According to the University of California Davis, herbicide resistant plants have "the ability to survive and reproduce following exposure to a dose of herbicide that would normally be lethal to the wild type." Larger, stronger plant species that do not respond to the chemical herbicide glyphosate are "the single largest threat to production agriculture that we have ever seen," according to Andrew Wargo III, the president of the Arkansas Association of Conservation Districts, in a May 3, 2010 New York Times article. Agriculturalists have identified several species that have become herbicide resistant.
Pigweed is also known as Palmer amaranth and waterhemp. Pigweed grows 3 inches a day and competes with crops for soil nutrition and water. Farmers began to report in 2000 that pigweed was not dying in their soybean fields, even after using eight times higher doses of Roundup herbicide. Roundup Ready soybean seeds are genetically altered specifically to allow the herbicide to kill the weeds but not the soybeans. There is little incentive for farmers to buy the more expensive seeds if they do not lead to the desired weed-control.
Marestail and Canada fleabane are other common names for horseweed, which has evolved since 2001 into a plant highly resistant to glyphosate herbicides. Herbicides have become ineffective in over 20 states, including California and most of the Midwest. Use of glyphosate herbicide increased 752 percent between 1997 and 2003, and the plant's resistance paralleled that increase. Rather than reduce the need for herbicides, genetically modified food seed has increased their use, according to a Center for Media and Democracy report on Monsanto and Roundup Ready crops.
"Giant ragweed is one of the most problematic weeds in the eastern corn belt," according to Max Loux of Ohio State University. Farmers sustain losses of 30 to 50 percent when ragweed is uncontrolled, and ragweed has become resistant to Roundup weed killer, driving farmers back to expensive methods such as hand picking and tilling. Many farmers have resorted to using older herbicides that are more toxic than Roundup. "The biotech industry is taking us into a more pesticide-dependent agriculture when they've always promised, and we need to be going in, the opposite direction," said Bill Freese, a science policy analyst for the Center for Food Safety in Washington."