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More Glyphosate Resistance Ahead For U.S. Farmers

Weed scientist predicts United States will overtake Australia as worst region for resistance problems. Mike Wilson

The world's worst herbicide resistance problems are in Australia right now, but the United States could soon win this dubious honor if farmers here do not begin to diversify their weed control programs.

That was the message offered by Australian weed scientist Stephen Powles at the Commodity Classic, held in Austin, Texas Feb 23-26. The Classic is a joint meeting of 3,000 farmers from National Corn Growers Association and American Soybean Association.

Powles, Professor at University of Western Australia, outlined the growing problem in his home country where most grain farms are 6,000, 10,000, even 50,000-acre spreads. Glyphosate resistance is appearing in intensive cropping systems - particularly resistant-prone species such as ryegrass in long-term glyphosate usage for burn-down in no-till cropping. For years Australian farmers would apply glyphosate on no-till wheat over and over and only now are beginning to realize that resistance is an issue.

"It's a recipe to get glyphosate-resistant weeds," he says.

Now glyphosate resistance is showing up in North America, particularly in soybeans, he says. According to a recent Syngenta Crop Protection study, glyphosate-resistant horseweed (marestail) has been found in 10 states and the acreage is increasing. Common ragweed resistance was recently confirmed, and waterhemp, lambsquarters and giant ragweed are being investigated for possible resistance to glyphosate.

"Australia has the world's largest herbicide resistance problem, but by 2008, the U.S. will overtake Australia as the number one nation in herbicide resistance problems," predicts Powles.

Massive adoption rate

Why? Massive adoption of Roundup Ready technology over the past eight years, Powles says, in both North and South America. Glyphosate made up over half of the total U.S. herbicide market in 2002.

"It's the most rapid adoption ever of an agricultural innovation, because it's got clear benefits to growers," he says.

"We've all been blinded by the fantastic technology. I've listened to big farmers say 'It's almost impossible for my workers to screw up. If they get the rate wrong or the time wrong, it still works.' It's going to be hard to change that thinking.

As a result, there's zero consideration of resistance risk, Powles says. "There's a lack of diversity because most of the U.S. acreage is being treated with the same chemical."

In Australia, farmers are waking up to the serious nature of the potential problem. Powles believes they will begin to change management practices to preserve the efficacy of glyphosate.

"In the United States, grain growers are not sensitized to resistance," he says. "Everyone wants the easiest, most profitable system," but as a result, "resistance is going to come, big time. If the entire Midwest is sprayed with glyphosate year after year, we'll reap the biological repercussions."

U.S. farmers may be getting the message, though. According to the Syngenta study, 76% of Midwest farmers indicated a strong concern that continuous use of glyphosate will lead to resistance when asked that question in 2003; the figure grew to 85% in 2004. In the study, 52% of respondents said they plan to adopt (or already have adopted) a weed resistance management strategy such as rotating herbicides, rotating Roundup Ready and conventional crops, and using cultivation.


U.S. farmers may be complacent because they believe another wonder product will come along after glyphosate. But that's not going to happen, Powles predicts.

"Glyphosate is a one in a hundred year discovery," he says. "It's like penicillin - that also was a one in a hundred year discovery. Not in your lifetime will you see another herbicide as effective as this. There's nothing on the horizon that would be equivalent, in terms of new technology, to herbicides that we have now.

New herbicides may come on the market, "but there are no new modes of action," notes Chuck Forsman, technical business manager of non-selective herbicides, Syngenta Crop Protection.

Powles agrees. "If we drive that product to the point where it is no longer useful, farmers will adjust -- but there won't be one U.S. grower who doesn't lament the day they allowed that to happen," he says. "It won't be the end of the world, but there will never be anything as good."

Diversify weed control

Powles' recommendation? Involve more diversity in your herbicide arsenal. That's the best way to preserve glyphosate's usefulness, even though it means using conventional herbicides in conventional weed programs that require more management.

Liberty Link herbicide (Bayer) or Callisto (Syngenta) represent different chemistries. "The solutions will be different for individual farms, depending on what part of the world you farm in," he says.

If that doesn't happen he believes time will eventually run out for glyphosate. "It's coming over the next 5 to 10 years, big time," he warns. "While we don't know where we are on that curve, the next 10 years will see a major increase in glyphosate resistance in weeds. We should do everything to minimize that increase, and diversity is the only way."

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