Superweed is a misleading term used to describe weeds that are resistant to an herbicide. It is conceptually possible that certain, herbicide-resistant GMOs could contribute to an increase in herbicide-resistant weeds; however, such concerns are not limited to GMOs, but are relevant for any crops used in conjunction with herbicides.
What is a superweed?
First of all, what is a superweed? According to the Weed Science Society of America (WSAA), there is “no science-based definition for superweed,” but the term is often used to describe herbicide-resistant weeds. Just as overuse of one particular antibiotic promotes the spread of bacteria resistant to that antibiotic, overuse of an herbicide can result in herbicide-resistant weeds. According to Rick Boydston of the USDA-ARS:
Resistance is a natural phenomenon which occurs spontaneously in weed populations, but is only noticed when a selection pressure is applied to the weeds via herbicide application
Boydston further explains that herbicide resistant plants within a given population of weeds are rare: “1 in 100,000 to 1 in 1,000,000”. Treating this population with an herbicide provides selective pressure, so that only the few resistant plants naturally present in the population go on to produce the next generation, thus passing on the resistance trait.
Weed scientists like Dr. Andrew Kniss find use of the term superweed to describe herbicide-resistance “exceptionally frustrating” because it “indicates that the weed is super in some way”. However, despite claims that “Some runaway weeds in the southern U.S. are said to be big enough to stop combines dead in their tracks,” herbicide-resistant weeds are not any bigger or nastier than their non-resistant counterparts.
What causes herbicide resistance?
Anti-GMO activist groups like Food Integrity Now call herbicide-resistance “a direct result of growing GM crops”. Where do these claims come from?
There are two ways that specifically herbicide-tolerant GMOs could hypothetically contribute to a rise in herbicide-resistant weeds. It is important to note that not all GMOs are herbicide tolerant. The following points apply only to crops that have been genetically modified to tolerate the herbicide glyphosate. It has been proposed that herbicide-tolerant GMOs could contribute directly to the rise in herbicide-resistant weeds through cross-pollination with closely related weed species. This explanation is embodied in the oxford dictionary definition of a “superweed”:
A weed which is extremely resistant to herbicides, especially one created by the transfer of genes from genetically modified crops into wild plants
However, According to the WSAA, “The transfer of resistance traits from genetically modified crops to weeds growing in the field is rare.” And according to Dr. Brad Hanson, Cooperative extension specialist at UC Davis, “to date no herbicide-resistant weeds in corn, cotton, or soybean production regions appear to have become resistant due to traits moving from the crop.”
If GMOs did cross with weeds and pass on herbicide-tolerant traits, this problem would not be unique to GMOs, but could also result from crops that have been conventionally bred for herbicide resistance. For example, it is documented that imidazolinone-resistant jointed goat-grass arose from a cross-pollination event with wheat bred to tolerate imidazolinone.
Pesticide use and GMOs
Herbicide-tolerant GMOs could also contribute to the rise in herbicide-tolerant weeds indirectly by encouraging an increase in the use of glyphosate. Glyphosate use has increased with the planting of GE crops, and numbers of glyphosate resistant species are highest for crops for which an herbicide-tolerant GE variety exists.
However, not all cases of glyphosate resistance have been identified in fields where herbicide tolerant GMOs are grown. To determine if glyphosate-resistant weed species identified for crops that are not genetically engineered for herbicide tolerance arose first in fields where herbicide resistant GE crops were planted, Dr. Andrew Kniss compiled data from weedscience.org and plotted new cases of glyphosate-resistant weeds that initiated in GM-crop sites versus those that originated in non-GM crop sites.
Remarkably, cases of glyphosate resistant weeds are no more likely to be documented in association with GM fields than non-GM fields. This is likely because glyphosate is also used in conjunction with crops that are not genetically modified for herbicide tolerance. For example, glyphosate is used to control weeds between rows in orchards and to encourage mature wheat to dessicate so it is ready to be harvested more quickly. If GMOs drastically contribute to the rise of herbicide tolerant weeds, we would expect to see a spike in herbicide tolerance corresponding with the introduction of GMOs; however, this is not the case.
Herbicides cause “superweeds”
In conclusion, herbicide-tolerance can arise from many sources. Current data do not seem to definitively point to GMOs as a significant cause for the rise in herbicide-tolerance. As Dr. Andrew Kniss puts it:
GM crops don’t select for herbicide resistant weeds; herbicides do. Herbicide resistant weed development is not a GMO problem, it is a herbicide problem.
The European Academies Science Advisory Council agrees:
Cultivating a GM crop variety with increased herbicide resistance, for example, may prove detrimental to the environment if the farmer over-uses that herbicide. But the same would be true of herbicide resistance introduced by conventional breeding.
Despite a lack of clear evidence that GMOs contribute to the rise in herbicide-resistance, many critics cite the development of “superweeds” as a reason for advocating a moratorium on GMOs. Chipotle cites as one of their reasons for removing GMOs from their menu that “GMOs engineered to…withstand powerful chemical herbicides…create herbicide resistant super-weeds.” Chipotle’s solution to the “super-weed” problem is to switch from using oil from soybeans genetically modified for herbicide resistance, to oil from sunflowers that are “naturally” resistant to the herbicide atrazine. As Dan Charles of NPR points out, use of herbicides in this system has led to far more resistance problems. This graph compares numbers of species resistant to the herbicide glyphosate (ESPS Synthase Inhibitors shown in light blue) with species resistant to herbicides used on sunflowers (ALS inhibitors shown in red).
Thus, prohibiting the use of herbicide-tolerant GMOs does not eliminate problems of herbicide resistance. Furthermore, according to the USDA, planting herbicide-tolerant crops “makes it easier to manage weeds using less tillage, which can help reduce soil erosion as well as improve soil quality and water conservation”
How to prevent superweeds
So if banning GMOs will not prevent the development of more herbicide-resistant weeds, what will? According to the USDA:
Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank.
The list of best management practices for managing herbicide-resistance includes “cultural practices that suppress weeds by using crop competitiveness,” following herbicide label instructions, and stacking herbicides with multiple mechanisms of action (MOA). Dr. Stephen Weller, professor of horticulture at Purdue, compares multiple modes of action for weed control with putting more than one lock on the same door: “The thief, in this case the weed, might manage to get by one of the locks, but if you have several, as in several modes of action, it is much harder. If you can control a weed with two or three mechanisms of action, the likelihood of resistance occurring to all the mechanisms used is greatly reduced.”
The USDA has also noted that “Economic Incentives May Encourage Grower Cooperation in Managing Resistance”.
Helpful resources on GMOs and superweeds:
Where the Superpowers of Superweeds Comes From. James Schnable, May 14, 2010
WSSA FACT SHEET: Dispelling Common Misconceptions about Superweeds
USDA: Managing Glyphosate Resistance May Sustain Its Efficacy and Increase Long-Term Returns to Corn and Soybean Production. Jorge Fernandez-Cornejo and Craig Osteen, May 4, 2015
What does Chipotle’s switch to non-GMO ingredients mean for pesticide use? Andrew Kniss, May 18, 2015
Where are the super weeds? Andrew Kniss, May 1, 2013
What are superweeds? & Do GMOs cause superweeds? Genetic Literacy Project, Accessed April 27, 2016
Escaping The Bench 
This is a conversation I often find myself having. I am often confused as to the link people draw between the use of GM crops and herbicide resistance. Some really useful data here to help, thanks for sharing!
Helen
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