By Julienne Isaacs
According to Peter Sikkema, professor of field crop weed management at University of Guelph’s Ridgetown Campus, glyphosate-resistant Canada fleabane was first found in eight fields in Ontario’s Essex County in 2010.
Glyphosate-resistant Canada fleabane is very difficult to control in soybean: studies have shown it emerges 11 out of 12 months of the year in southern Ontario – skipping only January.
For producers with multiple-resistant fleabane, Sikkema says, there are no post-emergence herbicide options for control in soybean.
But those producers with glyphosate-resistant Canada fleabane still have a few chemical options.
At the top of this list, Sikkema says, is use of a preplant burndown. “The best is glyphosate plus Eragon (saflufenacil, Group 14) plus Sencor (metribuzin, Group 5). In our studies, this tankmix provided 95 per cent control, which I think is acceptable,” he says.
If producers are growing Roundup Ready Xtend soybeans, they can use a preplant burndown of Roundup Xtend.
The best option for post-emergent control is FirstRate (cloransulam-methyl), a Group 2 herbicide that’s registered for pre- and post-emergence in soybean. But for producers with multiple resistant Canada fleabane, this herbicide will not be effective.
“Preplant burndown herbicides can’t be used on their own because they did not provide acceptable control in our trials,” Sikkema says.
A study led by Sikkema’s graduate student, Holly Byker in 2011 and 2012, found that burndown applications of Roundup plus saflufenacil (Eragon) provided the most consistent control of Canada fleabane, but even this tank mix did not provide acceptable control in all situations.
A study led by another graduate student, Christopher Budd, in 2014 and 2015, built on that research and found that the most efficacious tank mix for the control of glyphosate-resistant Canada fleabane was glyphosate plus saflufenacil plus metribuzin applied pre-plant in Ontario.
An additional study conducted at the University of Guelph Ridgetown Campus set out to determine the dose response of glyphosate-resistant Canada fleabane to Sencor in a tank mixture with Roundup applied preplant. Based on eight field experiments conducted between 2013 and 2015, the study found that that the highest label rate of metribuzin (1,120 grams of active ingredient or g a.i. per hectare) tank-mixed with glyphosate (900 g a.i. per hectare) does not always provide consistent control of the weed.
The metribuzin rate required to achieve 95 per cent control of glyphosate-resistant Canada fleabane was 1,720 g a.i. per hectare at four weeks after application and 2,237 g a.i. per hectare at eight weeks after application.
The 2014/2015 study also found that improved control of glyphosate resistant Canada fleabane was obtained when glyphosate plus saflufenacil when applications were made during daytime hours.
“Just altering your herbicide application timing during the day had an impact on glyphosate-resistant Canada fleabane control and soybean yield,” Sikkema says. But because of the risk of offsite movement of herbicide via drift, producers need to be cautious when considering daytime application.