2,4-D and dicamba-resistant crops and their implications for susceptible non-target crops

Scientists have documented that non-target terrestrial plant injury was 75 to 400 times higher for dicamba and 2,4-D, respectively, than for glyphosate.

Herbicide-resistant weeds are becoming a more widespread problem in the United States. Although herbicide resistance has most commonly occurred in the south in cotton and soybeans, it is increasing in other regions as well. According to a team of agricultural researchers from Pennsylvania State University, University of New Hampshire and Montana State University, too much reliance on glyphosate-type herbicides for weed control on U.S. farms has created a dramatic increase in the number of genetically-resistant weeds.

"I'm deeply concerned when I see figures that herbicide use could double in the next decade," said David Mortensen, professor of weed ecology at Penn State. "During the period since the introduction of glyphosate-resistant crops, the number of weedy plant species that have evolved resistance to glyphosate has increased dramatically." Mortensen said. This list includes many of the most problematic weed species, such as common ragweed, horseweed, johnsongrass and several of the most common pigweeds. According to the research team, despite company-sponsored research that indicated resistance would not occur, 21 different weed species have evolved resistance to several glyphosate herbicides, 75 percent of which have been documented since 2005.

"In practice, the problem of glyphosate resistance goes far beyond a species count," Mortensen said. "More important, perhaps, is the increase in acreage infested with glyphosate-resistant weeds”. A separate survey of thousands of U.S. farmers across 31 states conducted over three years by Stratus Agri-marketing, Inc., showed 49 percent of the farmers surveyed reporting glyphosate-resistant weeds on their farm in 2012, up from 34 percent in 2011. The reported acreage infested with glyphosate-resistant weeds has risen from 32.6 million acres in 2010 to 40.7 million acres in 2011 and 61.2 million acres in 2012. According to the survey, 27 percent of the farms have more than one species of glyphosate-resistant weed. Marestail (horseweed) was the most commonly reported weed with glyphosate resistance, followed by Palmer amaranth.

"We do understand why farmers would use the glyphosate and glyphosate-resistant crop package," Mortensen said. "It is simple and relatively cheap, but we have to think about the long-term consequences."

"Several species have developed amazing biochemical ways to resist the effects of the herbicide," said J. Franklin Egan, doctoral student in ecology at Penn State. "If weed problems are addressed just with herbicides, evolution will win." Egan noted that some weeds have evolved to make an enzyme that remains unaffected by the herbicide and still functions within the cells, while other weeds have developed ways for the plant to move the herbicide away from targeted enzymes. "For instance, glyphosate-resistant strains of horseweed sequester glyphosate in leaf tissues that are exposed to an herbicide spray so that the glyphosate can be slowly translocated throughout the plant at nontoxic concentrations," Egan said. "To the horseweed, this controlled translocation process means the difference between taking many shots of whiskey on an empty stomach versus sipping wine with a meal."

Due to the increasing number of weeds resistant to current applications, new generations of seeds under development are being genetically modified to resist multiple herbicides. "Specifically, several companies are actively developing crops that can resist glyphosate, 2,4-D and dicamba herbicides," said Mortensen. (See the related Michigan State University Extension article “Engineering crops with resistance to 2,4-D and dicamba.”)

"What is [more] troubling is that 2,4-D and dicamba are older and less environmentally friendly [than glyphosate]." Vapor drift of more toxic herbicides has been implicated in many incidents of crop injury and may have additional impacts on natural vegetation interspersed in agricultural landscapes, Mortensen stated. Scientists have documented that non-target terrestrial plant injury was 75 to 400 times higher for dicamba and 2,4-D, respectively, than for glyphosate. The continual insertion of more genes into crops is not a sustainable solution to herbicide resistance, according to the researchers. They add that companies are creating a genetic modification treadmill similar to the pesticide treadmill experienced in the mid-20th century, when companies produced increasingly more toxic substances to manage pests resistant to pesticides.

Egan said there are several problems with the treadmill response. First, weeds will eventually evolve combined resistance to dicamba, 2, 4-D and glyphosate herbicides. Globally, there are already many examples of weeds simultaneously resistant to two or more herbicides. Increased use of 2, 4-D and dicamba applied over the growing corn and soybean means much more of these herbicides will be applied at a time of year when many sensitive crops like tomatoes and grapes are most vulnerable to injury. Such injury results when these herbicides move from the targeted field during or following an application.

There is also increased risk that farmers will use the herbicide during inappropriate or non-recommended weather conditions, leading to herbicides drifting from the targeted area and killing or harming other plants and crops. Additionally, the proposed herbicide programs don’t substitute 2,4-D or dicamba for glyphosate, but advocate combining current rates of glyphosate with the other herbicides, leading to additional herbicide use. Egan also said that if farms become too reliant on herbicides, farmers will find it more difficult to use integrated weed management approaches. Integrated weed management includes planting cover crops, rotating crops and using mechanical weed control methods. Farmers can use herbicides in this management approach, but must use them in a targeted, judicious fashion.

The researchers said that in previous studies, integrated weed management had lowered herbicide use by as much as 94 percent while maintaining profit margins for the operations. "Integrated weed management is really the path forward," said Egan. "We believe these methods can be implemented and we already have a lot to show that they’re effective and straight forward to incorporate.”

The MSU Extension publication E2931, “Integrated Weed Management – One Year’s Seeding…”provides information about weed lifecycles, rotation, tillage and other practices related to managing weeds without depending entirely on herbicides.

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