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Does Roundup-resistant marestail illustrate problems with reliance on GM crops and pesticides? (February 22, 2001 --Crophchoice news) -- Scientists discovered glyphosate-resistant marestail (horseweed) in three fields of Roundup Ready soybeans in Delaware. The resistant weeds may be present in other fields there, and in New Jersey and Maryland, as well. Glyphosate is the active ingredient in the herbicide Roundup, which Monsanto has engineered its Roundup Ready soybeans to resist.
"It certainly looked like resistance," University of Delaware weed scientist Mark VanGessel told Farm Chemicals magazine. "Marestail control was random throughout the field, which ruled out sprayer problems or applicator error. With almost ideal weather conditions early in 2000, we also ruled out environmental or stress factors."
Members of the agricultural community disagree about the implications of this finding.
VanGessel dismissed the idea that Roundup Ready soybeans transferred their herbicide resistant genes to the marestail.
"Weeds go through frequent genetic changes," he said. "It's random chance that the marestail developed resistance to Roundup."
To deal with the problem, he suggested that growers spray 2,4-D (in combination with other herbicides) at least seven days before planting their soybeans. Then, after the Roundup Ready soybean plants have emerged, growers can spray Roundup to control the other weeds.
Others in the agricultural community contend that doing this points out the irony of escalating herbicide use at a time when the biotechnology industry is telling farmers that its genetically engineered plants require fewer herbicide applications.
Agricultural economist and consultant Charles Benbrook admitted that he couldn't address when the marestail resistance to Roundup emerged, but he does think that Roundup Ready crops "pushed it over the edge."
Since 1996, when industry first bioengineered crops for widespread commercialization, herbicide use has increased, Benbrook said.
"Of course Roundup resistant weeds can be sprayed with other herbicides," he said. "If that is the solution, what about those cost-savings to farmers and reductions in soybean herbicide use?"
"2,4-D goes on soybeans at about 0.4 pounds (active ingredient) per acre. Add in two applications of Roundup at the average 0.7 pound rate, and a grower is applying just under 2 pounds of herbicide per acre." No-till growers and some others apply Roundup three times, he said. This practice will grow more common as generic competition drives down the price of the herbicide.
Add to that the .2 to .5 pound of herbicide (again, active ingredient) that most growers are applying per acre just before or when they sow their seeds, he said.
In 1999, biotechnology and chemical conglomerates aggressively marketed 13 combinations of low-dose herbicides for use on conventional soybean plants, he said. (The average application rates for these herbicides are less than 0.1 pounds active ingredient per acre, based on 1999 USDA chemical use survey data for soybeans). One of the very low dose herbicides - thifensulfuron -- is applied at 0.002 pounds active ingredient per acre; growers sprayed it on 5 percent of the nation's soybean acreage in 1999.
In 1998, he said, growers applied an average of 1.4 pounds of herbicides (.92 pounds of glyphosate and .5 pounds of other herbicides) to genetically modified soybeans. Plus, Roundup Ready soybeans required about 40 percent more active ingredient per acre, measured by weight, than non-bioengineered soybeans.
In contrast, the average acre of conventional soybeans in 1998 required about 1 pound of herbicides.
Benbrook's conclusion is that Roundup Ready soybeans, on average, require much more herbicide per acre than non-bioengineered varieties.
"Roundup Ready technology has its virtues but sustainability and reducing herbicide use are not among them," he said.
Marestail is not the first weed to develop resistance to Roundup, which is interesting given that the biotechnology industry has long said it's impossible for weed species to develop such a trait, said E. Ann Clark, a pasture and grazing specialist at the University of Guelph in Ontario, Canada.
Three years ago, rigid rye grass in Australia developed resistance to Roundup after a farmer had applied too much of it. And on a Malaysian plantation, goose grass (one of the 20 worst weeds in the world) showed resistance, she said, noting that the tolerance was specific to that location.
Although the evidence is only anecdotal, Clark has heard reports from western Canadian farmers of weeds developing resistance to the herbicide 2,4-D, an endocrine disruptor.
"Overreliance on any single method of weed control will select for individuals tolerant to that method of weed control and Roundup is no exception," she said.
Purchasing expensive genetically engineered seeds and herbicides has increased the cost of production for farmers. The problem is not the crops themselves, but the way that we're growing them, she said. Planting corn, soybeans, and corn, year after year, and then spraying them with the same herbicides at the same time leaves a niche for weeds to grow and develop resistance.
Clark suggests that grain and livestock producers should instead plant winter cereals as part of a more complex crop rotation. This, combined with lessening or eliminating the use of herbicides, will reduce their production costs, widen their margins, and yield a higher net profit.
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