Multiple giant ragweed (Ambrosia trifida) populations in Nebraska have been confirmed as glyphosate-resistant by University of Nebraska researchers. According to UNL weed scientists, Lowell Sandell, Avishek Datta, Stevan Knezevic and Greg Kruger, greenhouse studies have confirmed resistant populations, collected from suspected plants in Butler, Nemaha, Richardson and Washington counties last fall.
Visual weed control ratings and plant biomass reduction curve comparisons clearly showed a glyphosate resistance level in giant ragweed tests ranging from two to six fold, depending on the population and plant size.
This is the second confirmed resistant weed in the state. In 2006, marestail, or horseweed, was found to be resistant to glyphosate. While marestail has become a management challenge in many fields, new resistant weeds are emerging in Nebraska, according to the researchers.
Knezevic, UNL Extension integrated weed management specialist in Concord, says that researchers are certain resistant giant ragweed populations exist elsewhere in the state, but they did not have the time or manpower to confirm it yet.
"Producers have started to complain this year all over the state," Knezevic says.
Resistance is usually the result from repeated use of the same herbicide. Widespread adoption of glyphosate-tolerant corn and soybeans across the Midwest, coupled with over-reliance on glyphosate-based herbicides, has resulted in the evolution of resistant weed biotypes, the researchers wrote. The selection process exerted on weed populations by the rapid adoption of glyphosate-tolerant crops is unprecedented in the era of herbicide weed control.
Today, almost 20 weed species worldwide, and 12 species in the U.S., are confirmed as glyphosate-resistant. In addition to giant ragweed and marestail confirmed in Nebraska, the U.S. list includes common waterhemp, common ragweed, kochia, palmer amaranth, hairy fleabane, junglerice, goosegrass, Johnsongrass, Italian ryegrass and annual bluegrass.
According to Knezevic, glyphosate-resistant kochia and waterhemp are being studied and most likely exist in the state as well, but have not been officially confirmed.
The researchers wrote that resistant giant ragweed populations are currently confined to small pockets, but the confirmation illustrates the necessity for farmers to adopt a diversified approach to weed control. Herbicide-tolerant crops, including those based on glyphosate, can remain a useful tool in crop production, but only with proper management.
For more information about glyphosate-resistant weeds, attend one of the University of Nebraska 2012 Crop Protection Clinics scheduled across the state in January or learn more at www.cpc.unl.edu.