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U of G Researchers Find Suspected Glyphosate-Resistant Weed

May 07, 2009 - News Release

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 Prof. François Tardif of the Department of Plant Agriculture. The plants were still able to grow after an application of the herbicide at recommended levels, whereas susceptible ragweed did not survive.

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Glyphosate-resistant horseweed confirmed in Michigan

MSU Diagnostic Services

Glyphosate-resistant horseweed (Conyza canadensis) has been confirmed in a Christmas tree plantation in Mason County, Michigan. This detection represents the 16th state in the United States with glyphosate-resistant horseweed (Heap 2006). Also known as marestail, horseweed is a troublesome weed in no-till field crops, fruit crops, tree plantations and nurseries throughout Michigan.

For the past several years, MSU Diagnostic Services has been screening horseweed populations for glyphosate, ALS, triazine and PPO resistance utilizing greenhouse whole plant assays. ALS-resistant horseweed has been documented in 35 field crop locations and seven Christmas tree plantations spanning 14 Michigan counties. Triazine-resistant horseweed has been documented in two field crop locations, 12 Christmas tree plantations and 4 blueberry plantations spanning five Michigan counties.

While glyphosate resistance was suspected by the growers in some of these populations, herbicide resistance proved not to be the case until now. The initial greenhouse screen of this Mason County horseweed population yielded survivors of glyphosate at 1X and 4X the labeled rate (22 and 88 oz/A Roundup WeatherMAX 5.5L, respectively, plus AMS at 17 lbs/100 gallons). These plants were stunted and delayed in development, but were able to bolt and produce seed. Seed was collected from these survivors and subsequent dose response experiments were established with the original field population and greenhouse population.

Horseweed plants were treated with glyphosate at 0.01X, 0.1X, 0.25X, 1X, 4X, 10X and 100X the labeled rate (0.22, 2.2, 5.5, 22, 88, 220, and 2200 oz/A Roundup WeatherMAX 5.5L, respectively, plus AMS at 17 lbs/100 gallons) when rosettes were 2 to 3 inches in diameter. Visual weed control ratings and dry weights were taken 21 days after treatment.

While GR50 values have not yet been tabulated, these experiments consistently show the ability for this population to survive glyphosate at 1X, 4X and occasionally 10X the labeled rate.