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War On Weeds

New crop traits that allow use of older herbicides may help farmers manage resistant weeds

by Melody M. Bomgardner
May 21, 2012 | A version of this story appeared in Volume 90, Issue 21


Credit: David R. Shaw/CAST
Herbicide-Resistant Weeds Threaten Soil Conservation Gains
The U.S. government has put several federal policies and programs in place to help determine the selection and implementation of crops and conservation programs in relation to herbicides and tillage.

Farmers, plant geneticists, chemists, and agronomists recently have been engaged in an arms race against weeds, particularly weeds that have evolved resistance to the common herbicide glyphosate. A second generation of herbicide-tolerant crops has been developed to battle resistant weeds, but they have sparked concerns about overreliance on chemical controls.

Introduced in the 1980s, glyphosate has been the best-selling herbicide since 2001. Monsanto, which markets glyphosate as Roundup, introduced crops engineered to be tolerant of glyphosate in the late 1990s, and farmers now plant Roundup Ready herbicide-tolerant corn, soybeans, and cotton on the majority of cultivated acres in the U.S. Thanks to the popularity of the firm’s Roundup Ready trait, last year 94% of soybean acres were herbicide-tolerant, as was 73% of cotton acreage and 72% of corn acreage, according to the Department of Agriculture.

Farmers liked glyphosate because it vastly simplified weed control. But it also led to the emergence of resistant weeds that are increasingly hard to kill.

Beginning in 2013, pending approval by USDA, farmers will be able to plant crops that have been genetically modified to also tolerate applications of the herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 3,6-dichloro-2-methoxybenzoic acid (dicamba).Both herbicides have been in use for more than 40 years. The traits will be “stacked” to include tolerance to glyphosate as well as to 2,4-D or dicamba.

Credit: Monsanto
Field Day Weeds in the Roundup Ready soybean field (left) survived a soil-applied herbicide and glyphosate. Weeds in a field of Monsanto’s Xtend soybeans (right) died after dicamba was also applied.
Photo of two Monsanto soybean fields demonstrating dicamba tolerance.
Credit: Monsanto
Field Day Weeds in the Roundup Ready soybean field (left) survived a soil-applied herbicide and glyphosate. Weeds in a field of Monsanto’s Xtend soybeans (right) died after dicamba was also applied.

Dow AgroSciences’ 2,4-D-tolerant corn, part of its Enlist Weed Control System, is the first of the new crops in line for USDA consideration and marketing to U.S. farmers. Monsanto plans to follow with its 2014 introduction of dicamba-tolerant soybeans, called Roundup Ready 2 Xtend. Both companies say that the emergence of weeds resistant to glyphosate will drive farmers’ adoption of the new seeds.

Farmers would still be able to manage most weeds through applications of glyphosate, but for any resistant weeds that remain, they will have the option of adding 2,4-D or dicamba without worrying about damaging their crops. The firms are promoting the seeds as a way to control weeds without having to resort to tilling or hand-weeding. So-called low-till or conservation tillage is a common soil conservation practice.

But in the long term, experts say, if farmers do not also use nonchemical methods for weed control, such as crop rotation, eventually weeds will emerge that are resistant to 2,4-D and dicamba as well as glyphosate.

The new crops will be a valuable tool to help diversify weed management programs, says David R. Shaw, a research professor at Mississippi State University and chair of a task force on weed resistance for the Iowa-based nonprofit Council for Agricultural Science & Technology. However, if farmers depend too much on the new technology, “evolutionary nature is such that when you put enough selection pressure on a species, it will develop resistance,” he says. That pressure would create weeds that could survive 2,4-D or dicamba applications.

The specter of weeds brandishing multiple resistances has made the new crops a target for groups promoting sustainable agriculture. Others have raised alarms about risks associated with an increase in use of 2, 4-D and dicamba. For example, growers of crops that are susceptible to the herbicides are worried that more drift of 2,4-D and dicamba from treated fields will weaken or kill their crops.

For their part, Dow and Monsanto insist that the lessons learned from overreliance on glyphosate are changing farming practices. Never again, they say, will it be the norm to use the same herbicide, year after year, on the same crop in the same location. They dispute estimates that the use of 2,4-D or dicamba will greatly increase. And both firms have developed new, low-drift formulations of these herbicides that they say will minimize off-field migration.

Credit: Dow AgroSciences
Farmers in the Midwest and South are battling glyphosate-resistant weeds. NOTE: Affected acreage in 2012 is more than 60 million acres. SOURCE: Dow AgroSciences
Map showing the states that have glyphosate-resistant weeds.
Credit: Dow AgroSciences
Farmers in the Midwest and South are battling glyphosate-resistant weeds. NOTE: Affected acreage in 2012 is more than 60 million acres. SOURCE: Dow AgroSciences

At Dow, plant scientists began the search for a new herbicide-tolerance trait almost 10 years ago, with the emergence of weeds resistant to glyphosate, says Mark Peterson, Dow’s global biology team leader for Enlist. The company settled on 2,4-D as the target herbicide because it is already commonly used, kills a wide range of weeds, and has a mode of action that’s different from glyphosate’s. Both 2,4-D and dicamba are synthetic versions of the plant hormone auxin. Putting additional auxin on weeds triggers uncontrolled growth that leads to death.

To find genes that confer a tolerance to 2,4-D, scientists looked in bacteria that live in soils where 2,4-D has been used. One bacterium, Ralstonia eutropha, produces enzymes that break down the molecule into constituents that are not lethal to plants.

Dow’s plant geneticists were able to insert the gene into corn, soy, and cotton. The resulting genotypes were tested for 2,4-D tolerance, and the successful plants were then tested for any impact on yield, grain quality, stress tolerance, or maturity. “Our plant breeders say they are very well behaved plant traits,” Peterson reports. Well-behaved traits confer the desired advantage without also bringing along other metabolic changes that would weaken the crops.

While the Enlist crops were showing their stuff in field trials, Dow chemists worked on a new formulation of 2,4-D, called 2,4-D choline, to minimize volatility. David E. Hillger, an application technology specialist at Dow AgroSciences, explains that rather than traditional ester or amine forms of the molecule, which can volatilize in the environment, the new version is a more stable quaternary ammonium salt.

In addition, Hillger says Dow’s proprietary manufacturing process produces a product with less particle drift when application directions are followed. Dow recently reported that field tests of the formula showed a 92% reduction in volatility and a 90% reduction in drift.

Crops that contain the 2,4-D tolerance- trait will also tolerate older versions of 2,4-D. However, Dow has developed a stewardship program that obligates farmers to use a premixed combination of 2,4-D choline and glyphosate. The program includes farmer education about using multiple herbicide modes of action, the requirement to use Dow’s new herbicide mixture, and labeling instructions for proper application. State pesticide regulations generally require farmers to follow labeling guidelines when using herbicides.

Soybean growers will have their first opportunity to use a synthetic auxin herbicide beginning in 2014 with the arrival of Monsanto’s dicamba-tolerant soybeans. Like Dow’s 2,4-D trait, the dicamba-tolerance gene was isolated from a soil bacterium, Stenotrophomonas maltophilia. Much of the plant genetics work was done by researchers at the University of Nebraska, Lincoln. The bacteria metabolize dicamba with the help of the enzyme dicamba monooxygenase, explains Cindy L. Arnevik, Monsanto’s North American soybean trait lead.

Monsanto licensed the trait technology in 2005 from the university, and Arnevik’s team tested more than 100 resulting transformations. “We can put the same gene in a plant and one will express in the right place and get the right tolerance; the next plant doesn’t,” Arnevik says. Years of field trials of the tolerant strains followed to make sure the trait does not affect yield, regardless of whether the field is sprayed with herbicide.

On the chemicals side, Monsanto worked with BASF to develop a new generation of dicamba that has reduced volatility compared with the common formulation available today. Current versions of dicamba are not labeled for use with soybeans, giving farmers added incentive to trade up to the new formula. Monsanto does not plan to require farmers to spray a dual-herbicide mixture; farmers in areas where resistant weeds are not a problem can stick with glyphosate.

On the other hand, farmers who do have glyphosate-resistant weeds should not depend just on a two-herbicide blend, even in cases where the mixture appears to kill all weeds. Monsanto and Dow promote varying the herbicides used and always including ones that have a residual effect when applied to the soil.

More herbicide-resistant traits are in the pipeline, which will increase the availability of diverse modes of action. For example, Syngenta and Bayer CropScience are collaborating on a p-hydroxyphenylpyruvate dioxygenase herbicide-tolerance trait for soybeans. And crops tolerant of three or more herbicides are not far behind.

Since the advent of Roundup Ready traits, however, farmers have not been in the habit of controlling for nonexistent weeds. “To some extent it is true that farmers might not change what they are doing until a resistant weed appears. Roundup Ready was so good, farmers could forget what they knew in terms of weed management. Before that, there were no silver bullets,” says MSU’s Shaw. “There is now a whole generation of farmers that haven’t known anything but this very simplistic system.”

In addition to using a changing rotation of herbicides, Shaw says farmers will need to change their agricultural practices to include crop rotation, cover cropping, and weed control after harvest. And in some areas, low-till practices will be more difficult to adhere to.

Bill Freese, science policy analyst at the Center for Food Safety, is not convinced that seed firms will encourage better habits. “All of this talk about stewardship and grower education is window dressing. Monsanto promoted Roundup Ready as a weed system, and that is how Dow is promoting its crops,” he says. What’s more, he is skeptical that new application guidelines to control herbicide drift will be followed. “A lot of pesticide application is done by commercial applicators.” With applications scheduled in advance, “there is a low likelihood they will hold off on applying if wind speed is too high.”

Freese also points to research from Pennsylvania State University that projects a fourfold increase in the amount of 2,4-D used on corn after growers adopt the Enlist system. “It is postemergence use that causes most crop injury” and selects for resistant weeds, Freese says.

For its part, Dow says that “rates of herbicide application per acre of corn will not increase with our new technology package.” Without the new traits, farmers would still need to apply an ever-greater amount of herbicides to control weeds resistant to glyphosate. In addition, farmers would have to resort to cultivation practices that could increase soil erosion and pollution.

A group of U.S. growers organized as the Save Our Crops Coalition has asked USDA to take a close look at the problem of damage from 2,4-D drift. In a statement, the group, which includes fruit and vegetable growers, says “SOCC appreciates Dow’s substantial efforts to develop a low-volatility formulation of 2,4-D.” But, it adds, “an environmental impact statement should be conducted regarding the approval of the entire class of synthetic auxin-tolerant crops.”

For now, groups for and against the introduction of the new herbicide-tolerant crops are waiting on a ruling from USDA, which wrapped up its public comment period on the 2,4-D trait at the end of April.

MSU’s Shaw is already looking ahead. “Herbicide resistance is a sociological problem more than a physiological problem. It’s about the practices that people choose or choose not to follow.” He says that companies bear a major responsibility to ensure the adoption of good stewardship practices in their direct sales role with the grower. “They have the best chance to educate them. It’s where the rubber meets the road.”


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