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Posted: March 2014

Increasing Weed Resistance to the Herbicide Glyphosate
Craig Holdrege

In the 1970s farmers began spraying the herbicide glyphosate to kill weeds. Glyphosate, which was developed by Monsanto Company and is sold under the name Roundup, proved to be an effective broad spectrum weed killer. Any time such a poison finds widespread use, the likelihood that the target organisms will develop resistance grows, since organisms adapt to the changing environment (Délye et al. 2013).

In two decades of moderate use, no weeds were detected that had become resistant to glyphosate. During this time, Monsanto scientists developed the first glyphosate-resistant genetically modified (GM) crops—corn, soybeans, cotton, and canola. These crops do not die when they are sprayed with Roundup. In a 1997 article—one year after the first commercial planting of these crops—Monsanto scientists stated that because in 20 years’ use of glyphosate no weed resistance had been found, “it is reasonable to expect that the probability of glyphosate-resistant weeds evolving will not increase significantly over that considered with current use” (Bradshaw et al. 1997). They stated that broader use of glyphosate with the advent of glyphosate-resistant crops would be a means to “reduce the potential for further spread of herbicide resistance” (by killing those weeds that had become resistant to other herbicides).

Nothing could have been further from reality. Weed resistance to glyphosate has risen dramatically since 2000, when farmers and weed scientists in the U.S. began noticing that some weeds in fields planted with glyphosate-resistant crops were becoming resistant to glyphosate—they didn’t die when sprayed. This problem has increased, and a total of 28 species of weeds show resistance to glyphosate (Heap 2014). The resistant weeds can be found in 23 different countries (fourteen species are resistant in the U.S., mainly in fields with GM crops).

The evolution of weed resistance was furthered by the widespread planting of glyphosate-resistant crops on millions of acres of farmland (mainly the commodity crops: corn, soybeans, and cotton). While 15 million pounds of glyphosate were sprayed on corn, cotton and soybeans in 1996, in 2012 159 million pounds were sprayed—a 10-fold increase (Food & Water Watch, 2013; based on USDA/NASS data).

Before herbicide-resistant crops were available, “weed control required a higher level of skill and knowledge” (Mortensen et al. 2012). Then farmers were offered one simple method to control weeds—spraying glyphosate—and they began planting the same crops year after year on the same fields. The monocultures of industrial agriculture became more and more pronounced.

Farmers are therefore facing a huge problem and, as an expert on herbicide resistance recently warned, “U.S. farmers are heading for a crisis” (cited in Service 2013).

Confronted with weed resistance, farmers first began spraying more glyphosate and then started to use additional herbicides to try to kill the resistant weeds. Farmer expenses for herbicides have risen significantly; for example, farmers who used to pay $25 per hectare for herbicides are now paying $160 per hectare (cited in Service 2013; see also Food & Water Watch, 2013).

But this is no long-term solution, since already some glyphosate-resistant weeds have become resistant to additional herbicides, making them even more difficult to kill (Heap 2014). The problems will only get worse; they resemble those the medical profession faces with infectious bacteria that are resistant to multiple antibiotics.

The biotech industry’s answer to this problem is more of the same. They have created new GM plants with resistance to multiple herbicides. They would like to bring more onto the market, and crops resistant to the herbicides 2,4-D (Dow Chemical Co.) and dicamba (Monsanto) are currently being reviewed by the USDA in connection with the potential negative environmental impact ( It is hard to imagine how scientists at Monsanto and other biotech companies can keep a straight face when representing their new technologies as the solution to problems that their previous technology of the same kind helped create. Crops with multiple resistances will only encourage the spraying of more and multiple herbicides and further the evolution of more weed resistance, not to mention the environmental and health impacts that accompany increased herbicide use and industrial-scale monocultures. “This trend would move us in the opposite direction of the reduced chemical inputs that scientist in sustainable agriculture have long advocated” (Mortensen et al. 2012).


Bradshaw, L. D. et. al. (1997). “Perspectives on Glyphosate Resistance,” Weed Technology, vol. 11, pp. 189-98.

Délye, C. et al. (2013). “Deciphering the Evolution of Herbicide Resistance in Weeds,” Trends in Genetics vol. 29, no. 11, pp. 649-58.

Food & Water Watch (2013). “Superweeds: How Biotech Crops Bolster the Pesticide Industry,” Food and Water Watch. Available online:

Heap, I. (2014). “The International Survey of Herbicide Resistant Weeds” (Mar.11). Available online:

Mortensen, D. A. et al. (2012). “Navigating a Critical Juncture for Sustainable Weed Management,” BioScience vol. 62, pp. 75-84.

Service, R. (2013). “What Happens When Weed Killers Stop Killing?” Science vol. 341 (Sept. 20), p. 1329.

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