Roundup-resistant ragweed suspected in Ont. field

Researchers at Guelph’s Ontario Agricultural College suspect they may have found Canada’s first population of a glyphosate-resistant weed.

OAC plant science professor Francois Tardif said in a University of Guelph release Thursday that a giant ragweed biotype found in an Essex County soybean field last year is able to survive glyphosate use rates that kill normal susceptible weeds.

“We’ve seen a difference in control of this giant ragweed biotype than what is normally expected when sprayed with glyphosate,” Tardif said, as the plants were still able to grow after an application of the Group 9 herbicide at recommended levels, whereas susceptible ragweed did not survive.

The OAC researchers stress that their results are preliminary and that so far, the suspected resistant ragweed biotype has been found only in one small identified area of a 580-acre field.

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

“Glyphosate has become a tool of choice for the control for many weeds, so the appearance of a glyphosate-resistant population can complicate management for growers,” said Peter Sikkema, a U of Guelph plant agriculture professor who conducted the research with Tardif.

Further trials

The giant ragweed population in question was brought to the researchers’ attention in late 2008, they said, and was found in a field planted to Roundup Ready soybeans in Essex County.

Weed seeds were collected from the area and used in greenhouse tests, OAC said. As well, researchers are collecting information on the field’s history including crops grown, tillage practice and the herbicide program used.

Further greenhouse and field trials will be conducted on the weed biotype to confirm resistance and to identify potential management options. Researchers will also work to understand the genetic and biochemical basis for resistance.

“This is a very serious situation,” Sikkema said. “In other jurisdictions, most glyphosate-resistant weeds biotypes have been effectively managed with other herbicides and cultural practices. We’ll continue our research so we can make recommendations to growers on effective control options.”

Monsanto has previously said that in other areas where glyphosate-resistant weeds have turned up, those populations have been “effectively managed” with tank mixes, residual herbicides, tillage, mowing, crop rotations and/or herbicide rotations.

Glyphosate has been in commercial use for over 30 years, the company said, and incidence of weeds resistant to it is low, particularly compared to other herbicide chemistries such as ALS inhibitors and triazines.

The company cites research indicating that the use of Roundup Ready genetics, either singly or sequentially, in “typical” Canadian agronomic rotations presents “low risk” of development of glyphosate-resistant weeds.

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