Pearce: Glyphosate-resistant weeds come to Huron County

Glyphosate resistance continues to be a hot topic as more Ontario growers head into the field to finish off their corn planting and begin work on their soybean acres.

Yet according to Dr. Peter Sikkema, a recent find of glyphosate-resistant Canada fleabane and giant ragweed in Huron County comes as something of a surprise.

Sikkema, a weed scientist with the University of Guelph’s Ridgetown Campus, was at the bi-weekly meeting of certified crop advisors (CCAs) and extension personnel with Ontario’s Ministry of Agriculture and Food/Rural Affairs (MAF/MRA) on Tuesday near Exeter.

He told the gathering that glyphosate-resistant weeds have been found on farms with solid and full rotations and sound management regimens.

The Huron County identification means glyphosate-resistant Canada fleabane and giant ragweed are moving both eastward and northward; the resistant biotypes are not confined to counties along the north shore of Lake Erie and into Niagara.

Again, the only real certainty is that glyphosate-resistant Canada fleabane is spreading, and it tends to be more of an issue in soybeans than in corn or wheat. The fact that its seeds spread in much the same manner as dandelion (with “parachutes” attached to the seed), and that it can germinate 11 out of 12 months, makes it a very difficult weed to control. Its resistance to glyphosate compounds that complexity.


Sikkema identified several herbicides effective for the control of resistant Canada fleabane, including Eragon (salflufenacil), Integrity (salflufenacil + dimethenamid-P), Broadstrike (flumetsulam) and Sencor (metribuzin).

The quick and easy approach now, according to provincial cereal specialist Peter Johnson, is to treat any Canada fleabane found on a farm as if it’s resistant to glyphosate.

There are 19 sites in Ontario with multiple-resistant Canada fleabane that Sikkema knows about, and those biotypes are resistant to glyphosate with FirstRate (cloransulam-methyl). Again, these were cases where the farmers had no indication that they had weed biotypes that were resistant to both glyphosate and FirstRate. What’s worse, there is nothing that can control Canada fleabane post-emergence in soybean, that is resistant to these two modes of action.

One practice that has to change, however, is the promotion of fall herbicide applications. That practice, said Johnson, is reportedly very limited in terms of its usage by Ontario farmers. By their estimates, it’s at about five per cent of the farming population that actually uses fall weed control as part of a “normal” management routine.

— Ralph Pearce is a field editor for Country Guide at St. Marys, Ont.

Related stories:
Glyphosate-resistant fleabane arrives in Ont., May 4, 2011
Roundup-resistant ragweed spread tracked in Ont., Nov. 17, 2010

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