Weeding Through the Facts on Herbicide Resistance: An Expert Q&A


For as long as farmers have been farming, crops have faced considerable damage from diseases, insects, and weeds. To combat these problems, producers use a range of crop protection products to keep their plants safe.

Lately, we’ve been hearing media reports about more and more weeds becoming resistant to herbicides. So what are these so-called “superweeds”? And what do they have to do with biotech crops or “GMOs”?

To shed some light on the issue, we reached out to L. Val Giddings, PhD, an independent consultant with PrometheusAB, Inc., to clue us into what resistance is and how science and sound farming practices can help farmers keep growing an abundant and safe food supply.

What does resistance mean?

Insect pests will often eat crops or infect them with plant diseases that reduce yields by weakening or killing the crops. Weeds compete with crops for moisture, sunlight, and soil nutrients. Farmers use a variety of methods to reduce crop losses to pests and weeds. The success rate with these methods is often less than 100 percent. Some pests or weeds may escape or avoid such control measures, or they may carry a gene that makes a particular pesticide or herbicide ineffective.

The ability to survive exposure to a control method (e.g., a pesticide) that works by killing a pest or weed is what is meant by “resistance” (although with weeds and herbicides, we usually say that a weed is “tolerant” to an herbicide if it can survive exposure).

Where does herbicide resistance come from?

Resistance to herbicides is a natural phenomenon found widely among plants and animals. Such resistance depends on specific aspects of the biology of an organism. These aspects are controlled by genes, depending on their DNA sequences. There are many things that change the DNA sequence of a gene. They are called “mutagens,” and they include exposure to sunlight, natural chemicals in the environment (e.g., alcohol), radioactivity, heat, salinity, and more.

Over many generations these mutagens have led to wide genetic differences between and among different plants and animals. These differences make specific weeds more susceptible to certain herbicides than others. Such genetic differences impact the way an animal or plant can process a chemical it encounters when eating or drinking, for example.

Through mutagenesis and natural selection, some plants and animals have acquired genes that protect them against control measures like pesticides or herbicides, making them resistant.

How does biotechnology affect resistance?

There is nothing about crops improved through modern biotechnology that impacts resistance any differently than with crops derived through traditional breeding.

Modern biotechnology takes techniques we find practiced by living organisms everywhere throughout nature and harnesses them to improve plants and animals for human use. There is nothing about biotechnology that changes the fundamental rules of life.

A crop improved through biotechnology to tolerate an herbicide or resist an insect is still subject to natural selection, and its analog artificial selection, by plant breeders and farmers, as well as the principles of inheritance discovered by Gregor Mendel. This is why experts in genetics and risk analysis around the world have found crops improved through biotechnology to be just as safe, if not safer, than crops developed through mutation breeding.  

What are “superweeds” and how do they relate to crops grown using biotechnology?

“Superweed” is a term widely used in the media, but rarely by weed scientists. It is often used to describe weeds tolerant, in particular, to glyphosate, the active ingredient in Roundup, which has displaced many other herbicide uses since the advent of crops improved through biotechnology.

The term is often used  to suggest that a weed cannot be killed by a particular or multiple herbicides and has a new superior characteristic, or something that cannot be controlled. But it has long been known that none of the herbicides on the market work on all weeds (which is, after all, the main reason multiple herbicides have been developed), and weeds that are tolerant or resistant to one herbicide are usually vulnerable to others due to their different modes of action.

The term is also used often to suggest herbicide-resistant weeds are new and specific to “GM” crops, though neither is accurate.

With the dramatic increase in the use of glyphosate-tolerant crops, weeds tolerant to glyphosate, once rare, have become more common. But the data nevertheless show that herbicide-resistant weeds have emerged at a slower rate since the introduction of crops improved through biotechnology than seen before they were deployed. In fact:

“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. Which makes sense, because GM crops don’t select for herbicide resistant weeds; herbicides do. Herbicide resistant weed development is not a GMO problem, it is a[n] herbicide problem.”

Andrew Kniss, PhD, weed management expert at University of Wyoming

What is most misunderstood about resistance?

 There are a number of widely held misunderstandings about herbicide-tolerant weeds that are contradicted by the data and by experience. Here are some facts:

·         Herbicide-tolerant weeds are not new—they’ve been well-known and understood for decades.

·         Herbicide-resistant weeds are not caused by GM crops. They result from weeds with natural genetic immunity, or that have adapted to the prolonged use of one herbicide using the same mode of action to control the same population of weeds over a number of years.

·         Herbicide-tolerant weeds are not “superweeds.” They can still be controlled, either through rotating with alternative herbicides having a different mode of action, crop rotation, green mulches, cover crop management, or mechanical cultivation.

·         Herbicide-resistance traits in crops allow growers to incorporate additional herbicide options into their weed management programs and could actually reduce the rate at which weeds evolve herbicide resistance.

What’s next for biotechnology and managing resistance?

Effective management of weed resistance has been supported through the responsible introduction of crops improved through biotechnology. When industry standard best practices to prevent the emergence of herbicide-resistant weeds are not followed in the field, it leads to an increase in resistant weed populations through natural selection.

To manage these resistant weed populations, we have seen a renewed emphasis on annually rotating herbicide active ingredients that have different modes of action for impacting the weeds. This will be made easier with the introduction of crop varieties tolerant to multiple herbicides. These crops will enable the use of a mix of multiple herbicides with different modes of action. Topical applications of herbicides can also be combined with green mulches, targeted grazing, cultivation, and other weed control measures.

In addition to the resistance management and refugia that have so far worked well with insect pests, researchers are exploring variations on the “sterile male” technique, which limits reproduction, that has been used historically with great success.

What best management practices can producers who face resistance in their fields use to control weeds?

Best management practices for agricultural weed control include a host of options for farmers. Not all will work for all farmers, but most farmers can combine a mix of approaches drawing from the following and more:

·         Planting seeds improved to be tolerant to multiple different herbicide active ingredients

·         Rotating herbicide active ingredients

·         Tank-mixing multiple herbicide products for applications

·         Use of residual pre- and post-herbicides with different modes of action

·         Crop rotation

·         Green mulches

·         Managed grazing

·         Flood irrigation

·         Cultivation

For more information on best management practices, the Weed Science Society of America published these guidelines in the journal Weed Science: Reducing the Risks of Herbicide Resistance: Best Management Practices and Recommendations.