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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 | APPEARED IN VOLUME 90, ISSUE 21

Herbicide-Resistant Weeds Threaten Soil Conservation Gains
Credit: David R. Shaw/CAST
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.

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.
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.
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
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

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.

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.

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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Allan Felsot (May 21, 2012 2:29 PM)
Growers need to practice more pre and post spraying surveillance and monitoring. Herbicides are not 100% effective, and either spraying when some weeds may be near reproductive stage, or not roguing out escapes that will later flower, is the problem. The geniuses at Monsanto and DowAgro can throw all the new technology their scientists can invent at the growers, but unless growers themselves practice one of the most fundamental principles of IPM, i.e., surveillance and monitoring, they will not surmount further cases of weed resistance. Yes, for family farms of 1000+ acres, labor is a premium, but failure to practice even the simplest of IPM techniques will not sustain the new technologies of the future. IPM is not an obsolete phrase but as always an important element of crop management.
Larry Powell (May 21, 2012 11:57 PM)
Several peer-reviewed studies have already revealed the dangers of Roundup to the health of crops, livestock, wildlife and even humans. Just how far down the "chemical road" must we go before we realize our folly. Please read my article, "Ottawa Dithers Over Health Concerns About Roundup," here...
The Truth (September 20, 2013 11:22 AM)
Fact- glyphosate does not work in the human body at all. It works on the shikimate pathway in plants. That is its site of action. We do NOT have a shikimate pathway for it to work on. Glyphosate is highly water soluble (Sw) somewhere around 15,700 mg/L and the octanol-water partition coefficient (Kow)is very low at 0.0006 meaning that it has a very limited potential to bio-concentrate as opposed to DDT which has a Kow of 7,943,282 which is very lipophilic meaning it will become very fat loving and bio-accumulate. I agree to a certain extent that the co-formulates i.e. inert ingredients in roundup and other glyphosate containing herbicides could cause harm. The EPA has a list of known and approved co-formulates and it is about 77 pages long, but the problem is that the chemical companies do not have to divulge these co-formulates because of patent and registration rights. There in lies the problem. We need to have these ingredients labeled and known to the public because it is only the active ingredient (a.i.) that we truly know about. To be fair, these chemical companies do pretty extensive work when bringing a new chemical on to the market, it will take anywhere from 8-10 years for a product to be tested and to be registered with millions if not billions being spent. And these tests are completed with the co-formulates as well, but sometimes after the registration as in the case with roundup, they will tweak the formulation to make it more rain fast or whatever and they still have to get that registration process again done by the federal government.
Melody Bomgardner (June 7, 2012 2:50 PM)
Thanks for your comments!
Allan - you are correct, farmers are now advised to walk their fields regularly and check for weeds, and to check for early signs of weed resistance. After the crop is harvested, farmers may have left weeds to grow and go to seed, but now they have to guard against that. It is more labor, and there aren't many hands available now in the industrial agriculture areas of the Midwest.
Beverly Johnson, LPN (June 20, 2012 1:53 PM)
If, as farmers had been told, GMO crops will simplify their routines, yield more crops, and make harvesting easier by the application, then why are we now seeing the advent of resistent weeds, and even insects? IS it possible that in the rush to provide easier agriculture, and more crop yields, (read into that more profit), some vital information has been over-looked? Studies have been done that prove, beyond doubt, that GMO crops are causing difficulties...why are they being ignorred? Is is inability on the part of those who are supposed to be able to interpret this scientific data in the FDA, or is it a decision based on dollar signs? These questions demand investigation, as does the revolvinmg door of employee placement from the FDA to chemical companies, and vice-versa.

It behooves the populace to begin to educate themselves, and then make a choice about how to safely feed thier family. Personaly, my family and I are opting out of buying foods that are produced by companies that use GMO crops. It may be inconvienent at times, but unitl we know for certain the effects of GMO foods, we will continue to do so.

I urge studies by reputable companies, independent of either FDA or chemical corp. ties. Perhaps then we will get a true version of exactly how these crops may be altering the environment, not to mention what they may be doing to us.
The Truth (September 20, 2013 10:36 AM)
what studies are you talking about? "Studies have been done that prove, beyond doubt, that GMO crops are causing difficulties..." and what difficulties? We've been eating genetically modified food in this country and around the world for thousands of years...what do you think Maize is? The Mayans genetically selected corn (once was a weed and the cobb measured about 2 inches) for thousands of years which spawned what we eat today. Whats the difference between GMO's and that? GMO plants are just plants that have been manipulated to express certain genes that overwhelm certain herbicide analytes and products of the herbicide once it enters the plant and is transformed by enzymes inside the plant. They don't change the way the plant produces its products i.e. seed. There have been many many many many many many studies showing this, (Google Scholar it sometime). The regulations are set in place in this country for a reason and its not always about money. A lot of people hate on these chemical companies but their R&D spend millions a day in a process that may take 8-10 years to actually hit the market. The reason these genetically modified crops are needed is because of the over-reliance of roundup for the past 16-18 years. Anything will begin to resist...i.e. mosquito's resistance to DDT in the 1960's, overuse will always push a population of organisms to defend its existence. As for your ignorant comments about opting out of GMO foods for your're already eating it and have been for a long long time. And as for your comment about studies by reputable companies, you should check your stats and do your homework because those studies are there. Universities, which I work for do these studies year in and year out and the work is there if you know what your looking for. I am so tired of people who don't know what they're talking about spouting off at the mouth and causing fear mongering to occur in our society. It all goes back to education and it all goes back to science. You're either educated or you're not, and the science doesn't back up your claims at all.
Debebe Birru (September 19, 2012 2:23 PM)
GMO crops might have aggravated the problems of weed resistance as it is difficult to grow these crops without chemicals. The more farmers are relying on chemicals, the greater threat they are facing. My experience in tackling weed resistance is the application of integrated weed management system. Nonetheless, once weed resistance is observed, no other perfect way than going back to the most traditional practices. Of course, I understand the challenge that farmers may face in terms of labor cost, cost of fallowing land, etc., but there is not easy way to get rid of weed resistance.
C. Morgan McNeil (September 29, 2012 8:09 PM)
Monsanto can and does pay people... that is researches, to say anything.
Compare two studies (links pasted at the end of my remarks) two only of the many studies, findable online without particular research skill... in necessary defence against disinformation, and in light of Monsanto's history, well documented, of suppressing negative test results... and for the sake of self, loved ones, man's world community and earth.. please read the many backgrounders on Monsanto's usurious and criminally culpable business practices.That a Monsanto executive is now a senior advisor to the FDA puts the credibility of the entire American political system in correct focus. One can only assume profit derived from mortality, and bilateral medical treatments of lethal and non lethal conditions/diseases resulting from Roundup and genetically modified food is factored to some advantage or otherwise intended. Bear in mind U.S. ranking on world health issues, post natal child mortality, deaths per capita due to various preventable and systemic diseases... as tabulated by both international and U.S. agencies, is far behind many third world counties. Just who does the FDA or U.S. Government elite care about? Michael R. Taylor's appointment has been vigorously opposed to no avail.

Pointedly, EU regulators and Monsanto have been exposed for hiding Glyphosate Toxicity. The European Commission approved glyphosate knowing, as Monsanto did, that it causes birth defects, while the public were kept in the dark, the herbicide must now be banned. Dr Eva Sirinathsinghji and Dr. Mae-Wan Ho
A damning report co-authored by an international group of scientists and researchers for non-government organisation Open Earth Source (OES) reveals that studies from industry including those from Monsanto since the 1990s showed glyphosate caused birth defects. The European Commission approved the herbicide in full knowledge of those findings.

Efforts by the U.S.Government to force Monsanto products into the EU have been undertaken.
The information available at this link must be known
By Catherine Lagrange and Marion Douet
LYON/PARIS | Mon Feb 13, 2012 3:29pm EST
LYON/PARIS (Reuters) - A French court on Monday declared U.S. biotech giant Monsanto guilty of chemical poisoning of a French farmer, a judgment that could lend weight to other health claims against pesticides.

D. Wells, Natural News,February 3, 2012.. One may read of the careers of former Monsanto executives, Taylor among them. Taylor 'worked for Monsanto before he left to join a law firm that has since gained FDA approval of Monsanto’s artificial growth hormone.
Links to important information can be found at this site

As referred to in my opening remarks..two studies.
Regulatory Toxicology and Pharmacology
Volume 31, Issue 2, April 2000, Pages 117–165
I have posted two more addresses, each with a brief excerpt from published research papers that find results opposite to Monsanto's recently published 'proof' of the supposed safety, therefore desirability of Roundup. Roundup is highly toxic.
Glyphosate is a broad-spectrum herbicide widely used to kill unwanted plants both in agriculture and in nonagricultural landscapes. Estimated use in the U.S. is between 19 and 26 million pounds per year.
Most glyphosate-containing products are either made or used with a surfactant, chemicals that help glyphosate to penetrate plant cells.
Glyphosate-containing products are acutely toxic to animals, including humans. Symptoms include eye and skin irritation, cardiac depression, gastrointestinal pain, vomiting, and accumulation of excess fluid in the lungs. The surfactant used in a common glyphosate product (Roundup) is more acutely toxic than glyphosate itself; the combination of the two is yet more toxic.
In animal studies, feeding of glyphosate for three months caused reduced weight gain, diarrhea, and salivary gland lesions. Lifetime feeding of glyphosate caused excess growth and death of liver cells, cataracts and lens degeneration, and increases in the frequency of thyroid, pancreas, and
liver tumors.
Glyphosate-containing products have caused genetic damage in human blood cells, fruit flies, and onion cells.
Glyphosate causes reduced sperm counts in male rats, a lengthened estrous cycle in female rats, and an increase in fetal loss together with a decrease in birth weights in their offspring.
It is striking that laboratory studies have identified adverse effects of glyphosate or glyphosate-containing products in all standard categories of toxicological testing.
Two serious cases of fraud have occurred in laboratories conducting toxicology and residue testing for glyphosate and glyphosate-containing products.
Advertised as herbicides that can "eradicate weeds and unwanted grasses effectively with a high level of environmental safety,"1 glyphosate-based herbicides can seem like a silver
bullet to those dealing with unwanted vegetation. However, an independent, accurate evaluation of their health and environmental hazards can draw conclusions very different than those presented by these advertisements. The following summary of glyphosate's hazards is intended to serve that purpose. It will appear in two parts: Part 1 discusses the toxicology of glyphosate, its metabolites, and the other ingredients of glyphosate products and Part 2 will discuss human exposure to glyphosate and its ecological effects.

Glyphosate, N-(phosphonomethyl) glycine (Figure 1), is a post-emergent, systemic, and non-selective herbicide used to kill broad-leaved, grass, and sedge species.2 It has been
registered as a broad spectrum herbicide in the U.S. since 1974 and is used to control weeds in a wide variety of agricultural, lawn and garden, aquatic, and forestry situations.3
Most glyphosate herbicides contain the isopropylamine salt of glyphosate. A related chemical, the sodium salt of glyphosate, acts as a growth regulator in sugar cane and peanuts and is marketed for that purpose. The monoammonium salt of glyphosate is also marketed as an herbicide and growth regulator.4
Glyphosate products are manufactured by Monsanto Company worldwide. The herbicide is marketed under a variety of trade names: Roundup (including Roundup D-Pak, Roundup Lawn and Garden Concentrate, and Roundup Ready-to-Use) and Rodeo are the most common U.S. trade names.2 The sodium salt is sold as Quotamaster. The monoammonium salt is sold as Deploy Dry.2 Other brand names used for the isopropylamine salt are Accord,5 Vision, Ranger, and Sting.2
As an herbicidal compound, glyphosate is unusual in that essentially no structurally related compounds show any herbicidal activity.
Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology
Volume 147, Issue 2, March 2008, Pages 222–231
"The toxicity of Roundup, a glyphosate-based herbicide widely used in agriculture, was determined for the Neotropical fish Prochilodus lineatus. The 96 h-LC50 of Roundup was 13.69 mg L− 1, indicating that this fish is more sensitive to Roundup than rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). These differences should be considered when establishing criteria for water quality and animal well-being in the Neotropical region."

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