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Increased planting of glyphosate-resistant crops and application of glyphosate causes increase in glyphosate-resistant weed species.

[For an update (2014) on weed resistance to glyphosate, see Increasing Weed Resistance to the Herbicide Glyphosate.]

Inserted Transgene and Intended Effect of Genetic Modification: CP4 EPSPS gene derived from the common soil bacterium Agrobacterium sp., Strain CP4, to convey resistance to the herbicide glyphosate. Glyphosate-resistant crops - especially soybeans, cotton, corn, canola - are the most prevalent genetically modified crops and are grown on millions of acres of cropland in the U.S. and other countries. Ninety percent of soybeans grown in the U.S. in 2007 were genetically modified glyphosate-resistant varieties.

Results of This Study: The wide spectrum herbicide glyphosate has been sold by the company Monsanto under the name Roundup since 1974. In 1996 the first glyphosate-resistant weeds were reported and in the decade since then - which coincides with the increased cultivation of genetically modified, glyphosate-resistant plants - ten different glyphosate-resistant weed species have been found in fields planted with glyphosate-resistant crops. These include common waterhemp (Amaranthus rudis), common ragweed (Ambrosia artemisiifolia), and horseweed (Conyza canadensis). The latter has evolved increased resistance to glyphosate since resistant populations were first detected in fields planted with Roundup Ready soybeans in Delaware in 2000. Since then, resistant populations have been found in fourteen other states as well as in Brazil and China.

In addition to these weed species that have evolved resistance in relation to glyphosate use, at least thirteen other species of weeds that have natural resistance to glyphosate are now being observed in fields grown with glyphosate-resistant crops in the U.S., Brazil, and Argentina. These include common lambsquarters (Chenopodium album), velvetleaf (Abutilon theophrasti), and species of morning glory (Ipomeoa).

In a greenhouse experiment, researchers found that glyphosate-resistant horseweed could form hybrids with its nonresistant close relative, dwarf horseweed (Conyza ramosissima), which is also a common weed. The hybrids were fertile, had superior resistance to glyphosate, and the resistance was inherited as a semi-dominant trait (Zelaya et al. 2007). "Glyphosate-resistant crop systems are suggested to be simple and without great environmental consequences. However, we have demonstrated that there are major ecological and economic consequences from these presumed simple systems. We propose that if hybridization of new taxa with glyphosate resistance as a semi-dominant trait can occur with relative ease, the current agroecosystem is at considerable jeopardy" (Zelaya et al. 2007, p. 669).

Additional Comments: Nandula et al. conclude: "High levels of adoption of GR [glyphosate-resistant] crops by U.S. farmers have dramatically increased the use of glyphosate, with a concomitant decrease in use of other herbicides. This has impacted weed communities. The problem of GR weeds is real, and farmers have to understand that continuous use of glyphosate without alternative strategies will likely result in the evolution of more GR weeds. Even in the short term, no one can predict the future loss of glyphosate efficacy due to weed species shifts and evolution of glyphosate resistance."

  • Cadeira, A. L. and S. O. Duke (2006). "The Current Status and Environmental Impacts of Glyphosate-Resistant Crops: A Review," Journal of Environmental Quality vol. 35, pp. 1633-58.
  • Nandula,V. K., K. N. Reddy, S. O. Duke, and D.H. Poston (2005). "Glyphosate-Resistant Weeds: Current Status and Future Outlook," Outlooks in Pest Management vol. 16, pp. 183-7.
  • Owen, M. D. K. and I. A. Zelaya (2005). "Herbicide-Resistant Crops and Weed Resistance to Herbicides," Pest Management Science vol. 61, pp. 301-11.
  • Zelaya, I. A., M. D. K. Owen, and M. J. VanGessel (2007). "Transfer of Glyphosate Resistance: Evidence of Hybridization in Conyza (Asteraceae)," American Journal of Botany vol. 94, pp. 660-73.
  • This website tracks and documents herbicide-resistant weed species.

Author Affiliations:
  • Cerdeira and Duke: Brazilian Department of Agriculture; USDA Agricultural Research Service.
  • Nandula et al.: Delta Research Extension Center, Mississippi State University; USDA Agricultural Research Service.
  • Owen and Zelaya: Iowa State University.
  • Zelaya et al.: Iowa State University; University of Delaware.

Funding: Not mentioned; for and its funding, see the website.

Product Status: Glyphosate-resistant crops have been commercially grown on millions of acres of farmland since the late 1990s.

Copyright 2008 The Nature Institute.

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