Monsanto and University of Missouri are in the process of investigating a potential case of glyphosate-resistant common ragweed on a single field in central Missouri. Preliminary findings of a field and greenhouse trials indicate lack of control; however additional findings such as hereditability, insect interactions and the unique morphology of this population all raise additional questions that merit further research.
"At this point we are seeing a difference in control of common ragweed in one isolate field, though there have been no other complaints locally and excellent control in other parts of the state," says Reid Smeda, University of Missouri weed scientist. "We are researching factors that may be playing a role in this specific situation."
It's critical that growers use the right rate of glyphosate for the right size weed at the right time, as well as additional weed control tools that may be necessary for the weed spectrum on their farm. The vast majority of weed performance inquiries are due to improper application or environmental conditions.
The development of resistance to glyphosate is less likely compared to other herbicides. Only two weeds have been confirmed resistant in the U.S., one of which is in a non-Roundup Ready crop system, after nearly thirty years of commercial use. Similar measures, such as Roundup tank mixed with FlexStar or Phoenix, will be put in place should this common ragweed biotype prove to be resistant.
Monsanto recommends that growers follow sound agronomic principles and report instances of unsatisfactory weed control to their sales representatives for follow up.