Cancellations of three dicamba labels on June 6, 2020 by the U.S. Environmental Protection Agency, left many farmers looking for options for effective weed control this summer. Options are available but, the wrong choice of herbicide can lead to poor weed control and lower grain yield. A critical fact to remember—most conventional post emergent products work only on small weeds, 2-4 inches. Glyphosate (Roundup and other generics) is very effective on several weeds but some weeds have developed resistance to the herbicide. Even in glyphosate-resistant weed populations, glyphosate may control 50 percent of the weeds.
Kocia should be of most concern by farmers and commercial applicators when considering use of glyphosate. Cobra and similar generic herbicides containing lactofen, is only effective on one to two-inch kochia. Waterhemp is another glyphosate-resistant weed of concern in South Dakota. There are three products commonly used to control glyphosate-resistant waterhemp, fomesafen (Flexstar and other generics), acifluorfen (Ultra Blazer and other generics), and lastly lactofen. These herbicides work well but weeds must be at the two to four-inch height for best results. More than one application of these herbicides may be needed because waterhemp can germinate throughout the summer.
Glyphosate resistant ragweed also is present in areas of the state. Fomesafen, acifluorfen, lactofen and cloransulam (Firstrate and other generics) all provide excellent control. To get more information on these products and other products for other weeds view the latest South Dakota Pest Management Guide - Soybeans. In the guide on page 34 is a weed response chart that lists the control rating of common weeds in South Dakota. For other questions regarding weeds and weed control, contact Paul O. Johnson, SDSU Extension Weed Science Coordinator.