Weed resistance a global threat

The evolution of herbicide-resistant weeds is not a new issue. But the problem has come to the forefront with widespread use of glyphosate and more difficulties with weeds resistant to the herbicide. A national summit in May identified strategies to address herbicide-resistant weeds where they have emerged and identify steps to act proactively (use best management practices) to preempt further evolution.

About 250 policymakers, farmers, herbicide manufacturers, social scientists, agronomists and weed scientists participated in the summit organized by the National Academy of Sciences in Washington. Charles Arntzen, chairman of the planning committee and an Arizona State University scientist, said the summit grew out of a 2010 report by the National Academy and the National Research Council citing weed resistance as a growing problem and that more research is needed.

A basic question is what happens when thousands of farmers use the same weed control? Many weeds, particularly in the South, have developed resistance. “There is currently no new mode of action in the pipeline to replace glyphosate,” he said.

“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 Extension weed scientist. “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.”

The weeds are there because they’ve adapted to the agri-ecosystem, he said. “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. That is, use multiple modes of action and manage each field separately. A return to tillage and/or cultivation is a possibility. He also says there hasn’t been adequate research to develop new solutions.

“Resistance isn’t new, and it’s not isolated to one product,” added John Soteres, global affairs weed resistance management lead at Monsanto Co. and 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.”

Some companies offer incentives to encourage diversification of herbicide use, including rebates and formulation of lower-cost premixes that have multiple modes of action, reduced handling and a higher certainty of correct application rate.

“The industry, in addition to incentives, is providing more options to manage resistance, including new chemistry, use of biotech, and funding for basic and applied research,” said Soteres. “Using innovative farmer outreach programs is another. Flying Midwest farmers to the Delta to see the impact of weed resistance in that region of the United States is an example.” He added, “We are making progress with the combined efforts of industry and the public sector, but the challenges are significant. Some farmers are evaluating and using other weed control management options such as preplant tillage and cover crops.”

Resistance Down Under

Michael Walsh, a University of Western Australia weed scientist, noted that “Australia has had claim to the title of No. 1 in the world in weed resistance. The United States is closing in fast on that title, and weed resistance is a threat to the global food supply. In Australia and the United States, it’s universally accepted that overuse of herbicides has resulted in weed resistance.”

Beginning in the 1970s when sheep prices were low, Australian farmers plowed up pastures and planted wheat. When sheep were king, lolium (ryegrass) 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. 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. 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 that collected 95% of the seeds.

A widely used practice was “narrow windrow burning.” Chaff is concentrated in a narrow windrow behind the combine and burned, with 95% of the weed seeds destroyed. Walsh is leading research on developing the Harrington seed destructor, which is used to crush the weed seeds in the chaff.

Other successes were a diverse herbicide rotation, mandatory mode-of-action labeling, full-dose herbicide mixtures and a “double-knock” strategy. That’s a knock-down herbicide followed by another non-selective herbicide, possibly with tillage. Herbicides are important but will not be used alone. “Resistance can be managed,” summed up Walsh, “but growers have to recognize that herbicides alone are not the solution. A diversity of weed control practices is needed for sustainability.”


This article published in the July, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.