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* Purdue Agriculture

May 8, 2007

Giant ragweed, resistant to herbicide, a threat to Indiana crops

Bill Johnson looks over marestail
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Hoosier farmers should take extra steps this year to manage a giant menace in Indiana fields that has the potential to cause great yield losses, according to a Purdue University weed expert.

Glyphosate-resistant giant ragweed was found in one Indiana county last year, but as of this spring, resistant varieties are now located in at least 10 counties. The rapid-growing weed has the ability to significantly reduce yields, and because varieties resistant to glyphosate are becoming more common, it's going to become increasingly difficult to manage for Indiana growers.

Bill Johnson, Purdue Extension weed specialist, said the threat from giant ragweed is greater than from other glyphosate-resistant weeds because giant ragweed could cause a farmer to lose up to 50 percent or more of his yield in an infested field if densities are high.

"One of the reasons why Roundup Ready technology is so popular is because when glyphosate - the active ingredient in Roundup - was developed, it was very affective against giant ragweed and the only economical tool for managing ALS-resistant giant ragweed," he said. "Now that the weed is developing resistance to glyphosate, we currently have very few options to deal with it."

Johnson said no new herbicide modes of action have been introduced in the last 25 years and there are no new ones in the pipeline. That means producers are going to have to rely on better management of glyphosate and a combination of herbicides with different modes of action in order to control weeds.

In studies conducted last summer by Purdue and Ohio State University researchers, glyphosate-resistant ragweed was controlled with measures that included using a non-selective herbicide to burn down weeds prior to planting, very timely glyphosate applications and higher glyphosate rates, and combining glyphosate with other herbicides.

"We suggest using at least two tactics or herbicides for dealing with the most troublesome weeds," Johnson said. "Reliance on one chemical helps weeds develop resistance and also lessons the odds that your weed-management efforts will be effective for the entire season."

Ninety percent of the soybeans planted this year in Indiana and about 50 percent of the corn will be in Roundup Ready varieties, Johnson said. Farmers like these varieties because glyphosate is a low-cost and very effective way to combat weeds. However, frequent and exclusive use of the herbicide increases the likelihood that more weed species will develop resistance to it.

This summer a multidisciplinary team of Purdue researchers and Extension specialists will try experiments in some 20 to 30 counties across the state of Indiana to determine best practices in weed management. The strategies will include use of different herbicides, sprayer application factors and learning more about the genetics of resistance.

In the meantime, Johnson lists some good rules of thumb regarding glyphosate use for weed control, which include:

* Using appropriate soil-applied residual herbicides in both Roundup Ready soybeans and corn.

* Applying the correct rate of glyphosate based on weed size. Glyphosate should not be expected to routinely control weeds that are 18 inches or more in height.

* Starting with a clean field. No-till producers should control all vegetation prior to planting.

Johnson said just because a weed does not seem to be affected by glyphosate does not mean that it's resistant. Factors that affect glyphosate effectiveness including application rates, weather conditions, weed size and the timing of applications. There are also some weeds that have a natural or inherent tolerance to glyphosate, such as velvetleaf.

If weeds continue to crop up, Johnson said there are many factors that producers can check in order to determine if resistance is a problem in their field. Those include:

* Noting whether rain could have washed the herbicide away prior to plants absorbing it.

* Determining if only one species seems unaffected but all other species in a field are controlled. If several species are growing in various patches, then it may be an application problem.

* Finding out if other farmers in the area are experiencing similar problems.

More information on weed-management strategies with regard to glyphosate resistance can be found on the Web at the Purdue weed science site at  or at The Web site compiles the expertise of researchers at 16 different land-grant institutions.

Writer: Beth Forbes, (765) 494-2722,

Source: Bill Johnson, (765) 494-4656,

Ag Communications: (765) 494-2722;
Beth Forbes,
Agriculture News Page

Bill Johnson, a Purdue Extension weed specialist, looks over marestail, a troublesome weed for farmers. This summer, a team of Purdue researchers and specialists will experiment in 20 to 30 Indiana counties to determine the best practices in weed management. (Purdue Agricultural Communications photo/Tom Campbell)

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