Tips to battle herbicide resistance

Farmers need to use different modes of action to avoid weeds developing resistance to herbicides in their fields, said Washington State University weed expert Drew Lyon. 

“We find something that works, we go out and use it over and over again, and before long, it doesn’t work so well,” said Lyon, a weed science professor at Washington State University in Pullman, Washington. 

The number of resistant weeds occurs through continued application of the same herbicide mode of action each year, Lyon said. 

Before a grower ever makes an herbicide application, one random resistant weed is in the field. When the farmer applies the herbicide, the number of resistant plants goes from one in a million to one in 100,000, Lyon said, using an example. 

The next year, the number goes to two in 10,000 plants. 

The next year, the number is three in 1,000 plants. 

The fourth year, 96 percent of the plants are still being killed off. 

The fifth year, about 60 percent of the weeds are now resistant, Lyon said. 

The length of time varies, depending on cultural practices, frequency of herbicide use and the biology of the weed species. Group 2 herbicides, or ALS inhibitors, tend to be overcome relatively quickly, Lyon said. That’s the class used most often in wheat production. 

Glyphosate resistance concerns developed because Roundup Ready corn and soybean crops were sprayed for glyphosate, enabling resistant plants to spread, Lyon said. 

In greenhouse testing, WSU researchers found resistant plants were four times more resistant to glyphosate than susceptible plants. A jointed goatgrass plant was found to be 144 times resistant to Beyond herbicide than susceptible plants. 

No herbicides with a new mechanism of action are in development trials, Lyon said, meaning a new commercially-available option would be at least eight to 10 years away. A new mechanism of action hasn’t been introduced in more than 25 years. 

“There’s nothing about to ride in and rescue us if we lose some of the herbicides working today,” he said. 

Just because a farmer is using a different herbicide doesn’t necessarily mean the mode of action is different, he said. 

A perennial problem 

The number of herbicide-resistant weeds in the U.S. and worldwide is increasing, Lyon said. 

Some weeds are developing with resistance to multiple modes of action, Lyon said. Some are resistant to seven or more types. 

The Pacific Northwest is early enough in the process that it could delay such a situation, he said. 

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Farmers who spray their fields will find that the herbicide controls everything it’s supposed to except for one species, which is a sign of developing resistance, Lyon said. 

Lyon advises farmers to make adjust-ments to their weed control methods and stop problems before they start, including using clean seed, tarping grain loads, controlling weeds on field edges, composting livestock manure and managing weed seed during and after harvest. 

Farmers should help their crop compete with weeds and use practices to keep weeds “off balance,” he said. One suggestion is to rotate tillage practices. 

Growers who suspect they have resistant weeds can submit samples to WSU researchers. Visit for submission instructions.