There have been a number of complaints concerning the lack of control of several weed species with Roundup Ultra. The weed species include velvetleaf, common cocklebur, and common waterhemp. In many cases, the lack of control is attributable to application rate, weed size, environmental conditions, or a combination of these factors. Reapplication of Roundup Ultra or another herbicide registered for application at the current stage of soybeans and weed development should resolve the problem. However, consider carefully the economic impact of weed escapes before making retreatments at this late date.
However, there have been situations where multiple applications of Roundup Ultra have failed to control common waterhemp. A field near Everly, Iowa, has an infestation of common waterhemp that has not been controlled by three applications of Roundup Ultra. Interestingly, there are some plants that have been killed, some that are injured and may or may not recover, and some that are seemingly not affected. The reason for the lack of control does not appear to be the result of misapplication, rate selection, or environmental conditions. We are beginning investigations to determine the reason for the variable response exhibited by this common waterhemp population. Answers to this problem will be presented in an upcoming ICM newsletter.
This article originally appeared on page 146 of the IC-480(19) -- July 20, 1998 issue.