Superweeds: How Herbicide-Resistant Weeds Spread, the Dicamba Saga, and What Works in 2026

Across the cotton and soybean fields of the American South and Midwest, a weed called Palmer amaranth can grow more than two inches a day, reach eight feet tall, and produce hundreds of thousands of seeds per plant. Many populations no longer die when sprayed with glyphosate, the herbicide that genetically engineered Roundup Ready crops were designed to tolerate. Farmers have resorted to hiring crews to hack weeds by hand, as they did generations ago, and states have asked regulators to allow older or more volatile herbicides to save their crops.

These herbicide-resistant “superweeds” are one of the most consequential side effects of the herbicide-tolerant crop era. They have driven a cycle of new engineered crops and new herbicides, including dicamba, whose drift has damaged millions of acres of neighboring crops and triggered repeated court battles. This article explains how superweeds evolved, why the response has been controversial, where dicamba regulation stands in 2026, and what research shows about more durable solutions.

What are superweeds and why are they a problem? Superweeds are weeds that have evolved resistance to herbicides, especially glyphosate (Roundup), making them hard to kill with chemicals. They spread because heavy reliance on one herbicide selects for resistant plants. This drives farmers to use more and stronger herbicides like dicamba, which can drift and damage other crops. The long-term solution is diversifying weed control rather than relying on any single chemical.

How Superweeds Evolved

Herbicide resistance is evolution in action. When the same herbicide is sprayed over vast areas year after year, rare plants that happen to survive pass their resistance genes to the next generation, and resistant populations spread. Glyphosate was long considered unlikely to produce resistance, and for two decades after its introduction in 1974, resistance was rare. That changed after glyphosate-tolerant crops were commercialized. A 2008 review in Pest Management Science by Stephen Powles of the University of Western Australia explained that the high adoption of glyphosate-resistant crops in the Americas meant unprecedented and often exclusive use of glyphosate over very large areas, and that resistant populations of damaging weeds, including Palmer amaranth, waterhemp, giant and common ragweed, and horseweed, had already evolved in U.S. regions where these crops dominated. He warned that glyphosate would remain effective only if used in diverse weed management systems.

The warnings proved accurate. A 2018 review in the same journal by Ian Heap and Stephen Duke reported 38 glyphosate-resistant weed species in 37 countries, with the greatest economic impact in glyphosate-tolerant crop systems in the Americas. Many weed populations now resist several herbicide classes at once, narrowing farmers’ options.

What Is the “Herbicide Treadmill”?

The herbicide treadmill is the cycle in which weeds evolve resistance to a herbicide, so farmers apply more or switch to stronger chemicals, which then drives resistance to those too, escalating chemical use over time. A 2016 analysis in Environmental Sciences Europe by Charles Benbrook found that glyphosate had become the most heavily used herbicide in history, with global use rising almost fifteenfold after herbicide-tolerant crops were introduced. As resistance spread, farmers increased glyphosate rates, added more applications, and mixed in other herbicides. Critics describe this as a pesticide treadmill: each new resistant weed prompts more or stronger chemicals, which in turn select for new resistance. Agricultural companies responded by developing crops tolerant to multiple herbicides, including dicamba, 2,4-D, and glufosinate, allowing farmers to spray combinations over growing crops.

Before dicamba-tolerant crops arrived, some farmers and state agencies sought emergency permission to use older herbicides not normally allowed on cotton or soybeans, and many farms returned to deep tillage and hand weeding. Hiring crews to chop weeds by hand added substantial costs per acre, and tillage increased soil erosion, undoing some of the conservation benefits that herbicide-tolerant crops had enabled through no-till farming.

Why Is Dicamba Drift a Problem?

Dicamba, used against glyphosate-resistant weeds, can volatilize and drift onto neighboring fields, damaging crops not engineered to tolerate it, which has caused widespread crop injury and disputes between farmers. Dicamba has been used for decades on corn and small grains, but it is prone to drifting as vapor and damaging nearby plants, especially soybeans, which are extremely sensitive. A 2012 study in Environmental Toxicology and Chemistry by J. Franklin Egan and David Mortensen at Penn State measured dicamba vapor drift from applications to soybean and detected drift at measurable concentrations 21 meters away when a volatile formulation was used, enough to injure sensitive plants. The authors warned that dicamba-tolerant crops would greatly expand dicamba use over new areas and seasons, increasing drift risks.

Those warnings were borne out. After dicamba-tolerant soybeans and cotton were introduced and over-the-top dicamba products approved in 2016, farmers reported damage to millions of acres of non-tolerant soybeans, as well as to orchards, vegetables, and trees. Neighbor-versus-neighbor disputes erupted in farm communities, and some farmers planted dicamba-tolerant seed defensively to avoid damage. In 2021 alone, EPA received thousands of drift incident reports. Federal courts vacated over-the-top dicamba registrations in 2020 and again in February 2024; as South Dakota State University Extension explained, the 2024 ruling found that EPA had failed to meet legal requirements for public notice and comment when it re-approved the products.

Dicamba Returns in 2026

After two growing seasons without over-the-top dicamba, EPA approved it again. According to Agri-Pulse, on February 6, 2026, EPA re-registered dicamba products sold by Bayer, BASF, and Syngenta for over-the-top use on dicamba-tolerant soybeans and cotton for the 2026 and 2027 growing seasons, calling the restrictions the strictest it had ever required for any dicamba product and describing the decision as a response to growers who said they needed it to control resistant weeds. The new labels include limits on application rates and number of applications, required drift-reduction and volatility-reduction additives, a prohibition on spraying above 95 degrees Fahrenheit, and limits on how many acres can be treated in a county on hot days. The registration marked the fourth time EPA had approved over-the-top dicamba since 2016, and a coalition of environmental groups promptly filed a legal challenge. Farm groups welcomed the decision as providing clarity for the season.

Is Dicamba a Health Risk?

Dicamba’s main documented harm is to non-target plants through drift; human health concerns exist but are less established, and regulatory fights have focused mostly on crop damage and proper use. As dicamba use expanded, researchers examined potential health effects. A 2020 analysis of the Agricultural Health Study in the International Journal of Epidemiology by Catherine Lerro and colleagues followed 49,922 pesticide applicators in Iowa and North Carolina, more than half of whom had used dicamba. Those with the highest exposure had an 80 percent higher risk of liver and intrahepatic bile duct cancer, an association that persisted even when exposures were lagged by up to 20 years, and a modestly elevated risk of chronic lymphocytic leukemia, while earlier suggested links with lung and colon cancer were no longer apparent. The authors noted this was the first evaluation of liver cancer and called for further research. These findings concern heavily exposed applicators rather than the general public.

How Do Farmers Actually Control Superweeds?

The most effective approach diversifies weed control: rotating crops and herbicides, using cover crops, tillage, and mechanical weeding, so no single method dominates and drives resistance. Weed scientists broadly agree that relying on any single herbicide, or even a single herbicide-tolerant crop system, invites resistance. Diversification is the most durable solution. A long-term Iowa field experiment reported in 2012 in PLOS ONE by Adam Davis and colleagues compared a conventional two-year corn-soybean rotation with three- and four-year rotations that added small grains, red clover, and alfalfa, with lower fertilizer and herbicide inputs and periodic manure. Grain yields, harvested products, and profits in the more diverse systems were similar to or greater than in the conventional system, weeds were effectively suppressed in all systems, and freshwater toxicity was two orders of magnitude lower in the diverse rotations. The authors concluded that diverse cropping systems can use small amounts of chemicals to fine-tune rather than drive production.

Practice	How it helps
Diverse crop rotations	Disrupts weed life cycles and reduces reliance on any one herbicide
Cover crops	Suppress weeds by shading and competing for resources; protect soil
Rotating and combining herbicide modes of action	Slows resistance compared with repeated use of one herbicide
Harvest weed seed control	Destroys weed seeds at harvest to shrink the seed bank
Narrow rows and competitive varieties	Close the crop canopy faster, shading out weeds
Targeted mechanical control	Cultivation or precision weeding removes survivors before they set seed

Lessons From Australia

Australian grain farmers faced herbicide-resistant ryegrass earlier than many U.S. farmers and developed innovative practices, including harvest weed seed control, which captures and destroys weed seeds in the chaff during harvest, as well as burning narrow windrows of chaff. These methods, promoted by researchers including Stephen Powles, reduce the number of weed seeds returning to the soil and have helped Australian farmers manage resistance. U.S. researchers and farmers have begun adapting these approaches, including seed-destroying devices attached to combines.

What Do Superweeds Mean for Consumers?

Superweeds raise farming costs and herbicide use, which can affect food prices and the environment, but the main impact is on agriculture rather than direct consumer health from the weeds themselves. Most consumers never see a Palmer amaranth plant, but the superweed problem affects them indirectly. It has increased the total amount and number of herbicides used on major crops, contributing to residues in food and water. It has raised farmers’ costs and spurred conflicts in rural communities over drift damage. And it illustrates how agricultural technologies can create new problems when used without diversity. Consumers who want to support alternatives can choose organic products, which prohibit synthetic herbicides, and support policies that fund research on integrated weed management. Our article on glyphosate residues in grain foods explains how to reduce dietary herbicide exposure.

Drift Damage Beyond Soybeans

Dicamba drift affected more than neighboring soybean fields. Growers of vegetables, fruit, grapes, and specialty crops, as well as homeowners and managers of natural areas, reported injury to plants and trees, characterized by cupped and curled leaves. In some states, scientists documented symptoms on trees along field edges and in parks. Organic farmers faced the risk that drift could damage crops and complicate their operations. States responded with their own restrictions, including cutoff dates for spraying and buffer zones, and some states such as Arkansas imposed stricter rules than federal labels. The disputes strained relationships among neighbors who had farmed side by side for generations.

2,4-D and Other Herbicides

Dicamba was not the only older herbicide brought back into wider use. Crops engineered to tolerate 2,4-D, an herbicide introduced in the 1940s and a component of Agent Orange (though the toxic contaminant in Agent Orange came from another component), were approved in the 2010s along with new, less volatile formulations. Glufosinate-tolerant crops also expanded. Stacking tolerance to several herbicides in one crop allows farmers to use combinations that may slow resistance, but it also increases the total number of chemicals used and raises concerns about drift and environmental effects. Weeds resistant to several herbicides, including some Palmer amaranth and waterhemp populations, have already appeared, showing that chemical stacking alone may not stop the treadmill.

How Organic Farmers Manage Weeds

Organic farmers, who cannot use synthetic herbicides, rely on the diversified practices that weed scientists recommend for everyone: crop rotation, cover crops, mulches, timely cultivation, flame weeding, and careful attention to preventing weeds from setting seed. Weed control is often their biggest challenge and a major source of labor costs. Organic systems provide a living laboratory for nonchemical weed management, and some of their techniques, such as roller-crimped cover crops that form a weed-suppressing mat, are being adopted by conventional farmers seeking to reduce herbicide use.

A Balanced View

Herbicide-tolerant crops made weed control simpler and supported no-till farming for many years, but overreliance on glyphosate led to widespread resistance. The industry response, crops tolerant to older herbicides like dicamba and 2,4-D, has helped some farmers control resistant weeds while causing significant drift damage and legal battles. EPA’s 2026 re-registration of dicamba reflects pressure from growers facing difficult weeds, while critics argue that it repeats past mistakes. Research consistently points to diversified systems as the most durable solution, combining rotations, cover crops, and multiple tactics so that no single herbicide carries the whole load.

Practical Takeaways

For farmers, diversify weed control with rotations, cover crops, and multiple modes of action, and follow drift-reduction requirements carefully. For neighbors and specialty growers, document drift damage promptly and report it to state agriculture departments. For consumers, support organic and diversified farming and stay informed about pesticide policies affecting your region.

Meet Palmer Amaranth

Palmer amaranth, a pigweed native to the American Southwest, has become one of the most troublesome weeds in U.S. agriculture. It grows extremely fast, germinates over a long season, tolerates heat and drought, and a single female plant can produce hundreds of thousands of seeds. Because male and female flowers grow on separate plants, it cross-pollinates readily, spreading resistance genes through pollen as well as seed. Seeds can spread through equipment, manure, and contaminated seed mixes, including wildflower seed used in conservation plantings. These traits make it ideally suited to evolve and spread herbicide resistance, and a few uncontrolled plants can infest a field within a few years.

The Costs of Resistance

Herbicide-resistant weeds impose costs on farmers through added herbicides, extra applications, tillage, hand labor, and yield losses when weeds escape control. Heavy weed infestations can sharply reduce cotton and soybean yields. These costs have encouraged many farmers to adopt multiple-herbicide programs and to reconsider practices such as tillage, which can increase soil erosion. Research and extension programs help farmers develop strategies tailored to their fields, but adopting more diverse systems may require new equipment, knowledge, and markets for additional crops.

New Technologies on the Horizon

Engineers and agronomists are developing tools that could reduce reliance on herbicides. Camera-guided sprayers apply herbicide only to detected weeds, reducing total use. Robotic weeders and laser weeding machines kill weeds mechanically or with heat without chemicals, and some are already used on vegetable farms. Harvest weed seed control devices destroy weed seeds during harvest. These technologies are expensive and still being adapted to large row-crop farms, but they offer possibilities for breaking the herbicide treadmill if combined with diversified practices.

Looking Ahead

The 2026–2027 dicamba registration will be closely watched for drift incidents, and a court challenge may again determine its future. EPA must make a new decision for 2028 and beyond. Meanwhile, weeds continue to evolve, and the long-term solution depends on diversifying farming systems, investing in research on nonchemical methods, and supporting farmers as they adopt new practices. How agriculture responds to superweeds will shape pesticide use, rural relationships, and environmental health for years to come. The lesson of the past three decades is that weeds adapt to any single tactic, so the farms best positioned for the future will be those that combine many tactics and keep herbicides as one tool among several.

No-Till Farming and the Resistance Trade-Off

One of the benefits credited to herbicide-tolerant crops was the expansion of no-till and reduced-tillage farming, which leaves crop residue on the soil surface, reducing erosion, conserving moisture, and building soil organic matter. Glyphosate made it easier to control weeds without plowing. As resistant weeds spread, some farmers returned to tillage to bury weed seeds and kill emerged weeds, reversing some of these soil benefits. Cover crops offer a way to suppress weeds while maintaining reduced tillage, and research continues on combining cover crops, targeted herbicide use, and precision tools to protect both soil and weed control.

The Role of Seed and Chemical Companies

A handful of companies sell both herbicide-tolerant seeds and the herbicides designed for them, creating business incentives to market new trait-chemical packages as solutions to resistance. Critics argue this model encourages chemical reliance rather than diversified management, while companies point to stewardship programs, training, and investments in new modes of action. Few genuinely new herbicide modes of action have been commercialized in recent decades, which limits options and increases pressure on existing chemicals. Public research on integrated weed management provides an independent counterbalance.

Policy Approaches to Resistance

Regulators and policymakers have considered measures to slow resistance, such as requiring resistance management plans as conditions of herbicide registration, monitoring for resistant weeds, and supporting farmers who diversify rotations through conservation programs and crop insurance adjustments. EPA has added resistance management elements to some herbicide labels. Advocates argue that stronger policies and incentives for diversification would reduce long-term costs for farmers and the environment, while industry favors voluntary stewardship and flexibility for growers.

Questions Farmers Can Ask

Which weeds in my fields show signs of resistance, and have they been tested? Am I relying too heavily on one herbicide or mode of action? Could adding a small grain, forage, or cover crop to my rotation reduce weed pressure? What drift-reduction requirements apply to the products I use, and how will I protect neighbors? What cost-share or technical assistance programs are available for diversifying my system? Extension specialists and crop advisers can help answer these questions.

Waterhemp, Ragweed, and Other Resistant Weeds

Palmer amaranth is the most notorious superweed, but it is not alone. Waterhemp, a close relative common in the Midwest, has evolved resistance to as many as six or more herbicide modes of action in some populations. Giant ragweed, common ragweed, horseweed, and kochia have also developed glyphosate resistance in many areas, and Italian ryegrass and other grasses resist several herbicides in parts of the South and West. Each species has its own biology and timing, so effective management must be tailored to the weeds present in each field.

Community Conflicts and Mediation

Drift disputes have pitted neighbors against each other in farm communities, sometimes with tragic consequences, including a 2016 dispute in Arkansas that ended in a fatal shooting. State agriculture departments have handled thousands of drift complaints, but investigations can be slow and compensation uncertain. Some communities have developed sensitive crop registries, where growers of vulnerable crops such as vegetables, vineyards, and organic fields can mark their locations so applicators can take extra precautions. Communication between neighbors before spraying remains one of the simplest and most effective ways to prevent conflicts.

Environmental Effects Beyond the Field

Herbicide drift and runoff can affect plants, insects, and waterways beyond farm fields. Dicamba and 2,4-D can injure non-target plants in field margins, hedgerows, and natural areas, reducing flowering plants that pollinators depend on. Herbicide residues in streams can affect aquatic plants and algae. Endangered species assessments have become an increasingly important part of EPA’s herbicide registrations, and the 2026 dicamba labels included runoff-reduction measures such as vegetative buffers and cover crops. Diversified farming systems that use fewer herbicides reduce these off-field effects, as the Iowa rotation study showed with its much lower freshwater toxicity.

Common Myths

“Superweeds are genetically engineered.” They are ordinary weeds that evolved resistance through natural selection.

“New herbicide-tolerant crops will solve resistance.” Weeds have already evolved resistance to multiple herbicides; diversification is needed.

“Dicamba drift is a rare problem.” Thousands of incident reports and millions of damaged acres were documented in some years.

“Hand weeding is a thing of the past.” Some farmers returned to hand weeding to control resistant Palmer amaranth.

“Only GMO farms face resistant weeds.” Any farm relying heavily on one herbicide can select for resistance, though herbicide-tolerant systems accelerated it.

Key Terms

Herbicide resistance: The inherited ability of a weed population to survive a herbicide dose that normally kills it.

Mode of action: The way an herbicide kills plants; rotating modes of action slows resistance.

Over-the-top application: Spraying herbicide over an emerged crop.

Volatility: The tendency of a chemical to turn into vapor and move off target.

Integrated weed management: Combining multiple control tactics to manage weeds sustainably.

The Bottom Line

Superweeds are weeds that evolved resistance to herbicides, especially glyphosate, after years of heavy reliance on herbicide-tolerant crops; a 2018 review counted 38 glyphosate-resistant species. Industry responded with crops tolerant to dicamba and 2,4-D, but dicamba’s drift damaged millions of acres and led courts to vacate its over-the-top registrations in 2020 and 2024. EPA re-registered dicamba for 2026 and 2027 with its strictest restrictions yet, and the decision faces a legal challenge. Research shows that diversified rotations, cover crops, and multiple control tactics can control weeds with far fewer chemicals while maintaining profits, offering the most durable path off the herbicide treadmill.

Frequently Asked Questions

What are superweeds?

Superweeds are weeds that have evolved resistance to one or more herbicides, especially glyphosate (the active ingredient in Roundup), making them difficult to kill with the chemicals that once controlled them. They are not genetically engineered themselves; rather, repeated use of the same herbicide kills susceptible plants and leaves resistant ones to multiply, spreading resistance across farmland.

What causes herbicide-resistant weeds?

Resistance develops through natural selection. When farmers rely heavily on a single herbicide, the rare weeds that survive it reproduce and pass on their resistance, while susceptible weeds die off. Over several seasons, resistant weeds come to dominate. Planting the same herbicide-tolerant crops year after year and spraying the same chemical accelerates this process, which is how superweeds emerged.

Why is dicamba controversial?

Dicamba is an older herbicide brought back to fight glyphosate-resistant weeds, but it can volatilize and drift onto neighboring fields, damaging crops and plants not engineered to tolerate it. This has caused widespread crop injury, lawsuits, and disputes between farmers. Regulators have repeatedly restricted and re-approved dicamba, making it one of the most contested herbicides in recent years.

Are superweeds dangerous to human health?

Superweeds themselves are not directly dangerous to eat around or to health; the main concern is agricultural. Their bigger impact is driving farmers to use more and stronger herbicides, which raises costs, can harm the environment, and increases chemical drift. So the health and environmental worries center on the escalating herbicide use that superweeds provoke rather than the weeds themselves.

How can superweeds be controlled sustainably?

The most effective, lasting approach is integrated weed management: rotating different crops and herbicides with different modes of action, planting cover crops, using tillage and mechanical weeding, and cleaning equipment to prevent seed spread. By not relying on any single chemical, farmers slow the evolution of resistance. Experts stress that diversity, not a new “silver bullet” herbicide, is the real solution.

References

1. Powles SB. Evolved glyphosate-resistant weeds around the world: lessons to be learnt. Pest Management Science. 2008;64(4):360–365. PMID 18273881
2. Heap I, Duke SO. Overview of glyphosate-resistant weeds worldwide. Pest Management Science. 2018;74(5):1040–1049. PMID 29024306
3. Benbrook CM. Trends in glyphosate herbicide use in the United States and globally. Environmental Sciences Europe. 2016;28(1):3. PMID 27752438
4. Egan JF, Mortensen DA. Quantifying vapor drift of dicamba herbicides applied to soybean. Environmental Toxicology and Chemistry. 2012;31(5):1023–1031. PMID 22362509
5. Lerro CC, Hofmann JN, Andreotti G, et al. Dicamba use and cancer incidence in the agricultural health study: an updated analysis. International Journal of Epidemiology. 2020;49(4):1326–1337. PMID 32357211
6. Davis AS, Hill JD, Chase CA, et al. Increasing cropping system diversity balances productivity, profitability and environmental health. PLOS ONE. 2012;7(10):e47149. PMID 23071739
7. Agri-Pulse. EPA approves over-the-top use of dicamba for two years. February 2026. agri-pulse.com
8. South Dakota State University Extension. Over-the-Top Dicamba Ruling Explained. 2024. extension.sdstate.edu

Last updated: October 6, 2026