By DOUG SCHMITZ
Iowa Correspondent

AMES, Iowa — Although the presence of glyphosate-resistant weeds haven’t yet been officially confirmed in the state, an Iowa State University field agronomist said there are patches of waterhemp that would take high rates of glyphosate to control, with reports of lambsquarters and giant ragweed that may be resisting the herbicide.

Jim Fawcett, who oversees seven counties in eastern Iowa, said these glyphosate-resistant weeds may already be present throughout the Midwest.

“Although fairly rare, I’m convinced we do have some problems with glyphosate-resistant waterhemp and likely have populations of other weeds, such as lambsquarters, where higher rates of glyphosate are now needed,” he said.

According to the November 2001 Weed Control Methods Handbook, glyphosate is a nonselective, systemic herbicide that can control most annual and perennial plants, and controls weeds by inhibiting the synthesis of aromatic amino acids necessary for protein formation in susceptible plants.

To date, horseweed is present in Indiana, Ohio, Illinois and Michigan; giant ragweed has been found in Indiana and Ohio; and waterhemp is now in Illinois and Iowa.

“Waterhemp first became a serious problem in Iowa after the floods of 1993, so I think where the most flooding occurred may be where the problem started, although there are many other reasons for waterhemp becoming an increasing problem,” Fawcett said.
“It is extremely prolific and there is much genetic variation within the species. There are male and female plants, so cross-pollination increases genetic variation. The result is that herbicide resistant biotypes emerge and spread quickly.”

Giant ragweed has gradually developed to better compete with our row crop practices, Fawcett said. “The weed is emerging later than it used to so that soil-applied herbicides no longer give season-long control and burndown herbicides and pre-plant tillage are no longer as effective, since much of the germination is occurring later in the season.”

But he added emergence patterns are different in Indiana and Ohio, where emergence is later than in Iowa.

“Giant ragweed emergence in eastern Iowa is later than in western Iowa, so the weed is much more of a problem in the east part of the state,” he said. “I’m not sure why the evolution has been more rapid in the east than the west. But I would guess the genetic variation was greater in the east to begin with, and/or the selection pressure has been greater.”

While not all glyphosate-resistant weeds are present in every state, Bill Johnson, assistant professor of weed science at Purdue University, said he’s concerned – especially since yield losses of up to 100 percent could be incurred in dense infestations of giant ragweed.

“If giant ragweed is not controlled early in the season, you will never catch up to it,” he said.

A report published in Weed Science, however, stated the discovery of new glyphosate and other herbicide-resistant transgenes is providing more weed management options to growers of crops, with soybeans, corn, cotton and canola as some of the glyphosate-resistant crops that have been grown commercially.

Mike Owen, ISU professor of agronomy and extension weed scientist, said while glyphosate-resistant weeds may already be in Iowa, they have the genetics to resist whatever strategy is used against them.

“Isolated fields are there, but we can still get ahead of the resistant weeds,” he said. “We need to think like this: It’s not broken and we need to fix it so it stays that way.”

Johnson advised producers to use herbicide management programs that target the worst kinds of weeds in a field.

“Use residual herbicides to keep weeds small and thin the infestation,” he said. “This will make postemergence herbicides more effective and sustain the current technology we are using.”
To further assist producers in the fight against glyphosate-resistant weeds, the Iowa Soybean Assoc. (ISA) in partnership with ISU has published Glyphosate Stewardship: Fix It Before It Breaks to help U.S. soybean producers keep glyphosate-resistant weeds out of their fields – and stay ahead of them.

With 14 different weed species that have developed an increasing resistance to glyphosate, Owen said these weeds could be in a field for up to two years before they are discovered.

“That’s why you have to fix a problem before you have it,” he said.
“Prevent resistant weeds from moving in, because once you have a problem, it can be managed, but never totally fixed.”

To obtain a free copy of Glyphosate Stewardship: Fix It Before It Breaks, contact the ISA at 515-251-8640 or visit www.iasoybeans.com