Missouri has the first known case of a summer annual weed resistant to glyphosate herbicides.

A biotype of common ragweed, the resistant weeds were found in a 20-acre field in central Missouri. Plants from the field were examined by Reid Smeda, University of Missouri weed scientist, and by officials of Monsanto Company.

They were resistant to 10 times the rate of glyphosate herbicide that normally controls common ragweed. Glyphosate is the active ingredient in Monsanto Roundup herbicides as well as other brands.

“While our year-long investigation has led us to conclude that resistance has occurred in this particular field, there have been no other complaints,” Smeda says.

“This case of ragweed is significant in that it is the first time a summer annual has developed resistance to glyphosate,” he adds. Marestail and ryegrass are winter annuals.