Written collaboratively by Eric Jones, Clarence Winter, Philip Rozeboom, Jill Alms, and David Vos.
Weed management is always a critical component of crop production, but in sunflowers, it poses unique challenges. Compared with many other crops, there is a limited selection of herbicides that are labeled for sunflowers, especially postemergence herbicides. Certain herbicides can be applied postemergence for effective control of grass weeds (i.e., clethodim [Select Max]). Additionally, there are herbicide traits that allow for the application of select ALS-inhibiting herbicides in Clearfield (i.e., imazamox [Beyond Extra]) and Express (i.e., tibenuron [Express]). While these herbicides can effectively manage many broadleaf weeds and some grass weeds, kochia and waterhemp in South Dakota are resistant to these herbicides.
Preemergence herbicides will be paramount to manage kochia and waterhemp in sunflower production. There are more herbicides available to apply preemergence in sunflowers: herbicides from group 3, 14, and 15 can be effective on kochia and waterhemp. While there are herbicides available to manage these weeds, the soil residual activity of these herbicides will likely not last all season. Therefore, applying another preemergence herbicide when the sunflowers have emerged but before the weeds emerge (referred to as post residual) may be necessary to minimize weed competition with the crop throughout the growing season. As the sunflower crop remains weed-free, the plants can grow vigorously and reach canopy closure faster to further reduce the amount of weeds that germinate later in the season (Figure 1).
Pyroxasulfone (i.e., Zidua), a group 15 herbicide, can be safely applied between the v1 and v8 growth stages. This herbicide must be applied before weeds emerge as there will be no activity on emerged weeds. Many small-seeded broadleaf (including kochia and waterhemp) and grass weeds are susceptible to this herbicide. Application of this herbicide should coincide with a rain event to move the herbicide into the soil profile where the weed seeds are germination; this herbicide requires at least 0.5” of rainfall for “activation”. If this herbicide is to be applied post residual, then different herbicides should be used in the preemergence application to reduce selection pressure on resistant weeds. Refer to the specific herbicide label and the most-recent South Dakota Pest Management Guide – Alfalfa and Oilseeds to determine usage and application restrictions.
