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Roundup-Resistant Ragweed Confirmed In Ontario

Glyphosate-resistant giant ragweed is officially in Ontario, weed scientist Peter Sikkema recently confirmed. Researchers grew plants from seeds of giant ragweed found in farmers’ fields in the southwestern part of the province in 2008 and the resulting weeds were resistant to glyphosate, Sikkema says.

During a recent farm meeting he showed a picture of two giant ragweed plants about a foot apart shortly after being sprayed with glyphosate. The resistant plant’s leaves turned black quickly so it could not translocate the chemical to the growing point.

Glyphosate-resistant giant ragweed plants were seen in several other locations throughout Ontario in 2009, including Pelee Island. “Now it’s even more important to implement practices to limit glyphosate resistance,” says Sikkema, a scientist in the department of plant agriculture at the University of Guelph’s Ridgetown Campus.

Guelph’s Ontario Agricultural College has been investigating the possibility of glyphosate-resistant ragweed since May last year. At that time, the suspected resistant biotype had been found only in one small identified area of a 580-acre field. The giant ragweed population in question was brought to researchers’ attention in late 2008, in a field planted to Roundup Ready soybeans in Essex County.

Monsanto, which markets the Roundup brand of glyphosates and owns the Roundup Ready genetics used to breed glyphosate tolerance into various crops, had previously noted just nine weed species in the U. S. and 14 globally with resistance to the chemical.

Maggie Van Camp is an associate editor with our sister publication, Country Guide, and is based in southern Ontario. Includes files from

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