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Control of Glyphosate Resistant Fleabane

Mike Cowbrough, OMAFRA - Guelph
Francois Tardif, University of Guelph

Glyphosate is an extremely effective herbicide for the control of Canada fleabane. Unfortunately, glyphosate resistant Canada Fleabane is now prominent in a number of American States. The tremendous increase in the use of Roundup Ready crops is perhaps the biggest reason why resistant weed populations have been selected in those states. In Ontario the use of herbicide tolerant crops continues to increase. Although there have been no confirmed cases of glyphosate resistant weeds, overuse of any technology will inevitably lead to either weed shifts or resistance.

Many producers are aware that rotation of crops and herbicides, to different modes of action, is the most effective approach for managing against the development of weed resistance. These approaches are more critical now since there are literally no new herbicides (with new modes of action) coming to the Ontario marketplace anytime soon.

A field trial was conducted in 2003 to evaluate herbicides with different modes of action that are effective at controlling Canada Fleabane in soybean. The results of the trial are shown in Table 1.

Table 1. Rate, Mode of Action and % control of 5 herbicides applied to Canada Fleabane at Woodstock, ON in 2003 & 2004.

Herbicide Product rate/acre Mode of Action

Control (%)

glyphosate 1 L/ac Group 9


Classic 14 g/ac Group 2


First Rate 4.25 g/ac Group 2


First Rate 8.5 g/ac Group 2


Amitrol 240 1 L/ac Group 11


Amitrol 240 2 L/ac Group 11


Broadstrike Dual II Magnum 2 L/ac Group 2, 15


Classic, First Rate, Amitrol 240 and Broadstrike Dual II Magnum all provided acceptable levels of Canada Fleabane control and would be suitable options in soybean as a tank-mix partner with glyphosate (where registered) or as an alternative to glyphosate.

The authors would like to acknowledge the financial support of Monsanto for this field trial.