Resistant weeds are spreading
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Herbicide Resistance, and Weeds, Are Spreading in the United States: Newly Published Research
Press Release -- July 12, 2011 Herbicide resistance is growing. At least 21 weed species have now developed resistance to glyphosate, a systemic herbicide that has been effectively used to kill weeds and can be found in many commercial products. Some weeds are now developing resistance to alternative herbicides being used. New occurrences of resistance are being noted in varying weed species and locations, creating challenges for weed scientists.
Several articles in the current issue of the journal Weed Science focus on the issue of herbicide resistance. The articles highlight first reports of resistance. “The herbicide resistance issue is becoming serious,” the journal’s editor, William K. Vencill, said. “It is spreading out beyond where weed scientists have seen it before.”
Palmer amaranth is a common weed that competes with cotton, soybean, corn, grain sorghum, and peanut crops in the southern United States. A density of 10 of these weeds per row of cotton has been shown to reduce yields more than 50 percent. By 2010, 52 counties in the state of Georgia had infestations of glyphosate-resistant Palmer amaranth.
Field and greenhouse tests conducted for the current study now confirm that this weed is resistant not only to glyphosate, but also to phrithiobac, an acetolactate synthase-inhibiting herbicide. This marks one of the first reports of multiple resistance to both glyphosate and pyrithiobac in Palmer amaranth. As multiple herbicide resistance becomes more common, a grower’s ability to be economically sustainable is threatened.
Another study in this issue conducted dose-response, ammonia accumulation, and enzyme activity tests on glyphosate-resistant Italian ryegrass populations taken from hazelnut orchards in Oregon. This research now confirms resistance of Italian ryegrass to another control alternative, glufosinate ammonium, a nonselective broad-spectrum herbicide.
In West Memphis, Ark., another study reports the first documented glyphosate-resistant johnsongrass biotype in the United States. A soybean field in continuous production over 6 years showed reduced control of johnsongrass with the recommended application rate of glyphosate. A greenhouse study was conducted with this johnsongrass to confirm this finding and determine any differences in absorption or translocation of the herbicide within these plants.
As herbicide resistance spreads, growers will need new weed management strategies. These could include herbicides with alternative sites of action within the plant or nonchemical methods such as tilling and mulching. Growers should prevent resistant weeds in a production field from reaching reproductive maturity to prevent spread of the trait through seed or pollen.
Full text of “Multiple Resistance in Palmer Amaranth to Glyphosate and Pyrithiobac Confirmed in Georgia,” and other articles in Weed Science, Vol. 59, No. 3, May-June 2011, are available at http://allenpress.com/publications/journals/wees
Environmental Impacts of Killing Weeds now resistant to Roundup
Herbicide tolerance and GM crops
Why the world should be ready to round up glyphosate
Glyphosate is the active ingredient in many herbicides sold throughout the world, including the well-known formulation, Roundup. Glyphosate based herbicides are used widely for weed control because they are non-selective; glyphosate kills all vegetation.
Glyphosate has been promoted as ‘safe’. However, mounting scientific evidence questions the safety of glyphosate and its most well known formulation, Roundup. The evidence detailed in this report demonstrates that glyphosate-based products can have adverse impacts on human and animal health, and that a review of their safety for human and animal health is urgently needed.
The report's conclusion
The rapid evolution of weeds that are resistant to glyphosate is a result of farmers becoming over-reliant on one herbicide for weed control. This is particularly associated with GM RR (genetically modified Roundup Ready) crops. Now that resistance to glyphosate is widespread in weeds within GM RR soy, maize and cotton crops, farmers have to resort to using mixtures of herbicides.
Thus, the promise of reduced herbicide use and cheaper and easier weed controls has not been delivered. However, it is clear that GM RR crops have brought about an escalation in the pesticides ‘arms race’, with an increasing toxic burden on the environment involving significant uncertainty about the overall safety of glyphosate for people and biodiversity.