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* Purdue University Department of Botany and Plant Pathology

June 29, 2007

Publication helps farmers cut giant ragweed down to size

WEST LAFAYETTE, Ind. - Giant ragweed is the subject of many tall tales, and none of them are good. A new publication could write a happy ending for farmers troubled by the gargantuan plant.

"Biology and Management of Giant Ragweed" is the latest in a series of booklets on glyphosate resistance and weeds that are quickly gaining immunity to the popular herbicide ingredient. The booklets are produced by Purdue University weed scientists and colleagues from 15 other universities that make up the Glyphosate, Weeds and Crops Group.

The publications are available for download online or can ordered for $3 per copy through the Purdue University Cooperative Extension Service.

The 16-page giant ragweed publication covers essential information farmers need to know to combat the towering nuisance, said Bill Johnson, Purdue weed scientist and one of the publication's lead authors.

"We've known for quite some time that giant ragweed is our most competitive broadleaf weed in corn and soybean production," Johnson said. "This weed is so competitive that yield losses can be in excess of 80 percent, and the penalty for missing the early control timing can be very severe.

"Over the last couple of years, we've had an increased frequency of fields that are not getting acceptable control after glyphosate use. So we felt it was important to create a publication that provides an update on the biology of this weed, why it is so competitive and specific management recommendations for producers who are growing not only Roundup Ready crops but also for those growing non-genetically modified crops."

Giant ragweed can grow as tall as 17 feet, but its height often depends on the size of plants it is competing with for sunlight. The weed usually stands 1-5 feet taller than crops.

The weed was a driving force behind the development and rapid adoption of Roundup Ready crops - hybrids able to withstand applications of Roundup-brand herbicide. Glyphosate is the main ingredient in Roundup.

Farmers became too dependent on glyphosate and, as a result, hastened the arrival of glyphosate resistance in some weeds, including giant ragweed.

"Research in Indiana and Ohio has confirmed what appears to be glyphosate resistance in several giant ragweed populations," Johnson said. "Plants from these populations have survived numerous glyphosate applications in field studies."

"Biology and Management of Giant Ragweed," Extension publication GWC-12, can be downloaded at the Glyphosate, Weeds and Crops Web site, located at Printed copies also can be ordered from the site or through Purdue Extension's Education Store, at

Six publications are available in the glyphosate and weeds series, with three others in the works, Johnson said.

"In addition to giant ragweed, we've created publications on the biology and management of horseweed - or marestail - common lambsquarter and wild buckwheat," Johnson said. "We've also created a publication that discusses proper use of glyphosate: carrier volumes, nozzle sizes, droplet sizes and those sorts of things. Another publication discusses some of the facts about glyphosate-resistant weeds: how they develop, how to look for them, how to diagnose them and some of the things that you need to be careful of as you are planning weed management programs for fields that don't have glyphosate-resistant weeds."

The publications are produced with financial support from herbicide manufacturers and commodity groups.

For additional information about weed management, visit the Purdue Weed Science Web site, at

Writer: Steve Leer, (765) 494-8415,

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

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

Note to Journalists: A publication-quality photo from the University of Illinois, a member of the Glyphosate, Weeds and Crops Group, is available at

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