Weeds Developing Resistance To Widely Used Herbicide
By Scott Canon
The Kansas City Star
August 20, 2001
As weeds go, it looks so ... common.
Broad but not huge leaves. A stalk that runs from pure green to half crimson, but never eye-catching.
Could this scrawny member of the pig weed family have stumbled upon the herbicide-resisting powers that agri-giant Monsanto implanted in crops only after years of laboratory gene tinkering?
Is this, the humble amaranthus tuberculatus, a superweed?
"Right now, I'm just calling it insensitive," said Reid Smeda, a University of Missouri-Columbia weed scientist.
He's studying whether a strain of water hemp -- no relation to the stuff used for rope or dope -- plucked from northeast Missouri and west-central Illinois can shrug off the popular herbicide glyphosate.
Weeds overcome herbicides all the time. But glyphosate, best-known as Monsanto's Roundup brand, kills virtually everything green.
That's why genes were custom-fitted into crops specifically to withstand the weed killer king. With such seeds, a farmer can plant a genetically modified crop, spray with glyphosate and expect all the pest plants to whither while grain grows unharmed.
Farmers, in fact, love the simplicity of it all.
Such trademarked Roundup Ready varieties account for four out of every five American acres planted with soybeans. Roundup Ready corn, canola and cotton have been popular as well. Monsanto soon could offer herbicide resistance in sugar beet, rice, wheat and lettuce. Already, the technology is locked solidly into the nation's food chain.
Water hemp could ruin all the fun.
It could, in fact, fulfill the prophecy of environmentalists that widespread use of herbicide-resistant crops eventually will lead to the rise of herbicide-resistant weeds -- breeds of super weeds.
"Given enough generations and enough exposure, any species will develop resistance to a herbicide," said Rebecca Goldburg, a senior scientist for anti-biotechnology Environmental Defense.
Monsanto is more than skeptical. David Heering, the company's lead expert, disagrees with the theory that continued use of Roundup Ready crops inevitably will lead to superweeds.
"I would challenge anybody," he said, "who says they're going to predict weed resistance to herbicides."
'The U.N. of plants'
So far, water hemp has given only an inkling of Roundup resistance. Native to the Grain Belt, it is the most common weed in Missouri soybean fields and is on the way to being the most common weed in the state's corn fields.
Its genetic background is highly diverse -- "It's the U.N. of plants," Smeda said -- and that increases its chances of coming up with a strain that will resist what farmers throw at it.
In 1999, a farmer near Monticello, Mo., and another near Sutter, Ill., reported that some water hemp was shocking them by surviving glyphosate, albeit at low rates.
Both locations were Roundup Ready soybean fields treated season after season with glyphosate.
At first skeptical, Smeda collected samples for dousing in herbicide at his University of Missouri-Columbia greenhouse along with tests in the field.
In both settings, he found that some water hemp plants survived sprayings of glyphosate -- sold as Roundup, Touchdown, Glyphomax, Glyphos -- at higher rates than other weeds.
In greenhouse conditions, 4 percent to 5 percent of the suspect water hemp plants survive sprayings with glyphosate, almost regardless of the dosage.
"Under those conditions, you'd expect a 100 percent kill," Smeda said.
He's puzzled, however, because the offspring of those survivors demonstrate the same 5 percent survival rate, when science might predict them to inherit their parents' sturdier character.
Smeda thinks farmers will need to rotate their herbicides, which would mean planting something other than Roundup Ready crops.
"The Roundup Ready technology is so good, we don't want to lose it," Smeda said.
In Delaware, weed scientist Mark VanGessel has a horseweed, commonly known as mare's tail and categorized scientifically as conyza canadensis, that appears to have whipped glyphosate.
When he first got reports from around the state of resistance in 1999, VanGessel admits he was dismissive.
Dousing samples with glyphosate, he found to his own surprise that "it stunts them, but it won't kill them."
"Farmers had been using Roundup for 25 years, and there was no resistance yet," said VanGessel. "It turns out the operative word was 'yet."'
But those decades before Roundup Ready was introduced five years ago were different. Glyphosate was used in rotation with other weed killers, depending on the weed problem, and the timing of planting cycles. With new gene-spliced designer Roundup Ready soybeans, however, there seemed little reason not to use glyphosate exclusively.
The problem, say agronomists, is that continually killing off the herbicide weaklings leaves less competition -- for nutrients in the soil, for water, for light -- for that random genetic mutation that can weather a rain of Roundup.
"This is kind of a wake-up call that we can't rely on glyphosate year in and year out for weed control," VanGessel said.
Monsanto greets such reports with skepticism. The company's patent on glyphosate expired Sept. 20, 2000. But it continues to sell the herbicide and license the sale of Roundup Ready seed.
Roundup is much more environmentally friendly than other popular herbicides. Atrazine, for instance, can pose a problem for soil and water pollution years after it is first applied. Evidence of glyphosate use, in contrast, can disappear in a matter of days.
Heering, Monsanto's expert, concedes that goose grass in Malaysia and rye grass in Australia have shown resistance to glyphosate. But that has nothing to do with Roundup Ready crops. There, glyphosate is used like other traditional weed killers, without any coordination with genetically altered seeds.
Even there, he said, the company is making recommendations about mixing Roundup with other varieties.
As for the mare's tail and water hemp, Heering is not ready to concede. The company reads the data on water hemp to detect no resistance and is awaiting some tests of the mare's tail.
The consensus among weed experts holds that if water hemp hasn't developed resistance to Roundup, some other American weed eventually will. Mare's tail, insists VanGessel and others, already has. Farmers then will be forced back to changing their herbicide from one year after the next or risk fields choked with superweeds.
"The simplicity of the Roundup Ready system is not going to last very long," said Dale Shaner, a co-author of Herbicide Resistance and World Grains. "Most simple systems just don't stand up. Mother nature has a way to get around that."
Still others concede that if Roundup is destined for a showdown with superweeds, it holds a considerable edge now.
"There's a lot of (Roundup) being used," said Michael Christoffers, a weed geneticist at North Dakota State University. "If it were easy for weeds to develop resistance to it, we'd be seeing that happen much more often."
Related link: GM Crops Failed