Herbicide-resistant weeds gaining ground in Iowa

Farmers will need to find new ways to manage crops

Orlan Love
Published: July 4 2013 | 5:30 am - Updated: 29 March 2014 | 4:54 am in
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Herbicide-resistant weeds — called “superweeds” in some quarters — are forcing Iowa farmers to adopt more diverse weed-management strategies.

“For starters, don’t call them ‘superweeds,’?” said Iowa State University agronomist Micheal Owen, an expert on the misnamed herbicide-resistant weeds that are becoming increasingly problematic in Iowa corn and soybean fields.

“There is nothing ‘super’ about them. They don’t grow any faster, get any bigger or produce more seeds than non-herbicide-resistant weeds of the same species,” Owen said.

Nor, Owen said, are they harder to kill when treated with an effective method.

The trouble is, the method many farmers thought was foolproof — spraying glyphosate on fields planted with seeds that are genetically engineered to tolerate the chemical — has accelerated the evolution of weeds that are increasingly resistant to the most popular weed killer in America, the broad-spectrum, systemic herbicide commonly known by the Monsanto brand Roundup.

Getting worse

“It’s not a crisis, but it’s going to get worse,” said Bob Hartzler, an ISU Extension weed specialist.

ISU agronomist Matt Liebman said Iowa is on the threshold of serious herbicide resistance problems.

“It’s a hot topic among Iowa farmers this year, second only to delayed plantings caused by record spring rains,” said Ed Anderson, senior director of supply and production systems for the Iowa Soybean Association.

“We have problems in Iowa, but they are still localized and minor” compared with the serious yield losses experienced by southern soybean and cotton producers, Anderson said.

Here in Iowa “we still have time to get ahead of the curve, and we are doing that with our Take Action Campaign,” which compiles information on available options and disseminates it to farmers, he said.

Rodney Williamson, director of research and development for the Iowa Corn Growers Association, shares Anderson’s confidence that Iowa farmers will successfully manage weeds’ increasing resistance to herbicides.

“Most growers are aware of the potential. It’s a matter of them taking steps to control it,” Williamson said.

In Iowa, waterhemp, giant ragweed and horse weed (also known as marestail) have documented glyphosate resistance. Several other weeds are suspected to have developed resistance.

It was predictable, said Owen, who predicted it in the late 1990s, not long after Monsanto introduced the first seeds genetically engineered to withstand Roundup.

“A guy by the name of Chuck Darwin talked about this. It’s natural selection on a fast track,” Owen said.

Weed evolution

The herbicide kills susceptible weeds, leaving only those with resistance traits to pass their genes to the next generation. With a new generation produced each year, it’s evolution in hyperdrive.

Crop acreage with weeds resistant to the most common herbicide, glyphosate, the active ingredient in Monsanto brand Roundup, has doubled from 32.6 million in 2010 to 61.2 million in 2012, according to a report by Stratus Agri-Marketing, which conducts annual glyphosate resistance tracking studies.

Since 1996, farmers have been spraying Monsanto brand Roundup on fields planted with seeds developed by Monsanto to tolerate the chemical. This “Roundup Ready” gene initially reduced the need to till fields or apply harsher chemicals, making weed control simple, flexible, economical and easier on the environment.

“Fifteen years of simplicity, farmers relying on one chemical to control weeds — that’s going to go away,” ISU’s Liebman said.

It’s already gone away, according to Jim Grief, who farms near Prairieburg and operates a custom spraying business. Grief, a district director of the Iowa Corn Growers Association, said all the glyphosate he applies is now supplemented with a second herbicide with a different mode of action.

Grief said he is also applying about 50 percent more glyphosate per acre than he did 15 years ago.

“I can still kill weeds with glyphosate, but they are getting harder to kill,” he said.

The use of glyphosate in agriculture has increased from less than 30 million pounds in 1995, the year before the first Roundup Ready crops were introduced, to about 185 million tons in 2007, the most recent year for which statistics are available, according to the U.S. Environmental Protection Agency.

Much of that increase is of course attributable to the widespread adoption of crops with the glyphosate resistant trait. In Iowa, about 98 percent of soybeans and 80 percent of corn incorporate that technology, according to Owen.

Tracy Franck, who farms 2,400 acres in Buchanan County with his dad and son, said they “are putting on more Roundup every year to kill the same amount of weeds.”

They, like most other farmers in their area, are also applying a pre-emergent residual herbicide to help control the glyphosate-resistant weeds that are just beginning to show up in their fields.

“We are starting to see some lambs quarter and giant ragweed that are tough to kill,” he said.

The severity of the problem, Franck said, “depends on how long you have been planting Roundup Ready seeds,” which for the Francks started about six years ago.

“Every year it will get worse,” he said.

Although Wayne Humphreys, who grows corn and soybeans near Columbus Junction, has not experienced any yield loss because of herbicide-resistant weeds, he said he is “definitely concerned” about the prospect. Besides supplementing his glyphosate use with residual herbicides in the spring and fall, Humphreys said he plans to introduce hay into his corn and soybean crop rotation.

ISU’s Owen said the solution is simple: “Stop doing what you are doing” and start deploying more diverse weed management strategies that incorporate some combination of crop rotation, cover crops, manual tillage and a broader range of herbicides less dependent on glyphosate.

Bill Freese, a science policy analyst with the Center for Food Safety in Washington, D.C., said the herbicide/seed industry plans to address the problem with a new generation of genetically engineered crops that will be resistant to established weed killers 2,4D and dicambra.

Both those herbicides are more toxic than glyphosate and tend to cause more collateral damage through wind drift and volatilization, Freese said.

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