April 22, 2004
Farmers facing rise of herbicide-resistant weeds SIUC Country Column
The world's most popular weed killer no longer kills all weeds.
"Marestail (also known as horseweed) that is resistant to glyphosate (the active ingredient in Roundup) was found in Delaware in 2000, and it's spreading," said Bryan G. Young, Southern Illinois University Carbondale's resident weed man.
"We don't have any in the state of Illinois, but it's in Indiana, western Tennessee, western Kentucky, Bootheel Missouri -- the threat is there. I wouldn't be at all surprised if we find glyphosate-resistant plants in Illinois this year."
The state's scientists are already debating whether waterhemp should be on that list. The amount of weed killer needed to dispatch the stuff has crept steadily upward since 1996; the current recommended application rate amounts to more than three times what it was starting out.
But while rate creep suggests a problem, scientists would have to find that some waterhemp weeds survived weed killer much, much better than their run-of-the-mill relatives before pronouncing them resistant.
"That's where the problem is," Young said. "Since our normal population of waterhemp varies widely in its response to glyphosate -- from 0 to 100 percent survival -- it's difficult to determine if a plant that survives is really different."
Farmers generally don't notice resistant weeds until they make up 30 percent of the weeds in a field.
"Anything below that, and they tend to think it's misapplication of herbicide or environmental conditions," Young said.
To fight resistant marestail, soybean farmers are turning to an old standby: 2,4-D.
"It's one of the first selective herbicides ever developed and was used a lot in agriculture beginning in the '40s," Young said.
Gramoxone, FirstRate, Amplify and Classic can also whack the glyphosate-resistant marestail that Roundup leaves behind. But what happens if those fail, too?
"One result of Roundup Ready technology (the development of crops that could withstand weed killer without dying), especially in soybean, is that it's reduced the profit opportunities for herbicide manufacturers," Young said.
"It's so cheap and so effective that there's been little incentive to develop new herbicides."
Down the road, demand from farmers forced to till and hand-hoe weedy fields could provide that incentive, but even so, new weed killers won't come cheap, Young believes.
"We could be back to $20 per acre for weed control," he said.
While nothing can stop the rise of resistant weeds, farmers can do a few things to stave them off as long as possible.
"We can change how we manage weeds -- and notice, I say 'manage,' not 'control,'" Young said.
"Management is reducing or limiting the impact of the weeds on crop yields. Having Roundup Ready beans has given farmers a way to kill tall -- that's 'control' -- weeds, but that's actually worked against production."
The taller the weed, the more likely some are to survive, Young said. That not only cuts yields (Young has found at least a four bushel-per-acre decrease in fields where farmers allowed weeds to get more than 8 inches tall before going out to spray), but it ups the odds for developing resistant weeds.
"These are plants that tolerated the herbicide," Young pointed out. "You don't want them to reproduce."
To protect yield and delay resistance, follow the old way: Apply a residual herbicide before weeds emerge and follow up with a post-emergence herbicide.
And don't cut the application rate as a way to cut costs.
"Herbicide is cheap right now; you're talking $10 or less -- the cost of a bushel of soybean," Young said.
"A small savings on the herbicide bill won't amount to a hill of beans if you wind up with glyphosate-resistant weeds or a reduction in yield."