Monsanto recently announced that a common ragweed in one field in central Missouri is resistant to glyphosate. Monsanto reported that it made the resistant determination with University of Missouri weed scientist Reid Smeda. According to the company, the resistant ragweed was confined to a 20-acre section of the field.
Common ragweed now joins some biotypes of marestail and ryegrass shown to be resistant to glyphosate. Monsanto points out that these resistant cases are managed with tankmixes, which is the suggested treatment for common ragweed. The company recommends tank mixing FlexStar or Phoenix herbicides with Roundup for postemergence applications in soybeans to treat resistant ragweed. Another option is Valor or Scepter for soil-applied programs. In corn, herbicides such as triazines and growth regulators may be added to the herbicide program.