A discouraging number of waterhemp plants have survived applications of postemergence herbicides throughout the state, said University of Illinois Extension Weed Specialist Aaron Hager.

"During the past 10 days, we have experienced an increasing number of calls and inquiries describing a 'noticeable' percentage of plants surviving applications of glyphosate (at rates ranging from 0.75 to 1.5 lb ae/acre) in soybeans," Hager said.

"Many have indicated that within approximately 7 to 10 days after glyphosate was applied, it became obvious the plants would survive."

Hager said in some cases the cause of plant survival could be attributable to an application rate too low for the size of plants present, precipitation that occurred soon after the application, or poor coverage of the target vegetation.

In other instances, the best explanation appears to be the evolution of a glyphosate-resistant population.

Remediation of these situations will likely be challenging, and it is altogether possible that in many instances herbicidal control of surviving plants will not be achievable, he added. 

"Options to control surviving plants regardless of their herbicide sensitivity/resistance profile include inter-row cultivation or hand rogueing," Hager said.

"Some may scoff at these suggestions, but in many areas of the Mid-South and Southeastern United States, these represent the few remaining viable options to manage emerged populations of herbicide-resistant weeds." 

Viable herbicide options for control of surviving waterhemp depend on the resistance profile of the plants.