Editor's note: This article was authored by Daniel Piller and published by the Des Moines Register.
Iowa State University agronomy professor Michael Owen receives calls and e-mails from farmers asking what to do about the threat of resistance by weeds to Roundup and other glyphosate herbicides.
"I tell them to quit doing what they're doing now, because it won't work for long," Owen says. "Iowa is probably about 2 years away from a serious problem with glyphosate resistance, and it's better to react now than to wait until it's too late."
Owen for the last year has overseen a 10-plot study of weed herbicide resistance on Iowa farms.
Weed resistance has yet to inhibit Iowa's corn and soybean yields, which have risen steadily in recent years to the 2009 harvest averages in Iowa of more than 180 bushels per acre for corn and 60 bushels per acre for soybeans. Two decades earlier, before Roundup was used widely, Iowa's corn crop averaged 126 bushels per acre and soybeans averaged 41.5.
Farmers have enjoyed good incomes in the last decade because prices have kept up with higher yields, due largely to increased demand from ethanol production.
A reduced crop would not only cut into grain farmers' incomes, but also likely raise the feedstock prices for livestock producers and ethanol plants. Corn and soybean sales amount to about two-thirds of the $25 billion that Iowa's farms generate each year.
"We can't say for sure how deep the damage would be from weed resistance, but it would happen," Owen says.
Time has run out in much of the South, where glyphosate-resistant weeds have damaged cotton and grain crops, Owen says.
"What's happened in the South is a good example of what shouldn't happen here," Owen says. "Farmers didn't change their practices. Roundup works so well, they just kept applying it year after year. You need to mix up the herbicides, put different types on the fields."
Owen says Southern farmers have been quicker to abandon traditional tillage.
"When you till, you're essentially decapitating the weeds," he said. "If you don't till, you're putting all weed management on the herbicide."
Less than 20% of Iowa's 24 million cultivated acres are no-till. Owen isn't a no-till opponent, but he says no-tillers should be especially vigilant.
"Roundup isn't going away," Owen says. "It will be around for a long time. So have other herbicides like 2,4-D, atrazine and Dicamba. They're still around and still can be used."
Since it came into widespread use on farm fields in the 1990s, glyphosate has been so successful that many farmers used it as a one-brand-fits-all herbicide against a wide variety of notorious weeds, led by waterhemp, giant ragweed and marestail.
"The trouble is that the weeds alter their own genes to resist Roundup as well," Owen says.
Weeds are enemies of crops because they suck water and nutrients from the soil but also hog the necessary sunlight.
"The corn or soybean plant is most vulnerable when it begins to emerge," Owen says.
For that reason, farmers usually apply herbicides right after planting, to prevent weeds from emerging. Owen suggests that farmers might want to start the practice of applying a spray other than glyphosate before planting, then follow up with glyphosate after seeds are in the ground.
But Owen is the first to acknowledge that Iowa's spring weather, from pre-plant in March to post-plant in May and June, is often uncooperative.
This year, for instance, high winds in May made spraying a problem and record rainfall in June washed herbicide off fields.
"It can be hard to get it right," Owen says.
Agronomists like Owen have warned for years that the heady days for Roundup and other glyphosates would end as weeds, as they inevitably do, build up resistances to the chemicals.
Farmers have become so confident in glyphosate that they can be tempted to skimp on it to reduce costs.
"It's a mistake to go half-rate" or cut back the suggested application rates, says farmer Pat Sheldon of Percival, Iowa, who says that so far he's avoided glyphosate resistance.
"Roundup has worked so well that some guys are tempted to reduce the rate on application. Then the weed has a chance to survive, and if it does, it will have a resistance," Sheldon says.
Technology can provide help.
Global positioning systems in tractors, which have software with information about herbicide needs on different parts of fields, can be used to adjust applications and herbicide varieties.
But Owen says weeds have a natural advantage over corn and soybean plants. Weeds don't have to produce the grains that make them valuable.
"All they have to do is grow," Owen says of weeds. "They have an easier job."