Glyphosate resistant weeds

Glyphosate resistant weeds threaten the crop ... and glyphosate's futureGlyphosate resistant weeds, once believed unlikely to occur, are now threatening not only cost-effective ways of controlling weeds, but also sustainable farming


Management and conservation of the natural resource base and the use of technological and organizational change in a manner that ensures continued agricultural production from the land for present and future generations. Such practices conserve land, water, and plant and animal genetic resources. They are environmentally non-degrading, technically appropriate, economically viable, and socially acceptable. Sustainability rests on the principle that we must meet the needs of the present without compromising the ability of future generations to meet their own needs. Therefore, stewardship of both natural and human resources is of prime importance.

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systems. Glyphosate has been called a ‘once-in-a-century’ herbicide because of its unique combination of high efficacy and low environmental impact1. It has followed paraquat as a major enabling factor driving the expansion of no-till


Also known as conservation tillage or zero tillage is a way of growing crops from year to year without disturbing the soil through tillage ie cultivating the soil usually with tractor-drawn implements.

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farming, replacing the need to plow.
No-till reduces soil erosion


Displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement.

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and increases the health and fertility of the soil. It provides habitats for pest predators and havens for wildlife. Less fuel is required to grow a crop and greenhouse gas emissions are reduced.
However, farmers who use glyphosate too often should heed the warnings of both weed resistance experts and fellow farmers who have experienced resistance problems. Australian Professor Steve Powles, director of the Western Australia Herbicide Resistance Initiative and one of the global leaders in weed science, has warned: “… glyphosate will be driven to redundancy in large parts of North America and South America, unless growers diversify weed control now”2. Paraquat, glyphosate and glufosinate have been the only commercially successful non-selective


A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds).

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herbicides in the whole history of the chemical crop protection industry. Glyphosate is the only one to control perennial weeds


Weeds that return year after year. Some die back in the winter but their roots remain alive and shoots reappear in spring; some don't die back and grow in size and stature the next season.

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The International Weed Science Society represents individual associations around the world.
. So it is essential to food security to ensure that glyphosate remains an effective option for the world’s farmers. This means understanding and overcoming the problem of glyphosate resistant weeds.
This article covers the state of the problem, understanding how it has arisen, and solutions to the problem - including the use of paraquat with its distinctive mode of action. Relevant articles posted on the Paraquat Information Center, illustrating how paraquat is being used to fight glyphosate resistant weeds, are listed for further information.

State of the problem

Introduced in 1974, glyphosate was free from weed resistance issues until 19953. The first recorded case of resistance was in Lolium rigidum (annual ryegrass) in a field in Australia where glyphosate had been used repeatedly over 15 years. Figure 1. Annual data on confirmed numbers of cases of glyphosate resistant weedsSoon afterwards, two events led to a huge increase in sales of what was already the biggest selling herbicide in the world by far. GM glyphosate tolerant crops were introduced in 1996 and glyphosate’s US patent expired in 2000. Since then, glyphosate resistant weeds have evolved in glyphosate tolerant crops including soybeans, cotton, canola and corn where glyphosate had often been the only herbicide used. The annual numbers of confirmed cases of glyphosate resistance are shown in Fig 1.3 The International Survey of Herbicide Resistant Weeds collates information on the subject and recognised 32 different species with glyphosate resistant biotypes in late 2015. The distribution of these around the world is presented in the following tables.

Table 1. Glyphosate resistant weed species: North America

Weed Species Common name Country
Amaranthus palmeri Palmer amaranth USA
Amaranthus spinosus Spiny amaranth USA
Amaranthus tuberculatus Common waterhemp USA
Ambrosia artemisiifolia Common ragweed USA, Canada
Ambrosia trifida Giant ragweed USA, Canada
Conyza bonariensis Hairy fleabane USA
Conyza canadiensis  Horseweed USA, Canada
Echinocloa colona Jungle rice USA
Eleusine indica Goosegrass  USA
Kochia scoparia Kochia USA, Canada
Lolium muliflorum  Italian ryegrass USA
Lolium rigidum Rigid ryegrass USA
Parthenium hysterophorus Ragweed parthenium USA
Poa annua  Annual bluegrass USA
Sorghum halepense Johnsongrass USA

Table 2. Glyphosate resistant weed species: Central and South America

Weed Species Common name Country
Amaranthus hybridus (quitensis) Mucronate pigweeed Argentina
Amaranthus palmeri
Palmer amaranth Brazil
Bidens pilosa Hairy beggarticks Mexico
Chlorus elata Tall windmill grass Brazil
Conyza bonariensis Hairy fleabane Brazil, Colombia
Conyza canadiensis Horseweed Brazil
Conyza sumatrensis Sumatran fleabane Brazil
Cynodon hirsutus Gramilla mansa Argentina
Digitaria insularis  Sourgrass Brazil, Paraquay
Echinocloa colona Jungle rice Argentina, Venezuala
Eleusine indica  Goosegrass Argentina, Bolivia, Colombia, Costa Rica
Leptochloa virgata Tropical sprangletop Mexico
Lolium muliflorum Italian ryegrass Argentina, Brazil, Chile
Lolium perenne Perennial ryegrass  Argentina
Parthenium hysterophorus Ragweed parthenium Colombia
Sorghum halepense Johnsongrass  Argentina

 Table 3. Glyphosate resistant weed species: Europe and Middle East

Weed Species  Common name Country
Conyza bonariensis Hairy fleabane Spain, Greece, Israel, Portugal
Conyza canadiensis Horseweed Spain, Czech, Poland, Italy
Conyza sumatrensis Sumatran fleabane Spain, Greece, France
Lolium multiflorum  Italian ryegrass Spain, Italy
Lolium perenne Perennial ryegrass Portugal
Lolium rigidum  Rigid ryegrass France, Spain, Israel, Italy

 Table 4. Glyphosate resistant weed species: Asia

Weed Species Common name Country
Conyza canadiensis Horseweed China, Japan
Eleusine indica
Goosegrass China, Malaysia, Japan
Hedyotis verticillata Woody borrereria Malaysia
Lolium multiflorum Italian ryegrass Japan

Table 5. Glyphosate resistant weed species: Australasia

Weed Species  Common name Country
Brachiaria eruciformis Sweet summer grass Australia
Bromus diandrus Ripgut brome Australia
Bromus rubens Red brome Australia
Chloris truncata Australian fingergrass Australia
Conyza bonariensis Hairy fleabane Australia
Echinocloa colona Jungle rice Australia
Lolium multiflorum Italian ryegrass New Zealand
Lolium perenne Perennial ryegrass New Zealand
Lolium rigidum Rigid ryegrass Australia
Raphanus raphanistrum Wild radish Australia
Sonchus oleraceus Annual sowtistle Australia
Urochloa panicoides Liverseed grass Australia

 Table 6. Glyphosate resistant weed species: Africa

Weed Species  Common name Country
Conyza bonariensis Hairy fleabane S. Africa
Lolium rigidum Rigid ryegrass  S.Africa
Plantago lanceolata Buckhorn plantain S.Africa

Why have glyphosate resistant weeds arisen?

To find solutions to the problem of glyphosate resistant weeds, the biochemistry of resistance and the ways in which resistant biotypes are selected need to be appreciated.

Biochemistry of resistance

Weeds can become resistant to herbicides by three common mechanisms4:
  • Target site mutations: the active site on the enzyme inhibited by the herbicide does not allow a good fit
  • Translocation and/or sequestration mutations: the herbicide does not reach the site of action
  • Enhanced metabolism mutations: the herbicide is metabolized before it can exert its effects
Currently, examples of resistance to glyphosate by the first two means are known, but not the third. Resistance mechanisms identified in particular species are noted in Table 7.

Table 7. Glyphosate resistance mechanisms identified in resistant weed species4.

Target site mutations Translocation and/or sequestration mutations
Eleusine indica Conyza bonariensis
Lolium rigidum Conyza canadensis
Lolium multiflorum Lolium rigidum
Amaranthus palmeri
Amaranthus tuberculatus   
Glyphosate blocks the biosynthesis of aromatic amino acids (phenylalanine, tryptophan, tyrosine) by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Knowledge gained about the genetics and biochemistry of this enzyme in the development of GM glyphosate tolerant crops has been applied to understanding resistant weeds. A common target site mutation in EPSPS is the substitution of the amino acid proline for serine at a key position in the molecule. This changes the shape of active site so that glyphosate can no longer bind effectively4. The normal catalytic function of EPSPS is unaffected, so that plants with this mutation are as ‘fit’ as the rest of the population. Glyphosate is readily translocated from leaves to growing points of shoots, roots and rhizomes. This is fundamental to its ability to control perennial weeds. Most studies on resistant weeds showing reduced translocation have demonstrated that in these biotypes glyphosate is rapidly sequestered in cell vacuoles and unavailable for translocation4. A few others have indicated that glyphosate is poorly absorbed into the leaves of resistant plants4. One other type of glyphosate resistance has been investigated, although the mechanism is unknown. In some glyphosate resistant populations of Ambrosia trifida, the leaves of treated plants rapidly desiccate and fall off, so limiting the amount translocated4.

Selection for resistant biotypes

Glyphosate had been widely used for 20 years before the first case of resistance was recorded. Given that resistance to herbicides such as sulfonylureas and ACC-ase inhibitors was soon widespread within a few years of their commercialisation, any mutated genes for resistance to glyphosate were thought to be very rare and individual plants possessing them likely to be poor competitors. In addition, having a unique mode of action and no soil residual activity means that the selection pressure from any one application is low and only applied to the cohort of weeds present at application. Later emerging flushes are unaffected. Before the introduction of GM glyphosate tolerant crops, glyphosate was generally only used as one part of a diverse weed management system involving other control methods: mechanical, cultural and a wide variety of selective herbicides. Using different methods ensures that any weeds surviving glyphosate are killed by other means5. However, in situations where glyphosate is used several times in one crop, and to the exclusion of other means of weed control, the selection pressure is much greater. An illustration on what has happened in glyphosate tolerant cotton in the US is presented as a case study.  

Case study: Glyphosate tolerant cotton6, 7, 8

GM glyphosate tolerant cotton has been grown in the US since 1997. Selective herbicides in cotton all had various issues: needing rain for activation; narrow spectra of weed control; or could damage the crop. Glyphosate was far superior and within 5 years 90% of fields were being planted with glyphosate tolerant varieties. Surveys have indicated cost advantages running to hundreds of millions of dollars per annum. Lower overall costs of weed control, eg labor, herbicides, machinery and fuel, have been the major contributors to the economic benefits from the technology, although less tangible benefits of simplicity, flexibility and freeing-up of labor and management time for other purposes have also been very important in its popularity. Unfortunately, over-simplification of weed control has resulted in the evolution of glyphosate resistant weeds. In cotton, resistant populations of horseweed (Conyza canadensis) were first recorded in Tennessee in 2001, followed by giant ragweed (Artemisia trifida) in 2007, while resistant Palmer amaranth first appeared in Georgia cotton fields in 2005 and within a few seasons was infesting millions of acres of cotton and soybean crops in that state alone.

Solutions: How paraquat helps

Glyphosate resistant weeds have come to dominate certain populations because of intensive use of glyphosate for weed control. Including a diversity of approaches to weed control is essential to avoiding the shift to resistant biotypes. An integrated system is best, taking advantage of opportunities to rotate crops and use herbicides with different modes of action, as mixtures or sequential applications. Cultural techniques such as the use of cover crops


Cover crops are primarily planted not to be harvested for food but to reduce soil erosion, control weeds and improve soil quality. They are usually plowed or tilled under before the next food crop is planted, in which cases the "cover crop" is used as a soil amendment and is synonymous with "green manure crop."

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can also help9.
Researchers in Australia used a computer model to calculate that after 20 years’ annual use of glyphosate in an arable rotation, more than one field in three with an annual ryegrass problem would be expected to have a weed population resistant to glyphosate10. Figure 2. Calculated probability of glyphosate resistant ryegrass evolvingFig 2 shows that the simulation calculated that if the herbicide used in land preparation was alternated between glyphosate and paraquat, only one field in five would be expected to have resistance after 30 years, compared to nearly 90% of fields sprayed only with glyphosate.10 However, the most successful approach would be to use the ‘Double Knock’ regime with paraquat cleaning-up after glyphosate. This was predicted to keep all fields free of glyphosate resistant ryegrass for at least 30 years. Some of the many ways in which paraquat is helping to fight glyphosate resistance can be discovered in the articles listed below:       More modes of action needed to support glyphosate tolerant crops US researchers have recently reported on a benchmark study designed to show how best management practices to avoid resistance, involving the use of more diversity in herbicide modes of action, can maintain the benefits of GT cropping systems …
        Paraquat wipes out superweeds Applying paraquat through weed wipers can very effectively kill glyphosate resistant Palmer amaranth plants up to 1.50 m tall …       Paraquat and new spray hood fight superweeds An advanced design spray hood enables glyphosate tolerant weeds over 1 m tall to be controlled by paraquat, while shielding late growth-stage cotton. As the sprayer moves over the crop, weeds are guided into the hood and knocked down by a horizontal bar before being sprayed …   Paraquat protects glyphosate in Aussie Double Knock Spraying paraquat in a ‘Double Knock’ system is a very effective way to prevent resistance to glyphosate developing. Survivors of glyphosate burndown are sprayed with paraquat up to two weeks later to hit them with two different modes of action …
  US growers must fight glyphosate resistance The threat posed by glyphosate resistant weeds has been highlighted by the publication of the US National Research Council’s report: Impact of Genetically Engineered Crops on Farm Sustainability in the United States …     Paraquat is glyphosate’s bodyguard In Brazil, glyphosate resistant weeds are estimated to infest three million hectares. To help ensure the benefits of glyphosate can be preserved, farmers continue to spray glyphosate for burndown, but follow just before or just after planting with a paraquat-based herbicide …     Dilemma for Brazilian Farmers Soybean growers in Brazil are facing a dilemma. More land under no-till is saving the soil from erosion and reducing energy use, but many no-till farmers are now finding that glyphosate resistant weeds are threatening future success …   Conservation tillage may collapse under resistant weeds threat Conservation tillage minimizes erosion, provides habitats for beneficial insects and other wildlife, and improves soil structure and fertility. However, many farmers rely too heavily on glyphosate and resistant weeds mean that a return to plowing fields could be the only way to maintain productivity …   Paraquat fights glyphosate resistant Palmer amaranth Palmer amaranth weeds in cotton not controlled by glyphosate can produce an alarming 450,000 seeds, storing-up huge problems for the next season’s crop …  


  1. Duke, S O and Powles, S B (2008). Glyphosate: a once-in-a-century herbicide. Pest Management Science, 64, (4) 319-325
  2. (2010)
  3. International Survey of Herbicide Resistant Weeds
  4. Shaner, D L et al (2012). What have the mechanisms of resistance to glyphosate taught us? Pest Management Science, 68, (1), 3-9
  5. Powles S B (2008). Evolved glyphosate–resistant weeds around the world: lessons to be learnt. Pest Management Science, 64, 360-365
  6. US Department of Agriculture
  7. Green, J M (2012). The benefits of herbicide-resistant crops. Pest Management Science, 68, (10), 1323 - 1331
  8. Robertson, R (2010).  Palmer amaranth super weed.  South East Farm Press, 18 October 2010. 
  9. Beckie, H J (2011). Herbicide-resistant weed management: focus on glyphosate. Pest Management Science, 67, (9),1037-104
  10. Neve, P et al (2003). Simulating evolution of glyphosate resistance. II. Past, present and future use glyphosate use in Australian cropping. Weed Research, 43, 418 – 427