Auburn researchers battle herbicide-resistant weeds to save farmers and their land
Think of the dustbowl that occurred in the American West in the 1930s or the boll weevil that devastated cotton production in the Southeast and increased production of the peanut – these challenges changed the face of American agriculture forever.
Today the challenge is herbicide-resistant weeds.
Auburn University Professor Scott McElroy says the primary problem is resistance to glyphosate, better known as Roundup, which has been an excellent product to kill weeds since the 1970s and has greatly enhanced the conservation practice of no-till farming, but that is in jeopardy if a solution can't be found.
"U.S. row-crop agriculture has survived on chemical solutions – herbicides, insecticides, fungicides, seed treatments, growth regulators – to combat problems," said McElroy, professor of weed science in Auburn's College of Agriculture. "The solution normally has been to use another chemical, but this thinking is coming to an end."
He says researchers and crop producers realize now that solutions have to be more holistic in nature, integrating multiple strategies to combat weed problems, which are literally driving some farmers out of business.
Finding out how and why weeds develop resistance is a goal of McElroy along with fellow Auburn researchers, Extension specialists and USDA scientists, as well as scientists at the University of Arkansas, Clemson University and the University of Georgia. The information will help them create and implement a solution for farmers.
"This is like bacterial resistance in human medicine when you keep using the same antibiotic, and then the bacteria develop resistance," he said. "Weeds are developing a resistance to the chemicals in herbicides, and this could wipe out the use of dozens of herbicides, not just Roundup."
The problem has risen dramatically since 2000, when genetically modified, Roundup-ready crops were introduced, including corn, cotton and soybeans. These plants were genetically modified to tolerate Roundup so only the weeds would be killed.
Because it worked so well and was cost effective, farmers began exclusively using Roundup, which is applied to kill weeds before planting and as the crop develops. Prior to 2000, farmers would use different herbicides with different chemical makeups at these various stages of crop development.
McElroy says areas with large, contiguous tracts of cropland have experienced huge problems, such as Georgia, Arkansas and west Tennessee. The state of Alabama is in much better shape than neighboring states because cropland is more dispersed throughout the state, which reduces the spread of seeds from herbicide-resistant weeds by pollen and birds.
"Southeast Alabama has a problem with Roundup-resistant Palmer amaranth weeds, but the Tennessee Valley is not as affected yet," McElroy said. "We are being proactive in battling it before it becomes a major problem in the state. But now common ragweed has developed resistance in a small area of Madison County in the Tennessee Valley."
In the southeastern United States, 20 species of weeds have become resistant to the chemicals in Roundup, the most recent being horseweed, various pigweeds, common ragweed, Johnson grass and goosegrass. "This is putting some farmers out of business in parts of Georgia because they can't grow crops and cotton," he said. "This is changing the landscape of row crops. Farmers have to change to other aspects of land-use."
In addition to Roundup being affected, other herbicides that kill weeds in different ways are also less effective.
"In the past, if a weed became resistant to a herbicide, producers could just switch to another herbicide that kills the weed in a different way," he said. "But in some cases, weeds are resistant to those herbicides, too."
Adding to the financial problem for farmers is that the cost of weed management has increased at a time when commodity prices are low. "Producers have to spend additional money on weed management at a time when they can ill afford it," he said.
Tracking the problem
"Why are weed populations becoming resistant? Why do they have genetic adaptations? The 'why' will dictate future management practices," said McElroy, who is studying weeds through targeted gene sequencing in his laboratory in Auburn's Center for Advanced Science, Innovation and Commerce, or CASIC, building.
"When we find a mutation, this adds to our information to help us manage the problem going forward and hopefully give us the next option for weed control," he said. "By understanding the underlying genetics and physiology of resistance we can better improve our weed management recommendations."
McElroy is doing greenhouse testing as well, but it takes three to six months to see if herbicide resistance occurs, whereas genetic testing takes only a few days to extract the DNA and see if a mutation occurs and to know if certain herbicides will be ineffective.
"To confirm a resistant population, scientists must collect plants from a treated field, propagate the plants in a greenhouse, collect seed, allow the seed to mature, germinate the plants and compare the resistant weed response to a known susceptible of the same species – applied at the same time, at the exact same growth stage," he said.
It is a time-consuming process that takes months to coordinate and it does not return a solution to producers in time for them to make a decision the next year. Doing this for every suspected weed population is just not possible, he says.
The quicker genetic testing is the key to the future.
"To kill the weed, we need to know its genetics and physiology, its likes and dislikes, the effect of maximum long-term use of herbicides," McElroy said. "At what point are they resistant? How tolerant are they and to what level?
"A weed is a non-model organism about which we typically know very little. Model organisms, such as crops, are ones in which biologists study the genetics and physiology. Historically, there has been less funding for weed management."
McElroy is primarily testing the weeds' resistance against the largest class of herbicides, called ALS inhibitors. ALS, or acetolactate synthase, is a protein found in plants that is key in the production of amino acids. The herbicides block ALS activity, thus starving the plant of amino acids and killing the weed.
"We will take the information from our genetic sequencing, establish a baseline of information and track that population of weeds," he said. "If we test a weed from another field and find the same mutation, we will know the resistant weeds have spread and that ALS-inhibiting herbicides will not work in that area."
McElroy is quick to point out the collaborative team effort of researchers and extension specialist at Auburn dedicated to solving this problem.
"We have assembled a team of people to combat this problem," he said. "Dr. Andrew Price at the USDA Soil Dynamics Lab focuses on non-chemical management strategies. Newly hired Extension specialists Dr. Steve Li and Dr. Joyce Ducar focus on developing immediate solutions for growers, and I am working on monitoring, screening and genetic sequencing of populations as they arise within the state."
Making the research count
McElroy relates how research can be implemented to fend off problems, using the example of horseweed, or marestail, which was one of the first species resistant to Roundup. It first occurred in Delaware in early 2000s and quickly spread across the country.
"Horseweed must be controlled before planting or it will greatly reduce yields," he said. "Traditionally, producers use Roundup prior to planting to kill horseweed. Roundup has no soil residual effects so producers can essentially plant after spraying."
When horseweed became resistant to Roundup, producers had to find an additional solution. They began using a combination of Roundup and an older herbicide called dicamba, which was a good solution because it had little soil residual effects, so producers could plant a month later.
In 2011, though, Ducar, then director of the Sand Mountain Experiment Station, and Charlie Burmester, Extension specialist at the Tennessee Valley Research and Education Center, began receiving complaints that the mixture of Roundup and dicamba was no longer working.
They and McElroy knew this was a major problem because there are very few other herbicides that could be applied to kill horseweed before planting and not damage the crop due to residual soil effects.
"Many producers were planning to switch to more risky weed control scenarios due to this problem," McElroy said.
McElroy and others soon discovered it was a two-fold problem, rather than a new form of horseweed resistant to both Roundup and dicamba. First, the horseweed is now even more resistant to Roundup, requiring more than 50 times the normal rate of Roundup to kill it. Second, horseweed plants were growing faster throughout the winter and, by the time treatments are applied in the spring, plants were simply too big for dicamba to have an effect.
"Our field specialists have been able to identify other chemicals to combat this new form of horseweed, but we know this is only a short-term solution," he said. "If history tells us anything, weeds will eventually figure out a way to tolerate any chemical we spray on them. Both non-chemical and chemical treatments are going to have to be developed that work in tandem for more sustainable weed management."
Finding the right solution
McElroy and fellow scientists continue to monitor herbicide-resistant populations, gather results and build a database of knowledge, they will take the information and pass it along to herbicide manufacturers to help in their formulations and they will inform farmers on how to control the weeds.
"Chemical distributors are very supportive of our work because this directly affects their business," McElroy said. "Not only that, but they have a strong relationship with the farmers and are concerned about their situation.
"The wrong solution is for the farmer to spray another herbicide that has a similar chemistry or to keep spraying the same product. The solution must be multifaceted."
The solution should include nonchemical aspects, McElroy says, like planting cover crops such as clover during winter, which will help manage soil erosion and maintain soil fertility and quality. "This is one step that can be done now by farmers as we continue to research the herbicide-resistant weeds," he said.
"Right now, the complete answer lies within the weeds themselves and in using research to find the key pieces of genetic information," McElroy said. "Weeds will continue to adapt to human management. The how and why of weed adaptation will help us in developing more sustainable practices."
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