How can a plant resist glyphosate (Roundup) herbicide?

How can a plant resist glyphosate (Roundup) herbicide?

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In my area, the most common weeds that strongly resists (N-(phosphonomethyl)glycine) (glyphosate) are the horseweed, or mare's tail, Conyza canadensis, and Canada thistle, Cirsium arvense There are several other weeds with similar resistance. I use the brand Roundup on jobs where a complete kill is necessary. However, I sometimes have to go through again, with glufosinate, to control these weeds. I'd prefer not to, as the glufosinate lingers much longer in the soil.

Glyphosate inhibits an enzyme used in the synthesis of the aromatic amino acids tryptophan, tyrosine, and phenylalanine. It is taken in by the stomata in the leaves, and is moved throughout the plant to all the points of growth, acting fastest on those plants which are undergoing fastest growth.

I can't seem to find an article on how the weeds mentioned can tolerate this treatment. How do these weeds resist the glyphosate?

I found a paper investigating the mechanism of glyphosate resistance in Conyza canadensis. They used 31P NMR to investigate the fate of the glyphosate in vivo. What they found is that the resistant plants are able to transport glyphosate into the vacuole:

The following view of horseweed resistance to glyphosate emerges from the data presented herein. Glyphosate enters the cytoplasm of both R and S plant variants at the same rate. Within hours, however, glyphosate begins to occupy the vacuole in the R but not the S biotype. The identical pH values of R and S vacuoles speak against the possibility of a pH-driven process. This, coupled with the preferential movement of glyphosate from the cytosol to the vacuole in R tissue but not in S, suggests the presence of a transporter for glyphosate either specific to R or at a substantially greater concentration in R than in S tissue.

Another paper titled "Two non-target mechanisms are involved in glyphosate-resistant horseweed (Conyza canadensis L. Cronq.) biotypes" found a different mechanism for glyphosate resistance in addition to the sequestration into the vacuole.

Glyphosate was metabolized faster in the R (C004) biotype than in the S biotype. The herbicide disappeared completely from the R (C004) biotype by conversion into glyoxylate, sarcosine and aminomethylphosphonic acid within 96 HAT.

The resistant plants seem to be able to metabolize glyphosate more effectively.

It is of course not guarenteed that these are the only mechanisms for glyphosate resistance, there are likely more variations.

Overview of glyphosate-resistant weeds worldwide

Glyphosate is the most widely used and successful herbicide discovered to date, but its utility is now threatened by the occurrence of several glyphosate-resistant weed species. Glyphosate resistance first appeared in Lolium rigidum in an apple orchard in Australia in 1996, ironically the year that the first glyphosate-resistant crop (soybean) was introduced in the USA. Thirty-eight weed species have now evolved resistance to glyphosate, distributed across 37 countries and in 34 different crops and six non-crop situations. Although glyphosate-resistant weeds have been identified in orchards, vineyards, plantations, cereals, fallow and non-crop situations, it is the glyphosate-resistant weeds in glyphosate-resistant crop systems that dominate the area infested and growing economic impact. Glyphosate-resistant weeds present the greatest threat to sustained weed control in major agronomic crops because this herbicide is used to control weeds with resistance to herbicides with other sites of action, and no new herbicide sites of action have been introduced for over 30 years. Industry has responded by developing herbicide resistance traits in major crops that allow existing herbicides to be used in a new way. However, over reliance on these traits will result in multiple-resistance in weeds. Weed control in major crops is at a precarious point, where we must maintain the utility of the herbicides we have until we can transition to new weed management technologies. © 2017 Society of Chemical Industry.

Keywords: glyphosate resistance glyphosate-resistant crops herbicide resistance mode of action multiple resistance weeds.

The Boundless Thicket

The Shikimate pathway is usually found in the actively growing part of the plant. Glyphosate is absorbed through the foliage or leaves of plants and transported through the cells to the growing points, because of this glyphosate is only effective on growing plants. You may be wondering since this inhibits amino acid synthesis, how the desired crop survives while the weeds do not. In regular plants the EPSP synthase catalyzes the transfer of PEP to the hydroxyl group at the five carbon position.

In the 1990s there was a stand of E. coli that was found in a waste area at a glyphosate production facility, that was glyphosate resistant, called Agrobacterium sp strain CP4. From this the company was able to form a synthetic molecule that included the amino acids Ala-100-Gly CP4.

(The Jmol below shows the ESPS synthase that is the mutant glyphosate resistant system) &ampltp&ampgtIn the &ampampampampltbr&ampampampampgt&amplt/p&ampgt
In the study done by Funke about the molecular make up of herbicide resistant Round up ready crops shows that the effect of potassium ions on the enzyme’s activity appear to be selective toward PEP utilization, showing that CP4 EPSP synthase is the prototypic class II EPSP synthase, because the catalytic efficiency of the reaction remains basicially unaltered in the present of glyphosate. This allows the CP4 EPSP synthase to work to allow plants to be herbicide resistant (Funke, Han, and et al). This new EPSPS gene encode for a polypeptide that contains a chloroplast transit peptide which enables the EPSPS polypeptide to be transported into a chloroplast inside of a plant cell. Once it is in the chloroplast it has the ability to work properly allowing the plant to make photosynthesis (Shah). So if we have found a way for our crops to be resistance is there a way that plants could become resistant to glyphosate?

Glyphosate had been a great advancement in the control of weeds for farmers and gardeners. With it being a broad spectrum weed controller it allows farmers to only have to spray twice a growing season instead of spraying every three weeks, like it was before Round up. RoundUp Ready plants, plants that are resistant to glyphosate, have been a great advancement for farmers as well. Farmers can plant these crops and not have to worry about the crops being damaged from the glyphosate that kills the weeds. Even with the problems that are created by glyphosate resistant weeds, glyphosate has been a great product that has made farmer’s lives easier and has allowed for advancement in agriculture.

Funke, Todd, Huijong Han, et al. "Molecular Basis for the herbicide resistance of Round up Ready crops ." Proceddings of the National Academy of Sciences of the United States of America. 103.35 (2006): 13010-13015. Web. 1 April 2012.

Problem Species

The known species that have developed glyphosate-resistant populations include common and giant ragweed, horseweed, Palmer pigweed, Italian ryegrass, tropical spiderwort, water hemp, johnsongrass, annual bluegrass, and others. This does not mean that every time you encounter one of these species, it is glyphosate-resistant. Instead, genetic strains of these species have developed resistance and have become a problem in certain areas of some states. To view an updated list of glyphosate-resistant species and where they occur, click here.

About the Author: Brian Sheppard is a food plot consultant from Georgia and owner of Wildlife Landscape Services. This is one of several articles he has contributed to QDMA’s Quality Whitetails magazine.

Our journalism depends on you.

You can count on TVO to cover the stories others don&rsquot&mdashto fill the gaps in the ever-changing media landscape. But we can&rsquot do this without you. looks at how glyphosate is used in two different Ontario industries &mdash agriculture and forestry &mdash and the pros and cons of existing alternatives.

Glyphosate in agriculture

As many farmers did in the 1990s, Blake Vince began relying more on the herbicide to control weeds after glyphosate-resistant, genetically engineered corn crops had been introduced and glyphosate prices had dropped as the original patents on it expired. Previously, he and other farmers had used the chemical sparingly &mdash often as a spot spray &mdash to treat hard-to-get-rid-of weeds as a crop grew, or before planting or after harvest so it wouldn't kill crops. But Vince, who farms 1,200 acres west of Chatham, was able to use glyphosate throughout the growing season on genetically modified varieties of corn (and, later, soybean seeds).

&ldquoIt is a whole system,&rdquo he says. &ldquoThey sell you the seed with the traits that allow you to use the chemical to ensure you have an immaculate field free of anything other than the intended crop to harvest.&rdquo

Glyphosate-resistant seed might have cost more than the non-engineered alternatives when it was introduced &mdash and still does &mdash but it slashed farmers&rsquo weed-control budgets from $50 an acre to $10 to $15, says Tardif. By the early 2000s, however, weeds that were resistant to glyphosate were appearing stateside and, in 2008, resistance was confirmed in giant ragweed near Windsor. Today, in Ontario, four species of weeds, some of which are widespread across the province, have glyphosate resistance, Tardif says: "That's when it really struck home for farmers here."

For Louise Hénault-Ethier, the David Suzuki Foundation&rsquos science-projects manager in Quebec and Atlantic provinces, the concern is that farmers will respond with more chemicals to tackle more resilient weeds. &ldquoI fear from a scientific standpoint that it will increase pesticide usage in the long term,&rdquo she says. &ldquoIt&rsquos so easy to rely on spraying something over a whole field that people don&rsquot necessarily know how to take advantage&rdquo of other strategies.

​ Blake Vince now uses less than a litre of glyphosate per acre each year on his farm. (Mary Baxter)

However, methods that limit or eliminate herbicide use in agriculture are evolving. Some farmers use equipment that employs cameras that, using artificial-intelligence technology, activate a nozzle to spray herbicide if weeds are encountered. Selective flamers, which direct flames to weeds, are another approach. And using drones to spot and treat weeds also looks promising, Hénault-Ethier says. Other technologies under development combine robotics and AI to detect weeds and either spray them or pull them out, says Tardif: &ldquoIt sounds like science fiction, but a lot of teams are working on that.&rdquo

Many solutions involve farmers rethinking how they manage their fields, such as by planting tightly packed crops to crowd out weeds. "Glyphosate was the giant hammer that will kill the weed now we're trying to replace that with a bunch of little hammers that, combined together, would get the same effect," Tardif explains. "The problem is that these tend to be more unpredictable, and sometimes we don't know what are the best combinations."

Since 2013 on the Vince farm, the primary strategies to reduce glyphosate use have included abandoning genetically modified seed and adding cover crops &mdash plants such as rye, crimson clover, hairy vetch, and peas grown not for sale but rather as barriers between for-market crops. The idea, Vince explains, is that cover crops reduce the growth of weeds by cutting off sunlight or releasing natural chemicals that can suppress &mdash and even kill &mdash unwanted plant growth. &ldquoI&rsquom less than a litre per acre per year,&rdquo he says of his farm&rsquos current glyphosate use (he once used three litres per acre).

Hénault-Ethier wants to see Canada adopt national regulations similar to those of the European Union, which require farmers to use integrated pest management &mdash a system through which farmers must look at other solutions before introducing pesticides. &ldquoIt&rsquos good practice to seek alternatives to pesticides,&rdquo she says. Asked whether Health Canada has considered mandating integrated pest management, a spokesperson for the department says it is included in the National Standard for Pesticide Education, Training and Certification in Canada. &ldquoCertification and training of pesticide applicators is done by provincial regulators based on the National Standard,&rdquo the spokesperson says. &ldquoFarmer training and certification requirements may vary across provinces depending on the class of pesticide being used.

&rdquoIn Ontario, the Ministry of the Environment, Conservation and Parks supports integrated pest management. "Ontario promotes the use of Integrated Pest Management (IPM) principles for the management of pests in agriculture," a ministry spokesperson says by email. "Farmers need to be trained through Ontario&rsquos Grower Pesticide Safety Course before they can buy commercial pesticides, including glyphosate, for use on their farm," the spokesperson adds, noting that the safety training includes the management approach and provides related resources. But Hénault-Ethier says the solution must be binding and federal, as the Canadian government registers the pesticides that farmers are allowed to use. Provinces and municipalities can have more stringent regulations, and many do, but this approach is inconsistent and inefficient compared to one authority setting the rules, she says: &ldquoIt&rsquos not uniform.&rdquo

Glyphosate in forestry

In forestry, glyphosate is used to make room to grow desired trees by killing competing vegetation. According to the Ministry of Natural Resources and Forestry, &ldquoHerbicides are only used when necessary to renew the forest,&rdquo and they&rsquore applied on about 0.2 per cent &mdash or 1,120 square kilometres &mdash of Ontario&rsquos 560,000 square kilometres of Crown forests.

In a natural setting, Natural Resources Canada research scientist and eco-toxicologist Chris Edge says, regrowth areas play an important role in an ecosystem. But in a forest-management unit &mdash a designated area licensed to a forestry company &mdash the ultimate goal is to regrow a certain tree on particular site. Having studied glyphosate&rsquos direct impact on wildlife, Edge says, &ldquoWe can detect glyphosate in new vegetation &mdash after application, we can detect it for a year. But after about 18 days, the concentrations are so low that they're below any sort of threshold set by Health Canada.&rdquo

​ Aerial view of the Pinelands forest around Foleyet, where gylphosate has been used. (Courtesy of Joel Therriault)

Chief Marcus Hardy, of Red Rock First Nation, which co-owns Lake Nipigon Forestry Management with Animbiigoo Zaagi'igan Anishinaabek and Biinjitiwaabik Zaaging Anishinaabek First Nations, says the communities strongly oppose the use of glyphosate, particularly via its common application &mdash aerial spraying &mdash within the Nipigon forest. &ldquoThere are a few elders that were adamant on explaining exactly what their concerns were,&rdquo he says, adding, &ldquowe saw less growth in the bush we saw less plants, medicine plants, blueberries and all that.&rdquo Regional organizations such as Nishnawbe Aski Nation, which represents 49 First Nations in Ontario Mushkegowuk Council the North Shore Tribal Council and individual band councils within these organizations have also released resolutions opposing its use. Hardy says the community relies on the forest as part of their food supply. &ldquoOur people use the bush for their grocery store,&rdquo he says. Instead of aerial spraying, the forestry management company spot sprays glyphosate so that it can use less and be more precise in its application.

Despite the lack of direct adverse impacts on species, Edge acknowledges that the principles of sustainable forestry take other factors &mdash such as social acceptance and broader ecological impacts &mdash into consideration. &ldquoCrown lands are a public resource,&rdquo he says. &ldquoIf people don't want their forests managed that way, then that should definitely be taken into account.&rdquo A Ministry of Natural Resources and Forestry spokesperson tells in an email that &ldquopublic input on sustainable forestry regulations and policies are sought and considered when elements of the policy framework are revised or new pieces added, including those related to the use of herbicides.&rdquo However, the province notes that the Pest Management Regulatory Agency &ldquohas determined glyphosate does not present unacceptable risks to human health or the environment when used as directed.&rdquo

Jurisdictions such as Quebec have banned the application of herbicides in forestry, and

​ Signs are put up in areas that are about to be sprayed with
herbicides such as glyphosate. (Courtesy of Joel Theriault)

MP Jenica Atwin has proposed a private member&rsquos bill aimed at banning glyphosate nationally. Fellow NRC researcher Nelson Thiffault says that Quebec&rsquos legislation has led to the use of alternative methods, including replanting more mature seedlings &mdash which are stronger and more capable of beating out competing plants &mdash and hiring crews to manually remove competing plants within the regrowth areas. However, he says, these alternatives cost more while aerial spraying costs $200-300 per 10,000 metres squared, the costs of the alternatives are roughly $500-600 per 10,000 square metre (although that can increase depending on site conditions). In Quebec, these costs are offset by the province, which contracts the seedling operations and pays the crews on public lands, according to Thiffault. Costs in Ontario would typically fall on the forestry company, according to MNRF.

Hardy says that, because MRNF is a licensed forester, the communities can take ownership of how their forest is managed: &ldquoIt&rsquos great financially, but the most important thing is the longevity of the forest. We're stewards of the land, and we need to take care of it. We need to make sure that things are done right &mdash safely.&rdquo

What's next for glyphosate?

Environmental groups are calling for the Pesticide Management Regulatory Agency, the Health Canada branch that regulates pesticides, to reconsider its 2017 decision to renew glyphosate&rsquos approval for use in Canada without new restrictions. (The chemical is next scheduled for review in 2032, although Health Canada has pledged to monitor for new information about the chemical and take action if necessary.) In March 2020, Canadian non-profit Safe Food Matters launched an appeal of an earlier Federal Court judgment that upheld Health Canada&rsquos decision. A spokesperson for EcoJustice, which, as an intervenor in the case will argue alongside Safe Food Matters, says by email that no date has yet been set for the hearing.

A Health Canada spokesperson notes by email that the 2017 review took into account 1,300 studies that totalled more than 89,000 pages. "This science based review indicated that potential risk to human health and the environment is acceptable when the label directions" are followed, the spokesperson writes. "It would be inappropriate," the spokesperson says, "to comment on the specifics of this case while it is before the court."

Ontario Hubs are made possible by the Barry and Laurie Green Family Charitable Trust & Goldie Feldman.​​​​​​​

Prevention and management strategies

The North Central Weed Science Society (NCWSS) Herbicide Resistance Committee developed the following strategies for avoiding and managing problems with herbicide-resistant weed biotypes.

Keep in mind that relying on any one strategy isn’t likely to be effective. You must use the following strategies in carefully selected combinations to avoid or properly manage herbicide-resistant weed problems:

1. Only use herbicides when necessary

Where available, base herbicide applications on economic thresholds. Continued development of effective economic threshold models should be helpful.

2. Rotate herbicides (sites of action)

Don’t make more than two consecutive applications of herbicides with the same site of action to the same field unless the management system includes other effective control practices. Two consecutive applications could be single annual applications for two years, or two split applications in one year.

3. Apply herbicides that include multiple sites of action

Apply herbicides in tank-mixed, prepackaged or sequential mixtures that include multiple sites of action. For this strategy to be effective, both herbicides must have substantial activity against potentially resistant weeds.

Remember that in the past, weeds selected for herbicide resistance often weren’t the primary target species. It may be expensive to apply herbicide combinations that duplicate a wide spectrum of weed control activity. Many of the more economical herbicide combinations may not be adequate.

View the Take Action Herbicide Classification guide for a list of premixes and their corresponding sites of action.

4. Rotate crops, particularly those with different life cycles

An example is winter annuals such as winter wheat, perennials such as alfalfa and summer annuals such as corn or soybeans. Also, remember not to use herbicides with the same site of action in these different crops against the same weed, unless you also include other effective control practices in the management system.

5. Avoid more than two consecutive herbicide applications with herbicide-resistant crops

Planting new herbicide-resistant crop varieties shouldn’t result in more than two consecutive applications of herbicides with the same site of action against the same weed unless other effective control practices are also included in the management system.

6. Include mechanical weed control practices

Where feasible, combine mechanical weed control practices such as rotary hoeing and cultivation with herbicide treatments.

7. Consider primary tillage

Where soil erosion potential is minimal, consider primary tillage as a component of the weed management program.

8. Regularly scout fields to identify the weeds present

Quickly respond to changes in weed populations to restrict the spread of weeds that may have been selected for resistance.

9. Clean tillage and harvest equipment

Clean before moving from fields infested with resistant weeds to those that aren’t.

10. Encourage others to adopt prevention management strategies

Encourage railroads, public utilities, highway departments and similar organizations that use total vegetation control programs to use vegetation management systems that don’t lead to the selection of herbicide-resistant weeds.

Resistant weeds from total vegetation control areas frequently spread to cropland. Chemical companies, state and federal agencies and farm organizations can all help in this effort.

Green, M.B., LeBaron, H.M., & Moberg, W.K. (editors). (1990). Managing resistance to agrochemicals: From fundamental research to practical strategies (Symposium Series No. 421). American Chemical Society, pp 496.

Gressel, J. (1992). Addressing real weed science needs with innovations. Weed Technology, 6, 509-525.

Gunsolus, J.L. & Curran, W.S. (1992, revised). Herbicide Mode of Action and Injury Symptoms CD-ROM.

Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. Tuesday, May 29, 2018. Available

Maxwell, B.D., Roush, M.L., & Radesevich, S.R. (1990). Predicting the evolution and dynamics of herbicide-resistance in weed populations. Weed Technology, 4, 2-13.

Jeff Gunsolus, Extension weed scientist


I’d like to acknowledge the members of the 1991 and 1992 North Central Weed Science Society (NCWSS) Herbicide Resistance Committees who developed the ten management strategies for avoiding and managing herbicide-resistant weeds and extensively reviewed the content on this page.

The members are: Thomas Bauman, T. Robert Dill, Ray Forney, R. Gordon Harvey (Chair, 1991), Nick Jordan, Rex Liebl, Michael Owen, Jamie Retzinger, Dave Stoltenberg, G. Chris Weed, Phil Westra, Gail Wicks and Bill Witt. It’s been my pleasure to serve as vice-chair and chair of this committee in 1991 and 1992, respectively, and to work with these dedicated people.

Spread of Glyphosate Resistant Weeds

After more than a decade of discussions about glyphosate resistant (GR) weeds, three GR species (horseweed, waterhemp and giant ragweed) have been verified in Iowa. Research conducted by Iowa State and others has confirmed glyphosate resistance in isolated fields across Iowa. Although GR weeds are found throughout the state, at this time it is estimated less than one percent of Iowa's corn and soybean fields are infested with GR at levels of economic concern. This article will discuss how GR weeds can appear in fields where they currently are not problems.

Herbicide resistance can become a problem due to two distinct mechanisms. The first involves independent selection of a resistant biotype from the weed community already present in the field. The other involves movement of the resistant trait (gene flow) from a field with an existing resistance problem to fields without resistance. This spread can occur due to either pollen or seed movement.

Waterhemp, shown here as a seedling, is one of three weed species verified to be glyphosate resistant in Iowa.

Independent selection

Herbicide resistant biotypes are present at low frequencies within the weed community prior to the discovery and introduction of a herbicide. The initial frequency of resistance is the percentage of the weed population possessing the resistant trait at the time of herbicide introduction. Repeated use of an herbicide results in resistant biotypes becoming more prevalent, and eventually they may become the dominant biotype and thus the weed is no longer effectively controlled by the herbicide. Evolution of resistance within a field is a direct result of the weed management program used within the field and the initial frequency.

The initial frequency of herbicide resistance varies among herbicide classes. With ALS inhibitors, resistance is present at a relatively high initial frequency and it was likely that resistant biotypes of certain weed species were present in most Iowa fields at the time of introduction of these herbicides (mid 1980s). It is believed that the spread of ALS resistant waterhemp across Iowa was largely due to selection of resistant biotypes from native infestations within individual fields.

Glyphosate resistance in plants is a rare trait, and thus it is likely that GR weeds were not present in most fields when we began to rely on glyphosate for weed control. Even though glyphosate has been used in a way favoring the selection of GR in the majority of fields across Iowa, GR weeds will only appear due to independent selection in those fields 'unlucky' enough to have one of the rare GR biotypes.

Gene flow

Now that GR biotypes have evolved in isolated fields across the state, the risk of resistance appearing in 'clean' fields is probably as great due to movement from fields with existing GR problems as it is from independent selection. Gene flow may occur either from pollen or seed movement. The likelihood of these two mechanisms of gene flow varies with characteristics of the resistance trait, biology of the weed and the weed management systems used in adjacent fields.

Although the inheritance of GR has not been characterized in all GR biotypes, where it has been identified, heritability is an incompletely dominant trait. Thus, cross-pollination between a resistant (R) and susceptible (S) weed biotype will result in increased resistance to glyphosate in the hybrid plant (S x R). Both waterhemp and giant ragweed produce large quantities of pollen that move long distances by wind currents — thus pollen movement is a threat to spread GR in these two species.

Researchers at the University of Illinois developed a model to help understand gene flow of GR in waterhemp via pollen movement (Liu et al. 2010). Their model predicted that GR could spread approximately three miles per year by wind dispersal of pollen, thus any field within a three mile radius of a field with GR waterhemp is at risk of resistance becoming established.

It is important to understand that GR will not move into every field within this radius due to several factors. The GR trait can only become established in fields where receptive waterhemp plants are present to intercept the pollen. Also, the greater the distance from the source field, the less likely a pollen grain is to pollinate a susceptible waterhemp plant. Finally, in order for GR to become established in the field, the weed management program being used must rely heavily on glyphosate. If the weed management program includes herbicides having different mechanisms of action than glyphosate that are effective on waterhemp, there is no advantage for the GR trait and it is unlikely that the resistant biotype would become permanently established in the field.

Herbicide resistant traits can also spread via movement of seed from the source field to fields free of the resistant biotype. Neither waterhemp nor giant ragweed have seed possessing long distance dispersal mechanisms. However, the lack of dispersal mechanisms is not a limitation since seeds can easily be transported by machinery. GR waterhemp was found more than 25 miles from the source field two years after resistance was identified (Liu et al. 2010), thus it moved much farther than the pollen transfer model predicted. The researchers speculated that seed movement by farm machinery or independent selection was responsible for the appearance of GR a long distance from the source field.


Glyphosate has been heavily relied upon for weed management since the introduction of Roundup Ready soybeans in 1996. This widespread use has resulted in the selection of GR biotypes throughout Iowa. Now that resistant biotypes are present, the spread of GR across the landscape is probably greater due to gene flow rather than the independent selection of resistant biotypes. This is due to the low initial frequency of glyphosate resistance within weed populations.

Does the threat of resistance movement via gene flow mean it is futile for individual farmers to prevent GR from appearing in fields they manage? The answer is no, and in fact, it places greater emphasis on the value of diversified weed management systems. A diverse herbicide program continually places different herbicide stresses on the weed community. This greatly reduces the benefit of the GR trait to weeds, and therefore reduces the likelihood of resistant biotypes becoming established in fields currently free of the resistant trait. Use of diverse management programs will maintain the value of glyphosate by reducing the spread of GR weeds.


Liu, J., P.J. Tranel and A.S. Davis. 2010. Modeling the spread of glyphosate resistant waterhemp. Proc. North Cent. Weed Sci. Soc.

Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on January 20, 2011. The information contained within may not be the most current and accurate depending on when it is accessed.

Beyond Roundup: Alternatives to consider adding to your weed management plan (Update)

What is your go-to postemergence herbicide? If you answered Roundup (glyphosate is the active ingredient in Roundup), you would be in the majority for landscape and nursery professionals. Though glyphosate works very well on most weed species, there are times when other products may be more effective or offer a less phtytotoxic (damage to ornamental plants) alternative (Fig.1). We should also keep in the back of our minds to continue rotating herbicides to prevent herbicide resistant weeds.

Utilizing several different modes of action in the nursery and landscape can aide in reducing resistant weeds, as well as being more effective on certain weed species.

Roundup has been a household name for over 20 years. It’s most likely the only herbicide that the general public can name. For several years, the most widely used herbicide in the world has been glyphosate (many trade names). There is a reason for the popularity of this herbicide. Some of the positive attributes include non-selective/broad spectrum (kills many types of plants), systemic activity (travels in the vascular system, both xylem and phloem), low mammalian toxicity (relatively safe for humans), limited soil activity, non-volatile, low environmental impact, and the efficacy of the product (how well it kills weeds). With these attributes, it’s not hard to wonder why this product has become a mainstay in the industry.

National media outlets are reporting that there are growing concerns regarding the potential link between glyphosate-containing products and increased risk of cancer. Jury verdicts in California have been covered in great detail by the media, thus elevating the dialogue of the cancer risk with consumers. Purdue Weed Scientists are trained and tasked with studying weed biology, weed competition, various methods of controlling weeds with herbicide and non-chemical tools, and herbicide resistance in weeds. They are also responsible for developing weed control best management plans for a number of crops grown in Indiana and the Midwest. They are not trained to be cancer scientists and thus do not conduct research on the potential risk of cancer. We rely on toxicologists to conduct the appropriate research regarding the toxicology of any pesticide to mammals and amphibians and use their expertise to develop recommendations on the risks associated with various pesticides. To that end, toxicologists affiliated with the National Cancer Institute conducted a review of the literature and found no link between glyphosate and increased risk of cancer. This is the most recent and most prestigious research paper available on the topic. The journal article can be found here: A scientific organization wrote an article about this subject in March that discusses the glyphosate-cancer topic in much detail:

With that in mind, what is going to be your answer when your client request that you stop using glyphosate on their property? Do you have a backup plan? These questions are the reality going forward and some landscape companies are having to devise a new plan that doesn’t use glyphosate on some properties.

Before discussing alternatives to glyphosate, always remember to utilize preemergence herbicides (fall and spring) as your primary method of weed control in nurseries and landscapes. Relying on preemergence herbicides will reduce labor, reduce the chances of phytotoxicity to ornamentals, and reduce total herbicide usage. Postemergence herbicides should be relied upon only when necessary to control weeds that have escaped your preemergence program.

Considering alternatives to glyphosate will require a knowledge of what ornamental plants are on the property, as well as what are the dominant weeds. There is no herbicide that will completely ‘replace’ glyphosate due to the attributes mentioned above. Careful planning by developing a property-wide herbicide plan will assist in determining the optimum solution for reducing the use of glyphosate.

When you are trying to control grassy weeds in ornamentals, there are several options that are very safe on most ornamental plants. The grass specific herbicides, such as fluazifop (Fusilade/Ornamec), clethodim (Select, Envoy), sethoxydim (Poast, Vantage, Grass Getter), and fenoxaprop (Acclaim), can control many grass weeds effectively with little phytotoxicity to most ornamental plantings (Fig.2). These herbicides will only kill grass, so they can be sprayed over the top of many broadleaf ornamentals, as well as plants such as liriope and iris, since these are not grasses (Fig.3). Always check the label to ensure the ornamentals are labelled for over the top or directed sprays. More information about controlling grasses in non-grassy ornamental plants can be found here:

Figure 2. Grass specific herbicides can be utilized over the top on many ornamental plantings in nurseries and landscape to reduce the chance of phytotoxicity on ornamentals.

Contact herbicides are an option and are most effective on annual weeds, especially while small. Since contact herbicides are not translocated throughout the plant, coverage of the weed needs to be sufficient enough to kill. Most large or mature plants tend to outgrow contact herbicide applications. There are some contact herbicides labelled in nurseries and landscape, including Scythe (pelargonic acid), Reward (diquat), and Finale (glufosinate). These products are broad-spectrum, so damage can occur if applied on ornamental plantings. Basagran (sodium salt of bentazon) is a contact that is effective on nutsedge, as well as many broadleaf weeds.

There is some confusion in the industry that glufosinate (Finale and other trade names) is a ‘replacement’ for glyphosate. Though their names are very similar, they are very different herbicides. As mentioned above, glyphosate is translocated throughout the plant, but glufosinate is a contact herbicide that does not translocate. Though glufosinate could be a replacement for glyphosate in some instances (i.e. small annual weeds), just remember that it will not have the same efficacy as a translocated herbicide on larger or more mature weeds.

Being that glyphosate is a translocated postemergence herbicide that is non-selective, there are a few ‘true’ alternative herbicides that can be used as a substitute. This is where the knowledge of your ornamental plants and types of weeds are very important. Some alternative postemergence herbicides that translocate include, Lontrel (clopyralid) Dismiss (sulfentrazone) and Sedgehammer (halosulfuron-methyl). These products each control hard to control weeds, such as thistle (Lontrel) and nutsedge (Sedgehammer) (Fig.4). These products have some over-the-top use of certain ornamental plantings, but can cause severe damage on certain ornamental plants. The label must be followed carefully when using these products around ornamental plantings due to the potential of severe phytotoxicity or death.

Figure 4. Some weed species, such as nutsedge, are better controlled with herbicides other than glyphosate.

Glyphosate is a product that is effective in many applications for your weed program, but there are alternatives. As described previously, no singular herbicide will totally replace glyphosate due to many positive attributes. Considering incorporating some of these alternatives will aide in the reduction of resistant weed populations, can be safer around ornamentals, more effective on some weed species, and make your clients more comfortable in having another option available.

There are 82 products available on CDMS containing glyphosate, with almost 1,400 total herbicide labels, that can be found at

Remember to always check labels prior to making any herbicide application.

Reference in this publication to any specific commercial product, process, or service, or the use of any trade, firm, or corporation name is for general informational purposes only and does not constitute an endorsement, recommendation, or certification of any kind by Purdue University. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer. Always refer to the label prior to making any pesticide application.

Environment | Big Business | Health and Safety


Herbicide Resistance | Seed Contamination | Risk Assessment

Most agree that it is very challenging to quantify the effect releasing a new organism into an environment will be. The ultimate effects of such an introduction are hard to predict, and careful analysis to minimize the risk associated is necessary. The main government organization responsible for determing the safety of the release of a genetically engineered organism into the environment is the Environmental Protection Agency (EPA). The EPA works closely with the The Animal and Plant Health Inspection service (APHIS) to determine if a crop is both safe to grow and eat.

Herbicide Resistance

One of the main concerns about genetically engineered crops such as Roundup Ready crops is the development of weeds and other plants that are also resistant to Roundup (glyphosate). An article recently published in Science Daily (link) suggests that farmers are becoming too reliant on Roundup. The use of Roundup Ready crops has become ubiquitous. This has resulted in weeds such as giant ragweed that are resistant to Roundup, when Roundup was the herbicide developed explicitly to combat these weeds. If a farmer were to grow on Roundup Ready crops, they would have to use Roundup to treat their fields, increasing resistance to the herbicide. It is recommended that farmers rotate Roundup Ready crops with other crops and use alternate herbicides to help prevent resistance.

Seed Contamination

Another concern with genetically engineered crops, and Roundup Ready specifically, is seed contamination. Through a variety of means, Roundup Ready genes have been introduced into the food supply. For example, as stated in a Greenpeace report (link), it has become virtually impossible for farmers to grow non-genetically engineered Canola in Canada. Read more about this case on the players page (link). Concrete evidence of seed contamination has been discussed in the Union of Concerned Scientist's report called Gone to Seed. Read more about Gone to Seed here.

Risk Assessment

A big concern about genetic engineering in general, and including Roundup Ready crops, is the fact that scientists do not know what the true effect of these organisms is on the environment before releasing them. Other issues stem from the idea that changing one gene does not necessarily change just one function. A variety of changes in the expression of such a gene could occur in the resulting organism.

Big Business

Terminator Seeds | Lack of Increased Yield

One of the issues many people have with Roundup Ready crops is the idea that they are only around to benefit big businesses like Monsanto. These crops require farmers to also buy Monsanto's herbicide Roundup to use, resulting in double the profits for the corporation. Monsanto, however, views Roundup Ready crops, and genetically engineered foods in general, to be the answer to world hunger. They feel that they are stepping in and helping the situation, whereas others think they are only in it for the profit.

Terminator Seeds

Roundup Ready seeds have what is known as "terminator technology" seeds that are grown for a second generation are sterile. Farmers need to purchase seeds from Monsanto each year if they want to continue to use their crops. Many cite the terminator technology as restricting and preventing farmers from reusing their best seed, requiring them to rely on the newest strain of Roundup Ready seed each year. Monsanto, however, argues that the terminator technology is used to help prevent the spread of the glyphosate (Roundup) resistance to other species.

Lack of Increased Yield

Monsanto claims to be addressing the needs of the world when it comes to the food supply. However, there is little to no evidence that the use of Roundup Ready crops increases the yield or profit of farmers who use their seeds. A press release from the New Soil Association released in April, 2008 shows that genetically modified crops do not result in higher yields than non-genetically modified crops. Realistically, however, most genetically modified crops (including Roundup Ready) are developed to be pesticide and herbicide resistant, rather than directly increase the yield of a given crop. This makes one question whether it is even important to have herbicide resistant plants at all, or if the focus should shift to provide other advantages that do increase yield. Many argue that because Roundup Ready crops do not increase the yield or profits of farmers, they only serve to benefit Monsanto. There is little to no benefit for the consumer, but high reward for the corporation. Farmers must not only purchase new seeds from Monsanto each year, but also Monsanto's own herbicide Roundup.

Health and Safety

Roundup | Allergies | Unsafe for Consumption


Concerns about the effects of genetically engineered foods on health have to do with both the crops themselves along with the toxicity of the herbicide Roundup. The EPA currently regulates the allowable concentration of glyphosate in the drinking water, and says that glyphosate is safe for humans in small concentrations. However, with the use of Roundup on the crops, consumers may be ingesting more of the chemical than they realize.


Introducing new DNA into the food supply may involve the introduction of new allergens. The issue is of greatest concern with transgenic organisms, in which genes from one organism to another. Soybeans genetically engineered to contain Brazil-nuts is one such example. These soybeans were shown to cause reactions in individuals allergic to Brazil nuts. If consumers are unaware of the fact that a brazil nut gene is present in the soybeans, they cannot avoid consuming the product. Read more about this on the players page (link).

Unsafe for Consumption

Most Roundup Ready crops are considered safe for consumption. One Roundup Ready crop, though, alfalfa, has been removed from the market. Read more about this on the players page (link). The Animal and Plant Health Inspection Service evaluates the safety of Roundup Ready crops. Specifically, they regulate the introduction of genetically engineered organisms that may be plant pests. One must apply for a biotechnology permit in order to market genetically engineered organisms. If an organism is considered a plant pest, it is given a regulated status, and cannot be sold in the US. Anyone can submit a petition to deregulate a plant pest.

Roundup herbicide research shows plant, soil problems

KANSAS CITY, Missouri (Reuters) - The heavy use of Monsanto’s Roundup herbicide appears to be causing harmful changes in soil and potentially hindering yields of the genetically modified crops that farmers are cultivating, a government scientist said on Friday.

Repeated use of the chemical glyphosate, the key ingredient in Roundup herbicide, impacts the root structure of plants, and 15 years of research indicates that the chemical could be causing fungal root disease, said Bob Kremer, a microbiologist with the U.S. Department of Agriculture’s Agricultural Research Service.

Roundup is the world’s best-selling herbicide and its use has increased as Monsanto, the world’s biggest seed company, continues to roll out herbicide-tolerant “Roundup Ready” crops.

Roundup Ready corn, soybeans and other crops are beloved by farmers because farmers can spray the herbicide directly onto their crops to kill surrounding weeds, and Roundup Ready corn and soybeans varieties make up the vast majority of those crops grown in the United States.

But as farmers have increased their use of Roundup Ready crops and Roundup herbicide, problems have started to rise. One of the biggest problems currently is spreading weed resistance to Roundup. But Kremer said the less visible problems below the soil should also be noted and researched more extensively.

Though Kremer said research to date has not shown that glyphosate directly causes fungal diseases that limit crop health and production, but the data suggests that could be the case.

“We’re suggesting that that potential certainly exists,” Kremer said in a presentation to the annual conference of the Organization for Competitive Markets, held Friday in Kansas City.

As well, Kremer said that research shows that these genetically altered crops do not yield more than conventional crops, and nutrient deficiencies tied to the root disease problems is likely a limiting factor.

Kremer said farmers should take heed and consider more crop rotations and tighter monitoring of glyphosate usage.

Kremer is among a group of scientists who have been turning up potential problems with glyphosate. Outside researchers have also raised concerns over the years that glyphosate use may be linked to cancer, miscarriages and other health problems in people and livestock.

Monsanto had no immediate comment on Friday, but has said in the past that glyphosate binds tightly to most types of soil, is not harmful and does not harm the crops.

The company has said that its research shows glyphosate is safe for humans and the environment.

Neither the USDA nor the Environmental Protection Agency, which is reviewing the registration of glyphosate for its safety and effectiveness, have shown interest in further exploring this area of research, Kremer said Friday.

Watch the video: Roundup Herbicide Causes Cancer (August 2022).