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University researchers confirm giant ragweed resistance.

December 22, 2006

3 Min Read

It's no longer 'maybe' or 'if'. Populations of giant ragweed resistant to glyphosate are now confirmed by university researchers in both Ohio and Indiana.

The news shouldn't come as a surprise to farmers who follow herbicide and weed control issues closely. Both Ohio State University and Purdue University weed scientists were suspicious of populations they were following a year ago, and commented about possible resistance, but couldn't yet confirm it. However, this positive confirmation may come as a wake-up call to farmers who battle giant ragweed, and who rely solely or heavily upon glyphosate products for control. Glyphosate is the active ingredient in Roundup, Touchdown, and a whole host of generic glyphosate products marketed under various names.

Bill Johnson, Purdue, and Mark Loux, Ohio State, have been monitoring suspect populations since '04. Three fields in central and southwest Ohio have now been officially labeled as fields with giant-ragweed resistant populations.

"The population in Indiana is located in Noble County, which is northwest of Ft. Wayne," Johnson says. "The field in which it is located had been in soybeans six out of the last seven years, and the producer relied solely on glyphosate for giant ragweed control."

Those are conditions tailor-made to producing resistance. Giant ragweed is the seventh weed to show resistance to glyphosate in the U.S., Johnson says. The other most notable weed farmers battle in the Eastern Corn Belt that can be resistant is marestail.

Three factors differentiate glyphosate resistance issues in giant ragweed from marestail, researchers say. Two are negative, one is positive.

First, ragweed can grow 15 feet tall if left undisturbed with huge canopies, much taller and bushier than marestail. Weed science research shows that as few as three to four giant ragweed plants per square yard can cut yield 70%.

Second, there are only four effective post-emergence treatments against giant ragweed, Johnson says. They include glyphosate, Flexstar, Cobra and FirstRate. Resistance to FirstRate, an ALS inhibitor, has been documented for several years in some fields. "If the populations are resistant to glyphosate and FirstRate, then we're left with either Flexstar or Cobra as a post-treatment."

The good news is that giant ragweed resistance to glyphosate may not spread as easily as in marestail. Giant ragweed seeds are relatively large and heavy and do not blow in the wind like marestail seeds. However, it's unknown if resistance could spread through giant ragweed pollen.

Both Johnson and Loux are urging farmers to rethink weed control strategies for '07, especially if they have giant ragweed problems. "If growers have fields with a history of poor control of giant ragweed with glyphosate, they need to change their management tactics," Johnson says. "One big key is to start out with a clean field, with tillage or an effective burndown, which includes 2,4-D.

"Other keys to control include using a residual herbicide and targeting the first in-crop postemerge weed treatment when the giant ragweed are between 6 and 12 inches tall."

He also recommends using the maximum 1.5 pound rate acid equivalent of glyphosate on the first post treatment of 6 to 12-inch ragweeds, or else substitute tank mix FirstRate, Flexstar or Cobra for glyphosate in that first treatment.

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