grain Farmers of Ontario.