Corn Wars

An article by Dan Charles for The Salt on Bt resistance in the corn belt actually does a good job of identifying the problem as an issue created by poor crop and pesticide management rather than blaming biotech. The insinuation is the title, but not the reporting. The comment section is a battle of bumperstickers vs. balance.

In May, Andrew Kniss asked the world to stop using the term “Superweed” and brought some perspective to the role of biotech in herbicide resistance.

Most of the time, the term superweed is associated in some way with herbicide resistance. So if we define superweed as a weed that has evolved resistance to herbicides, we can then test the hypothesis that “GM crops have bred superweeds.” (ASIDE: The way this statement is phrased, there’s no way it can possibly be true, because crops don’t “breed” weeds. There are some rare cases where crops and weeds cross pollinate, but those have not resulted in any herbicide resistant weeds to date. For the sake of argument, we’ll assume Ms. Gilbert really meant “GM crops have significantly increased the development of superweeds.”) Dr. Ian Heap has developed and maintained a website to document new cases of herbicide resistant weeds, and we can use the data at that site to get an idea of whether this statement is true or false using our definition of superweed.

If GM crops have contributed significantly to the development of herbicide resistant weeds, we would expect the number of unique instances of these superweeds to increase following adoption of GM crops. The figure below illustrates all unique cases of herbicide resistant weeds between 1986 and 2012. I have fit a linear regression to the data from 1986 to 1996 (time period before widespread GM crop adoption) and another regression to the time period 1997 to 2012.


The slope of the linear regression is an estimate of the number of new herbicide resistant weeds documented each year. In the eleven year period before GM crops were widely grown, approximately 13 new cases of herbicide resistance were documented annually. After GM crop adoption began in earnest, the number of new herbicide resistant weeds DECREASED to 11.4 cases per year. The difference in slopes between these two time periods is probably not very meaningful from a practical standpoint. But based on the best data available, we can be quite certain that adoption of GM crops has NOT caused an increase in development of superweeds compared to other uses of herbicides.

Perhaps this definition of superweed is too broad. Let’s define it instead as only “glyphosate-resistant” weeds. The first glyphosate-resistant weed was documented in 1996. This is approximately the same time GM crops were first being introduced into the market. But this first superweed evolved in Australia, where no GM crops were grown. So it is obvious that GM crops are not necessary for glyphosate-resistant superweeds to develop. Certainly, adoption of Roundup Ready crops (the dominant GM herbicide resistance trait) has increased the use of glyphosate in cropland, and therefore increased selection pressure for glyphosate-resistant weed populations. But even so, there are currently more instances of glyphosate-resistant weeds in non-GM crops/sites than in GM crops. The following chart (from illustrates the number of cases of glyphosate-resistant weed species in various crops/sites.


The only 3 GM crops on the chart are soybean, corn, and cotton. All of the other bars represent non-GM systems. If we add up the number of herbicide resistant species in GM crops and compare it to non-GM crops/sites, we should expect GM crops to have a higher number if GM crops are the primary contributor to evolution of superweeds. However:

  • 35 species of glyphosate-resistant weeds are present in GM crops (soybean, corn, cotton).
  • 40 species of glyphosate-resistant weeds are present in non-GM crops/sites (orchards, grapes, roadsides, wheat, fencelines, fruit, barley).

So again, there appears to be no strong difference between GM crops and other sites where glyphosate is used. So this data again suggest that GM crops are not any more problematic than other uses of glyphosate for selection of superweeds.


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About Marc Brazeau

Free lance cultural attaché. Writing at REALFOOD.ORG.

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