Do GMOs cause “superweeds”?

Almost any way you look at the data, it appears that GM crops are no greater contributor to the evolution of superweeds than other uses of herbicides.

Superweeds are overrunning America's farm landscape, immune to the herbicides that used to keep crop-choking weeds largely in check.

At a Glance

Critics of crop biotechnology often point to the growth of “superweeds” in corn and soybean fields over the past 20 years as illustrative of the ecological problems posed by crops genetically modified to be resistant to herbicides like glyphosate (trade name Roundup). But superweeds so named because of their ability to withstand specific herbicides predate the introduction of GMOs in the 1990s and glyphosate in the 1970s.

Scientists say weeds will eventually develop resistance to any chemical, including those used by organic farmers, through repeated exposure. Glyphosate resistance has gotten so much attention in recent years largely because of the popularity of the herbicide, which has helped farmers realize substantial yield improvements and lowered farming costs. But there is a consensus among weed scientists that GMOs do not uniquely cause the development of hardier weeds; other non GMO crops have more serious weed problems; and various technologies and management strategies can adequately manage the challenge.

Science and Politics

Crop biotechnology critics argue that the use of genetically modified seeds, which now represent nearly 95 percent of soybeans and 90 percent of corn grown in the United States, has led to a surge in so-called superweeds and that they do more harm than good.

“We don’t need pesticide-resistant GMOs to control weeds. There are natural ways to fight them,” said Bill Freese, with the Center for Food Safety, which is opposes GM crops. “The GMO industry likes to put a warm fuzzy glow on GMOs but we don’t see much use for them at all.”

Most scientists dislike the use of the word “superweeds” arguing that it polarizes the issue. Superweeds are not ‘super’ in any real sense of the word; they are weeds that have evolved to evade a particular weed management strategy. [See FAQ “What are superweeds?”) A dandelion that is so short that it has almost no stem could be considered a superweed. Its super power is crouching beneath the blades of lawnmowers.

Weeds are one of agriculture’s major challenges. That challenge is magnified when weeds develop resistance to a particular herbicide. Scientists believe this occurs through natural selection: The herbicide eliminates susceptible weeds, but doesn’t kill those with some natural immunity. Over time, those immune plants become more common and spread. Farmers contribute to this problem when they don’t vary herbicides usage, or when they use weaker-than-recommended doses of an herbicide.

There’s also the remote possibility, often cited by GMO critics, that modified crops can pass on their engineered tolerances to closely-related weeds growing nearby. However, there have been no documented cases of this, and scientists consider it unlikely. The real problem has come from the increased reliance on glyphosate both by farmers who plant GMO seeds and by those who use it for weed control around their fields. The pairing of glyphosate with GMO crops since 1996 has led to a steady increased in its use.

The problem is not the GMO seeds themselves, according to University of Wyoming weed scientist Andrew Kniss:

If GM crops have contributed significantly to the development of herbicide resistant weeds, we would expect the number of unique instances of these superweeds to increase following adoption of GM crops. In the eleven-year period before GM crops were widely grown, approximately 13 new cases of herbicide resistance were documented annually. After GM crop adoption began in earnest, the number of new herbicide resistant weeds DECREASED to 11.4 cases per year.

Glyphosate is a problem, for sure, particularly in soybeans, because of its popularity. Scientists have discovered 32 different weed species that have developed resistance to the herbicide. However, the link between glyphosate and the creation of ‘superweeds’ is not always so clear. As Kniss has written, “In corn, the Roundup Ready system resulted in a diversification of herbicide sites of action, which would be expected to decrease selection for herbicide resistant weeds.” Similar encouraging patterns have been observed in rice and wheat.

The first glyphosate-resistant weeds were found in Australia in 1996, where no GM crops were grown. And there are more instances of glyphosate-resistant weeds in areas with non-GM crops than in areas with GM crops. That’s likely the result of glyphosate being used by almost all conventional farmers for general weed-control. Some of the blame for that can be placed on Monsanto. When GMO crops were first introduced in the mid-1990s, the company encouraged farmers to look upon the herbicide as a cure-all, assuring them that there was little possibility of weeds developing strong resistance.

But not growing glyphosate resistant GMO crops will not solve the hardy weed problem. [See FAQ “What are superweeds?”). That has stopped opportunistic food companies from exploiting misconceptions about glyphosate, weeds and sustainability. When Chipotle launched its anti-GMO campaign, it linked its decision to switch from GMO soybean oil to non-GMO sunflower oil to its claim that GM crops damage the environment: “Evidence suggests that GMOs engineered to produce pesticides or withstand powerful chemical herbicides damage beneficial insect populations and create herbicide resistant super-weeds.”

The problem with that reasoning is that some of the worst cases of weed resistance show up in conventionally-developed crops in particular sunflower crops that are conventionally bred to tolerate a class of herbicides called ALS inhibitors. Many more weeds have become resistant to ALS inhibitors than to glyphosate.

The Takeaway

Herbicides, used by organic and conventional farmers, are critical to modern farming. But resistance ultimately develops to all chemicals, natural or synthetic, even if farmers manage their crops carefully. All herbicides eventually result in hardy weeds.

Most scientists say the major culprit is the over-reliance on single-mode weed management strategies and a lack of integrated pest management practices (multiple weed control modes). In recent years, a growing awareness has emerged among farmers to employ a greater variety of herbicides and not overuse any one chemical.

Growing weed challenges have prompted researchers to develop seeds with traits engineered to resist multiple herbicides, including glyphosate, 2,4-D, dicamba and glufosinate. Scientists expect these stacked herbicides to help counter the growth of hardier weeds. But using new and different herbicides to solve weed resistant problems, while critical to modern agriculture over the coming decades, is probably not a long term solution.

For the foreseeable future, technology is key. Tweaking herbicide resistance traits, either through biotechnology (Roundup system) or conventional breeding (ALS inhibitors) can either increase or decrease herbicide diversity depending on how they are used.

Many critics of “industrial agriculture” specifically target glyphosate, because of its wide popularity. They argue that pairing it with older herbicides that are more toxic, puts farmers on a ‘chemical treadmill.’ “Weeds that developed glyphosate resistance can develop resistance to the new herbicides as well and this has already begun to happen,” according to a 2013 paper by two scientists at the Union of Concerned Scientists In one key sense, supersede critics are right;

Would getting rid of GMO crops halt the spread of herbicide-resistant weeds? The data do not support claims that GMO herbicide resistant crops, as a whole, have made the weed resistance problem worse. The European Academies Science Advisory Council contends: “Cultivating a GM crop variety with increased herbicide resistance … may prove detrimental to the environment if the farmer over-uses that herbicide. But the same would be true of herbicide resistance introduced by conventional breeding.”

Weed scientist Andrew Kniss agrees: “Herbicide resistant weed development is not a GMO problem, it is a herbicide problem.”

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