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Weed resistance to herbicides: a disturbing trend

Conference participants learn about the development of herbicide-resistant weeds throughout the Americas and discuss how to maximize the long-term sustainability of herbicides.

Agricultural producers are facing the serious issue of escalating weed resistance to herbicides. The numbers of resistant weed species, locations affected, and herbicide modes of action involved are all on the rise. Compounding the problem, some weeds have developed resistance to multiple modes of action, making their control that much more difficult, if not impossible.

There is an urgent need to develop and use tools and methods that will make the herbicides that are currently available more sustainable over the long term. For this reason, close to 300 participants from 18 countries met for a three-day conference in Miami in late-January 2010 to discuss weed resistance issues. The meeting was hosted by Bayer CropScience and attended by representatives from industry, universities, governments, and the media.

Dr. Mike Owen, University of Iowa, works in the US Midwest corn/soybean belt, where the majority of acres are planted to glyphosate-tolerant crops and there is high pressure for the selection of glyphosate-resistant weeds. Photos courtesy of Bayer CropScience

Presentations of weed resistance cases from Canada, the US, Mexico, Brazil, Argentina and other areas drove home the fact that weed resistance is an increasing problem. Resistant weeds are present in all major crops, e.g., corn, soybeans, rice, cotton, cereals, orchards and vines, and for virtually all classes of herbicide. Despite this, “the growers’ perception is that the resistance threat is low,” said Dr. Mike Owen, agronomist and weed extension specialist at Iowa State University. “It is not the herbicides, but the way they are used that cause problems,” stated Owen, a statement that was echoed by other industry experts during the conference.

As an extension specialist, he is trying to get growers to recognize that proactive management would effectively eliminate the resistance problem. He thinks that growers have been misled by the convenience and simplicity of some herbicide systems and have disregarded the risks of neglecting herbicide and crop diversity. “Growers want to take steps, but many are unwilling to initiate programs because they don’t want to spend time planning and spend money for different, more costly herbicides,” he said.

“Growers need to start using mixtures and diversity measures before their current technology fails on a large scale, not after,” stated another presenter.

The longer growers wait to address the weed resistance issue, the worse the problem becomes. The chemical industry will not be able to develop new herbicides and modes of action to keep up with resistant weeds. “We need to recognize herbicides as precious chemical resources,” said Dr. Stephen Powles, plant and crop scientist at the University of Western Australia and director of the Western Australia Herbicide Resistance Initiative. “Glyphosate was a one in 100 year discovery,” he said, likening it to the discovery of penicillin. “We should do everything in our power to keep it working.”

Maximizing herbicide sustainability
“Weed resistance is a global problem, but it is controlled locally on each farmer’s field,” emphasized Dr. Harry Strek, head of integrated weed management and resistance biology for Bayer CropScience. He noted that herbicide companies could help growers by providing better herbicide information and tailored management recommendations.

The consensus is that the main way to lessen and manage weed resistance development is to use diversity in both herbicide application and crop rotation. This was stressed by various presenters. It will take more planning and more work by growers, and will likely have higher costs. It will also require the registration of additional useful tank mixes by herbicide companies. 

Dr. Stephen Powles, University of Western Australia, demonstrates target-site resistance vs. non-target-site resistance due to enhanced metabolism or reduced translocation of herbicide.

One way to facilitate herbicide rotation might be through mandatory labelling of herbicides with a clear indication of the herbicide group or mode of action, said Dr. Hugh Beckie, Agriculture and Agri-Food Canada weed scientist and adjunct professor at the University of Saskatoon, Saskatchewan. He noted that such labelling is being tried voluntarily in Canada. He also suggested that close management of resistant patches within a field to prevent new seeds from entering the seed bank could help in containing resistant populations.

Speakers also stressed the importance of using the correct strength of herbicide products. For example, participants learned that much of the early glyphosate resistance that developed in Brazil arose before glyphosate-tolerant crop technology was made legal. During that time, growers were using the technology without supervision or company technology support, said Dr. Pedro Cristoffoleti, weed scientist at the Luiz de Queiroz College of Agriculture, University of São Paulo, Brazil. He suggested that application rates that were too low likely caused the rapid development of glyphosate resistance. Fewer glyphosate-resistant weeds have been found since the technology was legalized, he added. Insufficient application rates are problematic because they leave weeds behind and facilitate the selection of resistant individuals. 

Herbicide must also be applied at the right time in the weed life cycle. If applied too late, weeds are too big to be killed completely and can grow back, facilitating the selection of resistant individuals. For some weeds, the correct timing of application can be a matter of a few days’ window, so growers must keep on top of scouting weed development.

“The era of total post weed control is over, and I don’t think it’s coming back,” said Dr. Larry Steckel, University of Tennessee extension weed specialist for row crops. He noted that glufosinate, a broad-spectrum, non-selective herbicide, is working with glufosinate-tolerant crops (e.g., LibertyLink) in some areas of Tennessee where glyphosate resistance is a serious problem. However, he cautioned that growers should be sure not to make the same mistakes they made with glyphosate by neglecting herbicide and crop diversity or else they will end up with no way of controlling some of the major, problematic weeds.