Charles J. Arntzen
Arizona State University
This summit grew out of a 2010 National Research Council report on the impact of genetically engineered crops on U.S. farm sustainability. A variety of topics were addressed in the report, and one of the findings was that weeds resistant to glyphosate were an emerging problem. Since the time the report was published, glyphosate resistance in weeds has more than “emerged”—it is now a significant problem.
Let me provide some background on this topic from my own personal perspective. When my dad was planting corn in the 1940s in western Minnesota, he used the Check Planting Method. You would have a quarter mile of heavy wire, which you would stake at either end of the field. A planter would go over it, and every time the planter hit a little button on this wire, the hopper would open and release a few seeds that would grow and make a hill of corn. It took my dad two days or more to plant a plot of land that today would take an hour and a half with a modern corn planter, but the end result for my dad was a beautiful corn field that looked similar to a checker board—all the rows were in perfect alignment. He followed this system because at the time there were no options for controlling weeds except cultivation. This planting method allowed him to cultivate both North-South and East-West from May through early July to optimally remove all weeds from the field. It was labor and time intensive. Along with my dad, I operated the tractor for many hours in many spring seasons, cultivating.
In the 1950s, the triazine herbicides were developed.1 Atrazine was particularly significant for corn. On our farm, my father was an early adopter of the new technology. He would spray triazine herbicides at the time of planting, and the herbicide would prevent weeds from emerging. This saved time and cost since manual cultivation was no longer needed. Controlling weeds with herbicides also freed me up from time spent sitting on a tractor, so I could get a summer job in town and earn money for college.
1Appendix D includes a table of herbicide classifications and mechanisms of actions for herbicides discussed in the proceedings.
I did not spend much time thinking about herbicides after I left the farm until 20 years later, when I was on the faculty of the University of Illinois, Urbana-Champaign. At the time, I was doing basic scientific research on chloroplast development, photosynthesis, and how electron transfer occurs in chloroplasts. Fred Slife, who was the “dean” of the weed science community in Illinois, asked me if I had heard about triazine-resistant weeds and told me I should take a look at them. Triazines were known to kill plants by blocking photosynthesis. But fields had been discovered which were full of pigweed that survived multiple applications of atrazine. When my fellow researchers and I looked at isolated chloroplasts from the resistant pigweed, we found that they could not bind atrazine and, as a result, the herbicide would not stop the electron transport that supports all photosynthesis. This led to cloning the gene for the triazine-binding protein from the resistant pigweed to show that there was a single base substitution in this one gene, and that this site controlled the binding of atrazine to its receptor. This type of mutation was also seen in lamb’s quarters and other weeds.
In the 1980s, I ended up working on this issue at DuPont, studying the genetic changes that could create resistance to the acetolactate synthase (ALS) class of herbicide. The idea was that if exposure to herbicides could trigger weeds’ selection-pressure response2 and thereby change their receptors to herbicides to block binding and herbicide action, the same change could be made in crops using biotechnology approaches. During this time, DuPont and Monsanto both were working on developing crops that combined superior crop-protection chemistry with superior genetics. It was an exciting time in plant biotechnology. Monsanto benefited from its earlier chemical screening program in which a molecule that resembles an amino acid was found to be toxic to virtually all plants. This was called glyphosate. A very attractive feature of this herbicide was that it did not get into the groundwater because it bound to the clays in the soil. Also, bacteria could degrade it in the soil, and it was nonselective. Glyphosate would kill all plants, including crops, unless the latter were made herbicide resistant through genetic engineering. Monsanto introduced Roundup Ready® soybeans, which were resistant to glyphosate, in 1996, followed the next year by cotton and canola, and corn the year after that.
If I could have asked my dad what he thought about the development of glyphosate-resistant crops, he very likely would have said it was a no-brainer. Glyphosate is environmentally friendly, it gives the farmer greater flexibility in the timing of herbicide application, and its use means farmers do not have to mix different chemicals. My dad would have adopted glyphosate-resistant crops. Tens of thousands of farmers made that decision.
The purpose of this summit is to ask “What happens when tens of thousands of farmers use the same form of weed control over millions of acres?” We are not going to deliver any silver-bullet solutions today at this summit, but we hope to emphasize issues of what we do next where herbicide-resistant weeds have emerged, and what we do in the parts of the country where the problem is not yet full blown.
2Selection pressure is the accumulation of numerous or single factors in an environment which differentiate the fitness of organisms that occupy the environment. In the case of weeds, the herbicide is the primary factor that differentiates for a specific trait, that is, resistance to the herbicide. Weed biotypes with the trait for resistance to a particular herbicide or herbicides will be favored in an environment where the herbicide is used and will become the dominate biotype.