Fighting Herbicide Resistant Weeds in the Midwest
By Ebeth Glickson
For something so pernicious, it’s rather delicate looking. A pest to agricultural crops, Giant Ragweed is one of the most competitive weeds plaguing the American Midwest. Standing flimsily at about 6 feet, it looks like any other nondescript crop weed. However, Giant Ragweed is unlike other common weeds, as it is considered a ‘superweed’. In other words, it has evolved the ability to withstand herbicide application.
The Giant Ragweed, commonly known as a “superweed,” is one of 14 herbicide resistant weeds. Photo by Elaine Haug, USDA-NRCS PLANTS Database.
Every year, when corn and soybean seeds are planted across roughly 19 million acres in Minnesota, herbicide is applied to kill weeds. The most well-recognized and widely used herbicide is glyphosate, otherwise known by Monsanto’s brand name, Roundup. While Roundup is valued for its wide-ranging weed killing abilities, its appeal has lessened as superweeds have begun to take hold.
Giant Ragweed isn’t the only herbicide resistant weed; there are currently 14 resistant weed strains affecting US crop production. Resistant strains of weeds first popped up in 1998, about two years after Roundup crops came to market. Initially, resistant weeds were mainly prevalent in the Southeastern states. Today however, these weeds are endemic. The International Survey of Herbicide Resistant Weeds shows the only states remaining without superweeds are Nevada and Alaska.
The chemistry of developing a herbicide is tricky: the herbicide must attack the weed, but leave the crop unharmed. Considering crop and weed are both plants, they tend to be genetically similar. Monsanto’s solution to this conundrum is genetically modified crops.
Glyphosate eliminates a plant by inhibiting an enzyme that is necessary for plant growth. Roundup crops have been altered to overexpress this enzyme, so even when glyphosate is applied there is enough enzyme left uninhibited that growth may continue as normal. Through random mutation, however, a weed can develop resistance to glyphosate. For instance, some weeds have mutated an enzyme in such a way that glyphosate is prevented from attaching and inhibiting.
When Roundup was first introduced to the market in 1996, there was little apprehension about the dangers of herbicide-resistant weeds. A number of scientists pledged their support for the product. As one weed scientist wrote in a Monsanto petition to the USDA, weed resistance usually occurs “as a result of continuous use of single herbicide for a number of years and glyphosate has not been used in this manner.” Perhaps it was unforeseen that Roundup would be used so widely, almost to the exclusion of any other methods of weed control.
Once introduced, Roundup was adopted rapidly because it offered clear advantages over other herbicides. For one, Roundup made it easier to manage weeds without tillage. (Tillage essentially amounts to moving the soil around, which beyond suppressing weeds, can help to prevent soil compaction that can impede root growth. However, tillage is time-intensive.) In addition, Roundup was considered safer than other herbicides. In studies conducted by the National Research Council, glyphosate was shown to be less toxic and less persistent in the environment.
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A lot of rhetoric around superweeds presents them as a source of a coming apocalypse. As Just Label It, a GMO Labeling organization, puts it, herbicide usage “leads to a dangerous, toxic dead end, one that will leave the landscape infested with ever more varieties of resistant superweeds.” Is this scenario realistic? Will superweeds truly bring conventional agriculture to its’ knees? Dan Hernandez, a biology professor at Carleton College, is skeptical of catastrophic scenarios involving superweed infestation. “There are selective pressures on those plants, and this might make them really hardy or it might make them really susceptible to other things.” In short, by evolving to withstand herbicide, they might also weaken their defense against drought or frost. Jeff Gonsolus, a Professor of Weed Science at the University of Minnesota, also seemed doubtful of the risk these weeds present. “There is nothing super about these weeds.” Gonsolus said. “They adapted to their new environment, just like any other plant.”
If these weeds aren’t all that threatening, then how will we control them? For some time, academia has been trumpeting the use of “diversified weed management” to reduce the risk of herbicide resistant weeds. This means using a variety of tools beyond glyphosate to beat weeds: pre-emergent herbicides, tillage, and mechanical cultivation to name a few. Tillage and mechanical cultivation essentially amount to disturbing the soil, while pre-emergent herbicide is herbicide applied prior to crop or weed emergence. Diversified weed management is really nothing new, it’s akin to weed control schemes used before Roundup was so abundant.
Why hasn’t diversified weed management been widely adopted then? Mostly, it’s a matter of scale. Andrew Ehrmann, a farmer who manages a CSA in Northfield, MN, depends on 6 acres of vegetables to support his family. His weed management is an idealized form of diversified weed management. His weed control methods include cover crop rotation, mechanical cultivation and hand weeding. Ehrmann’s weed management scheme is practical for 6 acres, but becomes impossible when managing hundreds or thousands of acres. For the conventional farmer, diversified weed management is not only complicated by farm size, but conflicts with other concerns, like topsoil. As Mike Peterson, a conventional farmer in Northfield, MN, explained: “Our goal is to dramatically reduce erosion, so the mechanical cultivation on the ground that I run would create more erosion, and I’d rather not farm that way.”
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Jared Goplen, a graduate student at the University of Minnesota, has been researching alternative, non-chemical forms of weed control. His work is mainly focused on Giant Ragweed, due to its prevalence in Southern Minnesota. Before the extensive use of herbicides, crop rotation was a recognized effective method of weed control. Goplen has been investigating the interactive effects of crop rotation and strategic herbicide use, hoping to pinpoint which crop rotation method works best. Goplen and his team examined six crop rotation systems, all of which included a mix of corn, soybeans, wheat, and alfalfa. They discovered that the total emergence of Giant Ragweed was reduced by an average of 38% when wheat and alfalfa were included in the rotation.
Goplen also investigated seed retention of Giant Ragweed, given that seed bank depletion is one of the most effective methods to reduce weed infestation. (The seed bank refers to the weed seeds in the soil; no one is planting weeds of course, but their seeds linger for years. An old adage dictates that for every weed gone to seed, comes seven years of weeding. ) Thus, depleting the seed bank makes a considerable difference. By setting up collection traps over two harvest seasons, Goplen measured the quantity of seeds produced during the growing season and the level of seed retention after crop harvest. The results were encouraging; the weeds retained 80% of total seeds during harvest season. This means farmers can remove Giant Ragweed while harvesting their crops, making management of the weed relatively simple, but perhaps costly. As a farmer drives a combine through a field to harvest their crop, the machine separates grain from waste. If the farmer purchases and attaches a Harrington Seed Destructor to the combine, the waste can be pulverized before being spit out onto the field, thus abolishing all seeds.
As the research continues, Goplen hopes to develop a safe and cost-effective model that will incorporate a variety of weed control methods. As Goplen admits, “You know alfalfa works really well to control Giant Ragweed, but can you make money doing it?” In the near term, no weed control scheme is as cost-effective and easy to use as Roundup.
That being said, farmers do understand that weed management will have to evolve. Between online chat lines, herbicide representatives, promotional sales meeting and educational meetings, farmers are getting the same message from all avenues. Besides, farmers are accustomed to acquiescing Mother Nature. “Mother nature, she’s gonna beat you,” Peterson said. “So I think it’s more of a time factor, we all knew it was coming.”
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A simpler solution to beating superweeds would be the introduction of a new herbicide. Of course, that would depend upon the discovery of a new ‘mode of action’ or a new biochemical site for a herbicide to attack. To date, only 22 ‘modes of actions’ for herbicide inhibition have been discovered. Are there more modes of action to be found? The answer is unclear. Maybe only 22 modes of action exist. Or there might be other sites, but they may not meet proper human safety and environmental standards. As seed companies have consolidated, less money and attention has been directed to discovery programs. Without more research, there is no conclusive answer.
While Monsanto hasn’t discovered any new modes of action, it has been developing a new herbicide called Xtend. This product combines on an older herbicide dicamba, with glyphosate. Will Xtend only replace one problem with another? Monsanto certainly hopes this won’t be the case. The cost of bringing a new product to market is considerable, meaning Monsanto is hoping Xtend will last for decades to come. “Growers will continue to get the message preached at them, of not relying solely on the new technology that’s coming at them,” Ross Recker, a weed representative from Monsanto, said. “Otherwise its effectiveness could be lost as well.” Even if weeds developed resistance to dicamba, Xtend would still be valuable. “You never have one weed out in your field,” Goplen said. “You have 10 or 12, so you have to think about controlling all of them, and not just one.”
Beyond Xtend, Monsanto has been working to combat superweeds more directly with RNAi. RNAi is a new form of technology, a part of epigenetics, which is a field so new that not much is known. RNAi is a natural biological process, a way in which the body regulates what becomes a protein. In order to not get bogged down in scientific terminology, it’s best to imagine you are a teacher who wants to bring a book to your class. This book can’t be removed from the library, so you photocopy it. The immobile book is the DNA, or genetic material, and the photocopy represents RNA. Before you can get to class, another teacher stops you and examines your material. He is the RNAi. He decides this book is not appropriate, and destroys your photocopies. In your body, this means RNA won’t become protein. If RNAi can be directed to target the correct RNA, or book, you can prevent the translation of certain genes. Ideally, Monsanto will be able to direct RNAi to destroy the genes that confer resistance to weeds.
In the future, technology may provide a way to supplement herbicides. Drones could pinpoint weed-infested areas, providing for surgical removal of weeds. Robots could drive through crop rows, identifying and cutting down weeds on the spot.
Ultimately, superweeds are not the end of conventional agriculture as we know it. Superweeds are true nuisance for the farmer, as a time-intensive opponent and thief of profits. Perhaps our expectation that weeds simply cease to exist is a little naïve. Yes, weeds may continue to adapt, but so can an informed farming community employing the best technology available.
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