Glyphosate Resistant Weeds

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Glyphosate Resistant Weeds are weeds that have the evolved the ability to withstand glyphosate, the active ingredient in Monsanto's herbicide Roundup. The Weed Science Society of America (WSSA) defines weed resistance as "the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type."[1] The evolution of Roundup-resistance in weeds is often linked to the use of genetically engineered Roundup Ready Crops. For example, between 1996 and 2006, the amount of glyphosate applied per planted acre of soybeans in the U.S. increased from less than 0.2 to about 1.2 pounds, a six-fold increase.[2]

"The first GR weed in row crops identified in the US was horseweed [Conyza canadensis (L.) Cronq.], and its appearance was possibly correlated with the cultivation of GR soybeans. Recently other GR weed populations have been reported... All these weeds are major economic problems in agronomic crops in the corn, cotton and soybean growing regions of the US and the distribution of glyphosate resistance in these weeds is increasing. GR horseweed is now wide-spread throughout much the US cropland."[3]

Impact of Glyphosate Resistant Weeds on Herbicide Use

"In general, farmers can respond to resistant weeds on acres planted to HT crops in five ways:
  • "Applying additional herbicide active ingredients,
  • "Increasing herbicide application rates,
  • "Making multiple applications of herbicides previously sprayed only once,
  • "Through greater reliance on tillage for weed control, and
  • "By manual weeding.
"[Between 1996 and 2008], the first three of the above five responses have been by far the most common, and each increases the pounds of herbicides applied on HT [herbicide tolerant] crop acres...
"GR [glyphosate resistant] horseweed, giant ragweed, common waterhemp, and six other weeds are not only driving substantial increases in the use of glyphosate, but also the increased use of more toxic herbicides, including paraquat and 2,4-D, one component of the Vietnam War defoliant, Agent Orange."[1]

Naturally Glyphosate Resistant Weeds

"Several weed species are inherently more resistant to glyphosate than most other weeds... Natural resistance of these species to glyphosate was not an issue until widespread adoption of GR crops, when many of these species became more problematic as they occupied ecological niches vacated by other weed species in fields of GR crops. This process has been termed “weed shifts.""[4] Naturally resistant species include:

  • Asiatic dayflower (Commelina communis L.)
  • Birdsfoot trefoil (Lotus corniculatus L.)
  • Chinese foldwing (Dicliptera chinensis (L.) Juss)
  • Common lambsquarters (Chenopodium album L.), a weed in Roundup Ready Soybeans
  • Field bindweed (Convolvulus arvensis L.)
  • Tropical spiderwort (Commelina benghalensis L.), a weed in Roundup Ready Cotton
  • Velvet leaf (Abutilon theophrasti (L.) Medic ), a weed in Roundup Ready Soybeans

Weeds With Evolved Glyphosate Resistance

The following species with evolved resistance to glyphosate have been documented in the USA:[4][5]

  • 1998: Rigid ryegrass (Lolium rigidum)
  • 2000: Horseweed (Conyza canadensis) in Roundup Ready Soybeans in 2000 and in Roundup Ready Cotton in 2003
  • 2004: Italian ryegrass (Lolium multiflorum)
  • 2004: Common ragweed (Ambrosia artemisiifolia) in Roundup Ready Soybeans
  • 2004: Giant Ragweed (Ambrosia trifida)
  • 2005: Palmer amaranth (Amaranthus palmeri)
  • 2005: Common waterhemp (Amaranthus tuberculatus) in Roundup Ready Soybeans
  • 2007: Hairy fleabane (Conyza bonariensis L.)
  • 2007: Johnsongrass (Sorghum halepense)
  • 2007: Kochia (Kochia scoparia) in 2007
  • 2008: Junglerice (Echinochloa colona)
  • 2010: Goosegrass (Eleusine indica)
  • 2010: Annual Bluegrass (Poa annua)

Additionally, the following species with evolved resistance to glyphosate have been documented in other countries:[5]

  • 1996: Rigid ryegrass (Lolium rigidum) in Australia in 1996; South Africa in 2001; France in 2005; Spain in 2006; and Israel and Italy in 2007
  • 1997: Goosegrass (Eleusine indica) in Malaysia in 1997; Colombia in 2006; and China in 2010
  • 2001: Italian ryegrass (Lolium multiflorum) in Chile in 2001 Brazil in 2003; Spain in 2006; and Argentina in 2007
  • 2003: Buckhorn plantain (Plantago lanceolata) in South Africa in 2003
  • 2003: Hairy fleabane (Conyza bonariensis L.) in South Africa in 2003; Spain in 2004; Brazil and Israel in 2005; Colombia in 2006; and Australia, Portugal, and Greece in 2010
  • 2004: Ragweed Parthenium (Parthenium hysterophorus) in Colombia in 2004
  • 2005: Johnsongrass (Sorghum halepense) in Argentina in 2005
  • 2005: Horseweed (Conyza canadensis) in Brazil in 2005; China and Spain in 2006; Czech Republic in 2007; Canada and Poland in 2010; and Italy in 2011
  • 2005: Wild poinsettia (Euphorbia heterophylla) in Brazil in 2005
  • 2005: Sourgrass (Digitaria insularis) in Paraguay in 2005; Brazil in 2008
  • 2007: Junglerice (Echinochloa colona) in Australia in 2007; Argentina in
  • 2008: Gramilla mansa (Cynodon hirsutus) in Argentina in 2008.
  • 2008: Perennial Ryegrass (Lolium perenne) in Argentina in 2008
  • 2008: Liverseedgrass (Urochloa panicoides) in Australia in 2008
  • 2009: Sumatran Fleabane (Conyza sumatrensis) in Spain in 2009 and Brazil in 2010
  • 2010: Tropical Sprangletop (Leptochloa virgata) in Mexico in 2010
  • 2010: Australian Fingergrass (Chloris truncata) in Australia in 2010
  • 2011: Ripgut Brome (Bromus diandrus) in Australia in 2011
  • 2012: Kochia (Kochia scoparia) in Canada in 2012

Articles and resources

Related SourceWatch articles


  1. 1.0 1.1 Charles Benbrook, "Impacts of Genetically Engineered Crops on Pesticide Use: The First Thirteen Years" and Supplemental Tables, The Organic Center, 2009.
  2. Testimony of Michael Owen, "Are ‘Superweeds’ an Outgrowth of USDA Biotech Policy? (Part I)," Hearing, Committee on Oversight and Government Reform, U.S. House of Representatives, July 28, 2010.
  3. Testimony of Stephen Weller, Professor of Horticulture, Purdue University, "Are ‘Superweeds’ an Outgrowth of USDA Biotech Policy? (Part I)," Hearing, Committee on Oversight and Government Reform, U.S. House of Representatives, July 28, 2010.
  4. 4.0 4.1 Vijay K. Nandula, Krishna N. Reddy, Stephen O. Duke, and Daniel H. Poston, "Glyphosate-Resistant Weeds: Current Status and Future Outlook," USDA ARS, August 2005.
  5. 5.0 5.1 Herbicide Resistant Weeds, Weed Science, Accessed August 9, 2012.

External resources

  • National Weed Summit, National Academies of Science, May 2012.
  • Vijay K. Nandula, Glyphosate Resistance in Crops and Weeds: History, Development, and Management, John Wiley & Sons, May 25, 2010.

2010 House Hearing on Superweeds

External articles




  • Peter Hunt, "Weed Warning on Super Crops," Weekly Times (Australia), October 16, 1996.