Herbicide-resistant crops can exacerbate ‘superweeds’ but new GM versions can help control problem


Herbicide-resistant weeds, sometimes referred to as superweeds, are not new: they have been around since farmers began using herbicides to control the weeds in their fields.

More recently, herbicide-resistant weeds have posed major challenges as they invade more fields and become more difficult to control. In Idaho and Oregon, growers were alerted mid-June to yet another new case of herbicide-resistant weeds in two sugar beet fields.

Many advocacy groups blame the use of chemical herbicides combined with crops genetically engineered to survive herbicide applications for the problem of the superweeds. GMO Inside describes the problem as such:

One major problem caused by the widespread use of GMOs, and the herbicides and pesticides they were developed to withstand, is the emergence of superweeds and superbugs – plants and insects now resistant to these chemicals.

But the growing problem of herbicide-resistant weeds has weed scientists and regulators pushing for more solutions to tackle the problem, including the judicious continued use of chemical herbicides paired with herbicide-resistant crops.

One example, Palmer Amaranth, more commonly known as pigweed, has already left farmers, scientists and regulators with few control options. Once easily controllable with glyphosate-based herbicides, pigweed has since developed resistance to glyphosate and now poses a problem in sections of croplands in the United States and worldwide.

In an attempt to provide farmers with another weapon against this difficult weed, Dow AgroSciences has developed a new generation of herbicide-tolerant crops that are able to withstand more than one herbicide.

The new crops, dubbed Enlist, have been criticized by anti-GMO advocacy groups who claim that the crops will create more herbicide-resistant weeds.

“Increased herbicide use on the new engineered crops will speed up weed resistance, leaving no viable herbicide alternatives,” said Doug Gurian-Sherman, a senior scientist with the Center for Food Safety. “This is a dangerous chemical cocktail that, when combined with the current farming system, it’s a recipe for disaster.”

Plant pathologist Steve Savage disagrees, arguing that chemical herbicides and herbicide-resistant crops continue to be an important tool for farmers:

Groups like the Center For Food Safety have generated furor by shouting an intentionally sensational half-truth. Ironically, this has put them in the position of advocating against a tool farmers need for environmentally sustainable farming.

As Brandon Keim reports in Wired, these crops are on the brink of receiving approval from the Environmental Protection Agency and the U.S. Department of Agriculture:

The agribusiness industry says the plants—soy and corn engineered to tolerate two herbicides, rather than one—are a safe, necessary tool to help farmers fight so-called superweeds. The U.S. Environmental Protection Agency and Department of Agriculture appear to agree.

With the ability to grow over 2.5 inches a day and produce 600,000 seeds, pigweed infests fields rapidly with no mercy. It is resistant even to mechanical weeding, having a tough stem that can damage farm equipment trying to uproot it. Currently, Monsanto’s RoundUp Ready genetically engineered crops that are resistant to glyphosate, a powerful herbicide, help farmers with weed control as they can spray the herbicide on their fields to kill the weeds and not their crops. But the reliance on a single herbicide placed enormous pressure on the weeds to develop some resistance to glyphosate, and they did.

“Glyphosate was so effective, and so cheap, and so easy, so that’s what we did. People thought it was a miracle,” said Larry Steckel, a weed specialist at the University of Tennessee.

“Life was good for a while, but we should have been smart enough to understand that we are going down the wrong path, relying entirely on one herbicide, and it eventually led to glyphosate-resistance,” said Allen York, professor of crop science at North Carolina State University.

And some are worried that the new Enlist crops would end up with the same problems. In a recent editorial, Nature, a leading scientific publication, cautions that genetically engineered crops with herbicide-resistant traits are but a short-term solution:

Stacking up tolerance traits may delay the appearance of resistant weeds, but probably not for long. Weeds are wily: farmers have already reported some plants that are resistant to more than five herbicides. And with glyphosate-resistant weeds already in many fields, the chances of preventing resistance to another are dropping.

In Keim’s Wired report, Dow Agrosciences claims that herbicide resistance in weeds would be “unlikely” with the use of Enlist crops engineered to tolerate two herbicides, glyphosate and 2,4-D:

 2,4-D resistance is a relatively difficult trait for plants to acquire and … the company is committed to promoting growing practices—such as crop rotations and non-chemical weed control measures—that reduce selection pressures favoring herbicide-tolerant weeds.

Other weed scientists agree that developing growing practices like crop rotations and non-chemical weed controls as part of an integrated pest control system will be essential in slowing herbicide resistance in weeds and maintaining the viability of chemical herbicides.

In Nathanael Johnson’s Grist report on the available ways to fight herbicide-resistant weeds, Andrew Kniss, a weed scientist from the University of Wyoming, points out that lack of crop diversity is the real problem. “This really is a symptom of the larger problem, of not enough diversity in our cropping systems,” he said.

But that does not mean that chemical herbicides should be taken completely out of the picture. “Herbicides to me are kind of like medicine,” Steckel said. “If used correctly they are not a danger to people or the environment.” In contrast, a shift to heavy reliance on mechanical tilling to control weeds in the absence of chemical herbicides would cause greater problems with soil damage and erosion.

Herbicide resistance expert Pat Tranel from the University of Illinois said that ideally, farmers would be “using herbicides as part of a system, and using other strategies such as crop rotation and more-diversified cropping.” But Nature notes in its editorial that farmers may be reluctant to plant crops that don’t bring in as much money as corn, and to invest in proper weed management when their neighbors aren’t doing the same.

Nature‘s editorial thus calls for regulations from the EPA on the new Enlist crops to ensure that farmers are using sufficient crop rotations and other weed control measures instead of letting Dow monitor the responsible use of its products:

When an insect-resistant variety of genetically engineered crop was released, US regulators required farmers to plant nearby refuges of non-resistant plants to ease the selection pressure on insects to develop resistance to the crops. Similar measures for herbicide-tolerant crops might require farmers to rotate crops or herbicides every few years — a familiar restriction, because many herbicides have limits on how often they can be used for environmental reasons.

Kniss disagrees that such regulations would be effective. “It’s hard to imagine a herbicide resistance plan that would work for more than one farm, or even more than one field,” said Kniss. “Let alone a whole country.” Instead, Kniss suggests that weed-management plans coordinated on a local level could be more effective.

Whether it’s centralized or local regulations, scientists agree that careful stewardship of the new Enlist crops will be necessary to maintain their viability against weeds in the future.

  • Mike

    XiaoZhi Lim , GMO’s caused the problems. The chemical corporations learned from politicians that first make a problem, then act like you the one to fix it. I see every article by you has a pro gmo slant. anyone before buying this spin watch the movies “Food Inc.” and “Seeds of Death” to hear the other side of this topic. GMO’s are bad OK.

  • Allan Felsot

    Crop varietals don’t cause or lead to resistant weeds; rather farmer practices lead to resistant weeds (and insects). XiaoZhi Lim, the problem is not the use of herbicide, but rather the misuse of the herbicide when IPM best practices are not followed. I predict that in five years, the Dow ENLIST cultivars, which are really resistant simultaneously to 2,4-D, a class of herbicides commonly known as “fops”, and glyphosate (yes, that is also a trait in the cultivars under scrutiny) will face the same problem as RR crops. The reason is that farmers in the vast acreages of the Corn Belt states where the problem of weed resistance disproportionately exists do not follow best IPM practices. IPM principles involve pre-spray and most importantly post spray monitoring and surveillance practices. If you spray, and do not follow up and rogue out the skips and misses (face it, no control practice is 100% effective), and you let these plants flower, you are the problem, not the spray. The development of weed resistance is simple to understand: if your plants have sex after you select them with a herbicide, then you have increased the probability of passing resistant alleles onto the next generation (yes, those alternative forms of a single gene already exist at very low levels of frequency in the population; they are unlikely contemporaneously de novo traits). And voila, in a very short time you have a much greater proportion of resistant alleles. I’m sick of seeing extension talk after extension talk that only emphasizes chemical rotation and does not focus on farmer behavior, such as getting your ass (or some high school kids) out to the field and roguing out your misses after you spray. I’ll defend herbicide use as safe from a toxicological/exposure standpoint, but what I won’t defend is farmers or pesticide applicators who do not take the responsibility to practice the monitoring/surveillance principle of IPM. If you use a technology, you must use it responsibly. At this point, I don’t think the big ag companies have realized they are dealing with a human behavior problem. Note that the above comments are related specifically to weed resistance. Insect resistance avoidance is more complicated but insect infestations are much spottier than weed infestations and can be resolved not only with chemical rotations but most importantly better population monitoring and more research that relates insect injury to damage thresholds.