Sustainable Agriculture Literature Review - Boulder County

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Glyphosate Resistance
Glyphosate, first commercialized in 1974, has been extensively used for weed control. 599
The first case of evolved resistance to glyphosate was reported in 1996 in rigid ryegrass
(Lolium rigidum) 600 and approximately 18 species have now evolved resistance (Graph
22). 601 According to the National Academy of Sciences, gene flow between herbicide
resistant crops and closely related weed species does not explain the evolution of
glyphosate resistance in U.S. fields because sexually compatible weeds are absent where
corn, cotton, and soybean are grown in the United States. Instead, a number of studies
have shown that the near exclusive use of glyphosate for weed control, a practice
accelerated by the widespread introduction of glyphosate-resistant crop varieties, has
caused substantial changes in weed communities. 602
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Graph 22: Number of Weeds with Evolved Glyphosate Resistance 603
(*As of March 2010)
The number of weed species evolving resistance to glyphosate is growing and the number
of locations with glyphosate-resistant weeds is increasing at a greater rate as more and
more acreage is sprayed with glyphosate. Though the number of weeds with resistance to
glyphosate is still small compared to other common herbicides, the shift toward
glyphosate-resistant weed biotypes will probably become an even more important
component of row-crop agriculture unless production practices (such as recurrent use of
glyphosate) change dramatically. 604, 605, 606 The evolution of glyphosate resistance and
weed shifts are estimated to lead to two important changes in agricultural practices, first a
general increase in the use of herbicides, and secondly reductions in conservation
tillage. 607 These changes are estimated to increase weed-management costs and
potentially reduce producers’ profits. 608
Insect Resistance
Similar to weeds, insects can adapt to toxins and other tactics used to control them.
When Bt crops were first considered for commercial introduction, the U.S. Environmental
Protection Agency (EPA) recognized the potential for rapid evolution of insect resistance
to Bt toxins produced by GE crops as a threat to the benefits provided by Bt crops and to
611, 612
the efficacy of Bt sprays in organic and conventional production systems alike.
Extensive monitoring of eleven major lepidopteran pests of corn and cotton over the last
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609, 610
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