This summer, Arkansas put a 120-day ban on the powerful weed killer dicamba after the chemical pitted neighboring farmers against one another. Produced by the agricultural corporation Monsanto, dicamba is meant to be sprayed on fields of soybeans that are genetically modified to resist the herbicide (the special soybean seeds are, of course, also a Monsanto product): the crop survives and the weeds die. Or at least that’s how it was intended to work. Unfortunately, many farmers who hadn’t purchased the herbicide/resistant-seed duo complained that dicamba from neighbors’ fields had drifted onto theirs and damaged their harvest — hence, the temporary ban to give scientists and the company time figure out if the problem was poor spraying technique or the nature of dicamba itself. Dicamba isn’t a new herbicide, but crop seed genetically modified to survive it is — and the extensive use of dicamba that comes with that innovation seems to point to problems with this herbicide. Why are farmers and agricultural companies moving to new weed killer that might be causing unacceptable levels of collateral damage?
Where's the evolution?
Dicamba is meant to replace Monsanto’s previous herbicide/genetically modified seed system that was based on the weed killer glyphosate, also known as Roundup. Monsanto’s Roundup Ready corn, soy, and cotton are genetically modified to survive Roundup spraying and are widely planted — but this ubiquity has had a downside. The more Roundup Ready crops are planted, the more Roundup can be sprayed, and the more strongly natural selection favors any weed that happens to carry a mutation that allows it to resist the effects of Roundup. Since 1998, scientists have discovered at least 24 different weed species in which glyphosate resistance has evolved! The more common these resistant strains become (and they are becoming more common), the less useful Roundup and Roundup Ready seeds are. Monsanto’s push to develop and sell dicamba-resistant crops is a direct response to the evolution of Roundup resistance.
So how did Roundup resistance arise? The evolutionary story is fairly straightforward. Populations of weeds are genetically variable — that is, they contain many different versions of their genes accumulated through generation after generation of random mutation. Many of the gene versions in a population at any given time are rare and likely have little particularly positive or negative effect for the individual carrying them, at least in usual circumstances. However, when the environment changes (for example, by being flooded with glyphosate), some of those variants may reveal their hidden utility, if they, for example, change the shape of a key protein just enough that it cannot bind a herbicide molecule. In the new environmental conditions (e.g., where glyphosate is present), individuals that do not carry the resistant version die and do not pass on the gene versions that they carry to the next generation; whereas, individuals that happen to carry a gene version that allows them to resist the herbicide survive and are able to reproduce, passing on the resistant gene version. Through this process, over the course of generations, gene versions that resist the effects of a herbicide can become common in a weed population, rendering the herbicide ineffective.
Roundup works by blocking a key enzyme (called EPSPS) that plants use to produce amino acids critical for survival. This challenge has been overcome by many different weed species through the same basic process of natural selection. However, the way that the resistant weeds have done this (i.e., the specific mutations and resulting physiological changes that allow them resist Roundup) varies from species to species. For example, some ryegrass strains have mutations that cause glyphosate to be isolated in cells’ vacuoles, much as hazardous waste is isolated in storage barrels, preventing it from doing much harm to the plant. Roundup resistant ragweed, on the other hand, survives by responding to Roundup with the rapid death of all mature leaves — within 12 hours of spraying. This decreases the amount of the herbicide that gets transported to other plant tissues, and the plant is able to re-leaf and survive. And some resistant strains of Palmer’s pigweed (a kind of amaranth) have lots of extra copies of the EPSPS gene, which allow them to make more of the enzyme, which means that the plant has some working EPSPS molecules even as others are blocked by Roundup. Natural selection simply favors gene versions that perform better — and in the case of resisting Roundup, a wide variety of different mutations and physiological mechanisms have proved useful for the task and have been favored in different weed lineages.
The story of the rise and fall of Roundup may contain some lessons for the future. While it took 15 years for glyphosate-resistant weed strains to evolve and another 15 or so for them to become common enough to reduce the effectiveness of this herbicide, it did happen. Weeds are plentiful and have a short generation time — two characteristics that produce large amounts of genetic variation. In this case, the power of random mutation ultimately overcame a powerful herbicide. Does the same fate await another powerful herbicide, dicamba? Possibly. Dicamba has been in use for about 50 years, and so far only one dicamba-resistant weed is at all common. On the other hand, the advent of crops genetically modified to resist dicamba will likely make use of this herbicide much more common — providing stronger selection favoring resistant strains. And if dicamba is indeed drifting onto neighboring fields (as has been charged in Arkansas), the odds tip even further in favor of resistant genes.
News update, June 2019
In the fall of 2017, we reported on a surge in the use of the weed killer dicamba, as soybeans genetically modified to survive the herbicide began to be marketed. Of course, widespread application of dicamba is a powerful selective force favoring the evolution of resistant weeds. And now, we have evidence that this evolution can occur in Palmer amaranth (Amaranthus palmeri), an aggressive weed that competes with soybeans for resources and can reduce a field’s yield by nearly 80%. In 2019, scientists from Kansas State University reported the discovery of a Palmer amaranth population resistant to dicamba, as well as another herbicide. Among plants grown from the seeds of this resistant population, 81% survived dicamba application at the levels recommended for weed control! While this trait is not yet widespread among weeds infesting soybean fields, it does suggest that, like Roundup, the useful life of dicamba is limited unless steps are taken to slow the evolution of resistance.
- Sammons, R. D., and Gaines, T. A. (2014). Glyphosate resistance: state of knowledge. Pest Management Science. 70: 1367-1377. Read it »
- A newspaper article on the Arkansas ban from the New York Times
- A 2010 article on the rise of Roundup-resistant weeds from the New York Times
Understanding Evolution resources:
- A tutorial on the process of natural selection
- A review of DNA and mutations
- An overview of applications of evolution in agriculture
Background information from Understanding Global Change:
- In your own words, describe the role of genetic variation in the process of natural selection.
- Review the process of natural selection. Use the four steps described on that page to explain how Roundup resistance could become common among a population of weeds growing near Roundup Ready soybean fields.
- Research and describe another example of the evolution of herbicide resistance by a weed population.
- In your own words, explain why different populations of weeds have evolved different means of resisting the effects of Roundup.
- Roundup and dicamba can also be used without genetically modified crops. What role does genetic modification play in the evolution of herbicide resistant weeds? Explain your answer.
- Teach about natural selection: In this classroom activity for grades 9-16, students simulate breeding bunnies to show the impact that genetics can have on the evolution of a population of organisms.
- Teach about rapid evolution resulting from human-caused changes in the environment: This news brief for grades 9-12 describes how global warming has already begun to affect the evolution of several species on Earth.
- Teach about genetically modified crops: This news brief for grades 9-12 describes approaches to slow the evolution of resistance to pesticides produced by genetically modified crops.
- Funke, T., Han, H., Healy-Fried, M. L., Fischer, M., and Schönbrunn, E. (2006). Molecular basis for the herbicide resistance of Roundup Ready crops. Proceedings of the National Academy of Sciences USA. 103: 13010-13015.
- Hakim, D. (Sept 21, 2017). Monsanto's weed killer, dicamba, divides farmers. The New York Times. Retrieved October 13, 2017 from https://www.nytimes.com/2017/09/21/business/monsanto-dicamba-weed-killer.html?mcubz=1&_r=0
- Kansas State University. (2019). Palmer amaranth that resists 2,4-D and dicamba confirmed in Kansas. Successful Farming. Retrieved May 30, 2019 from https://www.agriculture.com/crops/pesticides/palmer-amaranth-that-resists-24-d-and-dicamba-confirmed-in-kansas
- Sprague, C. (2013). Palmer Amaranth Management in Soybeans. Retrieved May 30, 2019 from https://weedscience.missouri.edu/publications/50737_FINAL_FactSheet_PalmerAmaranth.pdf