Four-way herbicide resistant common ragweed confirmed in Ontario

Common ragweed’s ability to resist the control efforts of different herbicide groups continues to grow across Ontario.

Last winter, University of Guelph weed scientist Dr. Peter Sikkema confirmed the presence of Group 14 resistance in common ragweed, making the broadleaf pest resistant to enzyme inhibitors, including branded weed control products such as Reflex and Blazer, which are key weed control products for identity preserved soybean and edible beans.

Earlier this week, Sikkema told agronomists, extension specialists, and crop retail representatives attending a virtual agribusiness meeting that his team has now identified the presence of four-way resistance in common ragweed in Ontario’s Bruce and Prescott-Russell Counties. That makes these weed seeds resistant to Group 14, Group 2 (ALS/AHAS inhibitor products, i.e. Pursuit), Group 5 (metribuzin) and Group 9 (glyphosate).

Sikkema found the four-way resistance in eight populations: six from Bruce County and two from Prescott-Russell. “That’s a big challenge for those affected growers in those two counties,” he said.

Sikkema noted that glyphosate resistant common ragweed has roots in Ontario that go back more than a decade — its presence in Ontario was confirmed in 2011. He agrees that the glyphosate resistance has not created a major weed control problem in most cropping fields across Ontario, but the growing cross-resistance will create bigger headaches for growers.

RealAgriculture agronomist Peter Johnson feels the cross resistance will make it increasingly difficult to control ragweed in soybeans and likely push growers to utilize more Enlist and Xtend soybeans. These soybean production systems rely on Group 4 herbicide chemistry, including 2,4-D from the phenoxy class (Enlist) and dicamba from the benzoic acid class (Xtend).

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