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
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
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).
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
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,
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.
Cerdeira and Duke: Brazilian Department of Agriculture; USDA Agricultural
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.
Not mentioned; for weedscience.org and its funding, see the website.
Glyphosate-resistant crops have been commercially grown on millions of
acres of farmland since the late 1990s.
Copyright 2008 The Nature