Herbicide Resistance, Weeds Are Spreading in the United States

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 in use. 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


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