Researchers at the University of Guelph's Ontario Agricultural
College have found a giant ragweed biotype that is showing resistance
to the popular herbicide glyphosate. The plants are 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," said
Professor François Tardif of the Department of Plant Agriculture.

plants were still able to grow after an application of the herbicide at
recommended levels, whereas susceptible ragweed did not survive.


"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," added Peter Sikkema, a plant
agriculture professor at the University's Ridgetown Campus, who
conducted the research with Tardif.

Currently, no weeds in Canada have been confirmed as resistant to
glyphosate, the most often used herbicide globally. But in other
countries around the world, 15 weed species – including giant ragweed –
have been confirmed as resistant to glyphosate. Eight of those species
are in the United States.

The giant ragweed population in question was brought to the
researchers’ attention in late 2008. It was found in a small portion of
a 580-acre field of Roundup Ready soybeans in Essex County. Weed seeds
were collected from the area and used in greenhouse tests. As well,
researchers are collecting information on the field’s history including
crops grown, tillage practice and the herbicide program used.

The researchers stress that the results are preliminary and that,
thus far, the suspected resistant biotype has been found only in the
one identified area. Further greenhouse and field trials will be
conducted on the weed biotype to confirm resistance as well as identify
potential management options. Researchers will also be working to
understand the genetic and biochemical basis for resistance.

Resistance evolves after a weed population has been subjected to
intense selection pressure in the form of repeated use of a single
herbicide. The herbicide controls all the susceptible weeds, leaving
only those with a resistant gene to reproduce.

"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."

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