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

Reid Smeda, University of Missouri weed scientist, discussed the finding this week during the annual convention of the North Central Weed Science Society in Columbus, Ohio.

The resistant weeds, a biotype of common ragweed, were found in a 20-acre field in central Missouri. Plants from the field, examined by Smeda and by officials of Monsanto Company, were found to be 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 yearlong investigation has led us to conclude that resistance has occurred in this particular field, there have been no other complaints,” Smeda said. Common ragweed is the third biotype in the United States to develop resistance to glyphosate, which has been used commercially for more than 30 years. Resistance also has been discovered within biotypes of the weeds marestail and ryegrass.

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

The development of these resistant weeds, as with other cases of weed resistance throughout history, is chiefly due to continual use of one weed-control strategy, Smeda said. It was found in a field with a history of continual soybean production and the repeated use of glyphosate in glyphosate-resistant soybean varieties.

Smeda will discuss proper weed control strategies to manage resistance with farmers and pesticide applicators at meetings throughout the winter.