Glyphosate resistant weed problems intensify

Bromus rubens: a new glyphosate resistant weedIn 2015, the problem of glyphosate resistant weeds continued to intensify in many countries, especially in the US. The president of the Weed Science Society of America recently described how glyphosate resistant weeds are spreading from the south into states including Illinois, Missouri, Nebraska and Kansas, where the problem has “exploded”.1 The problem is caused by the heavy exposure of weeds to glyphosate. Solutions lie in using different ways to control weeds, including cultural practices such as crop rotation and using herbicides with different modes of action. Paraquat, with its unique properties, is a good choice as this article explains.

Glyphosate resistant weeds

Many weed species around the world have become resistant to glyphosate. The latest official figures note that 32 different species of grass and broadleaved weeds


The leaves are "broad" as opposed to the "narrow" leaves of grasses. Also called 'dicots' having two seed leaves, while grasses are 'monocots' having one seed leaf.

Authoritative On-line References and Resources The International Weed Science Society.
now have resistant populations on thousands of farms around the world. Weeds species with new populations recorded for the first time in 2014 and 2015 are listed in Table 1.
Table 1. New glyphosate resistant weed species registered in 2014 and 2015 (published by end 2015).2    * = first appearance of this species.
Weed Species  Common name Country
Amaranthus hybridus  Smooth pigweed Argentina
Amaranthus palmeri  Palmer amaranth Brazil, United States
Amaranthus tuberculatus Tall pigweed Canada, United States
Ambrosia artemisiifolia Common ragweed United States
Bidens pilosa* Hairy beggarticks Mexico
Brachiaria eruciformis* Sweet summer grass Australia
Bromus rubens*  Red brome Australia
Chloris elata* Tall windmill grass Brazil
Conyza canadensis Marestail, horseweed Japan, United States
Kochia scoparia Kochia Canada, United States
Lolium multiflorum Italian ryegrass United States
Parthenium hysterophorus Ragweed parthenium United States
Sonchus oleraceus* Annual sowthistle Australia
Sorghum halepense Johnsongrass Argentina

Heavy use of glyphosate

Figure 1. Use of glyphosate in the US 1993-2013.According to the US Geological Survey, 130,000 tonnes of glyphosate were sprayed in the US in 2013 (Fig. 1). This figure compares with about 9,000 tonnes used in 1996, the year in which the first glyphosate tolerant crops were grown. Prior to 1996 corn and soybeans together accounted for about 25% of glyphosate use, but in 2013 this figure had risen to 75%. Over this period, the use of glyphosate increased around twenty-fold in these crops as it has become used, not only for clearing weeds before crops are planted, but also post-emergence without any risk to the GM herbicide tolerant crop. Furthermore, the US Geological Survey has found that glyphosate and its metabolite (i.e. a major product of its degradation), a molecule known as AMPA (aminomethylphosphonic acid), are common in the environment. Results from a survey of nearly 4000 sites in total, across 38 states, conducted between 2001 and 2010 showed that these molecules are often detected in surface waters (59% of 470 sites), but less often in groundwater (8% of 820 sites). Glyphosate readily binds to soil (but not as tightly as paraquat), so the pollution of ditches, streams and lakes is largely because of spray drift.

Paraquat can help

Heavy use of glyphosate is clearly causing problems, yet it is essential to food security because of its broad-spectrum of weed control. It has been called a ‘once in a century herbicide4 and farmers around the world rely on it, especially to control pernicious 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.

Authoritative On-line References and Resources

The International Weed Science Society represents individual associations around the world.
The key to avoiding the development of glyphosate resistant weeds is to limit its use. Paraquat, with its distinctive mode of action, can be used as a burndown spray alone or with various partner herbicides (including those adding a soil residual effect) in glyphosate tolerant crops such as cotton, maize and soybeans. This allows a more limited use of glyphosate, focused on later application to control weeds growing within the glyphosate tolerant crop. The best approaches to weed control include integrating practices such as:
  • Alternating use of herbicides with different modes of action
  • Using crop rotation and 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."

    Authoritative On-line References and Resources ATTRA is the US National Centre for Appropriate Technology's Sustainable Agriculture Information Centre.
    to smother weeds
  • Monitoring and controlling the spread of weeds in from field borders and ditches
An in-depth article on Glyphosate Resistant Weeds can be read in The Paraquat Information Center’s Knowledge Bank. This has been updated to cover recent developments.


  1. Reuters, 28 September 2015. Herbicide scrutiny mounts as resistant weeds spread in US
  2. International Survey of Herbicide Resistance Weeds
  3. US Geological Survey website. Glyphosate use in US by year and crop
  4. Duke, S O and Powles, S B (2008). Glyphosate: a once-in-a-century herbicide. Pest Management Science, 64, (4) 319-325


The brand name of the leading paraquat product is Gramoxone.