The evolution of herbicide resistant weeds is not a new issue. But the problem has come to the forefront recently with wide-spread use of glyphosate and more and more problems with weeds resistant to the herbicide. A national summit was held in May to identify strategies to address resistance where it has emerged and identify steps to act proactively to preempt further evolution.
More than 250 policymakers, producers, herbicide manufacturers, social scientists, agronomists and weed scientists participated in the summit which was organized by the National Academy of Sciences at The George Washington University in Washington, D.C.
Charles Arntzen, Arizona State University, chair of the planning committee, explained the summit grew out of a 2010 report by the National Academy of Science and National Research Council that indicated weed resistance was a growing problem and that more research was needed.
Not just glyphosate
"We need to focus on all herbicides, not just glyphosate and not just cotton in the South. Resistance has been around a long time – for 50 years," stated Mike Owen, Iowa State University professor of agronomy.
"All herbicides are at risk. It's not a glyphosate problem, a triazine problem or an ALS problem. It's a behavioral problem -- the way we use these herbicides. Our production systems have allowed a few weed species to adapt."
Weeds are there because they have adapted to the agri-ecosystem. "Even a few escapes can increase substantially over two or three years and the next generation will be even more resistant. Seed dormancy is key – once a weed is established, it will only get worse. Herbicides make it possible to farm more acres but it creates a Darwinian selection process."
Owen believes the solution is diversity – use multiple modes of action and manage each field separately. A return to tillage and/or cultivation is a possibility.
"Resistance is not new and not isolated to one product," added John Soteres, scientific global affairs weed resistance management lead at Monsanto Company and current chairman of the Global Herbicide Resistance Action Committee. "But we do have more knowledge today to manage it and a greater need to diversify products."
Soteres said some companies offer incentives to encourage diversification of herbicide use including rebates and formulation of lower-cost premixes which offer multiple modes of action, reduced handling and a higher certainty of correct application rate.
"The industry, over all, in addition to incentives, is providing more options to manage resistance, including new chemistry, use of biotech and funding for basic and applied research," continued Soteres.
"Implementing innovative farmer outreach programs is another. Flying Midwest farmers to the Delta to see the impact of weed resistance in that region is an example.
"We are making progress with the combined efforts of industry and the public sector but the challenges are significant," said Soteres. "Some farmers are evaluating and using other weed control management options such as pre-plant tillage and cover crops."
"Australia has had claim to the title of No. 1 in the world in weed resistance," noted Michael Walsh, University of Western Australia. "The U.S. is closing in fast on that title and weed resistance is a threat to the global food supply. In Australia and the U.S., it's universally accepted that overuse of herbicides has resulted in weed resistance."
In the southern tip of Australia the problem started in 1970 when sheep and wool prices dropped drastically and farmers shifted from sheep production to crop production. When sheep were king, lolium (rye grass) had been seeded and nurtured as a forage crop. But growers switched to wheat with no tillage, no crop rotation, no diversity of herbicides and low rates of application.
Targeting seeds at harvest
Lolium developed resistance to herbicides. The Australians were eventually forced into other forms of weed control including harvesting crop residue with weeds and removing them from the field (weed seeds are spread across the field with the chaff). "Ours is a unique approach to develop wide spread tools to control weeds," says Walsh. By collecting the chaff at harvest they were able to remove about 85% of the Lolium seed from the field. They also used a baling system which collected 95% of the seeds.
A widely adopted practice was "narrow windrow burning." Chaff is concentrated in a narrow windrow behind the combine and then burned – 95% of weed seeds are destroyed. Walsh is leading research on development of the Harrington seed destructor which is used to crush the weed seeds in the chaff.
Other things that worked included a diversity of herbicide rotation, mandatory mode of action labeling, full dose herbicide mixtures and what they call the "double-knock" strategy. That's a knock-down herbicide followed by another non-selective herbicide, possibly with tillage. "Herbicides are pivotal but will not be used alone."
He also advises farmers in the U.S. measure (count) weed seeds at harvest to know how many there are and consider what the next steps might be. "In Australia, change didn't come about until growers had no other choice." Plus, he noted, the resistance problem and laws enacted to control the problem played a major reduction in number of farmers, especially younger farmers who don't want to deal with the issues.