Producers should soon begin planning their program for controlling kochia. The spread of glyphosate-resistant kochia populations throughout western Kansas, and the difficulty growers have had controlling these populations, suggest that control measures should begin prior to emergence of kochia.
Large flushes of kochia emerge in late February to early March and into April. If allowed to emerge, postemergence herbicide applications often will not provide adequate control. Incomplete control of these dense populations is likely in these situations. When the kochia is glyphosate-resistant and complete herbicide coverage is not possible, results can be very poor when trying to use postemergence products to control dense populations. The dense populations also might be stressed, which reduces the effectiveness of postemergence herbicide applications.
The choice of herbicides for effective pre-emergence control of kochia in February and early March will vary depending on subsequent cropping intentions. Various cropping scenarios are discussed below.
Components of the herbicide program to effectively manage kochia at germination
Each herbicide program needs to consist of two components. First, a very water soluble and effective herbicide that can be incorporated with little precipitation, i.e. dicamba. Second, an herbicide that has longer residual and requires perhaps 0.75 inches or more precipitation for adequate incorporation. During January or February, precipitation events often are on the light side with heavier precipitation events more common in the spring months. Dicamba might persist for four to six weeks and the longer residual herbicide will resume controlling kochia once incorporated and perhaps if dicamba residual runs out. Included below are herbicides by crop that have longer residual control.
The best timing for this application is January through the first week of March but prior to kochia emergence which can vary depending on weather conditions. The later into the season, the more likely it is there will be some small, emerged kochia, which increases the risk of control failure. If producers wait until later to apply the burndown and preemergence herbicide in the same application, the kochia will be larger and most likely will not be controlled.
Fields going to sorghum or corn
A combination of glyphosate (using a minimum of 0.75 lb ae per acre) with herbicides that have pre- and post- activity on kochia is most valuable. Tank mixing 8 to 16 ounces of dicamba with 1 to 2 pints of atrazine will control existing broadleaf and grass weeds, and will provide extended preemergence control of kochia often into May. An application of Clarity alone, suggests a pint provides better control than 8 ounces. However, a combination of atrazine and Clarity is better than Clarity alone.
Dicamba plus Corvus or Balance Flexx are good residual herbicides but should be mixed with a little atrazine. December applications also have been effective in managing kochia. Corvus+atrazine, Scoparia+atrazine and Atrazine+Clarity were among the best treatments. Scoparia contains Isoxaflutole as does Corvus and Balance Flexx; however, is not labeled ahead of corn planting. The 24c Special local need label for use of Scoparia to control kochia in fallow or ecofallow has a four-month plant-back restriction to corn and a six-month plant-back restriction to sorghum. Sencor (metribuzin) or Zidua both have activity on kochia and can be applied in December through February ahead of planting corn.
On fallow fields going to fall-planted wheat
Atrazine should not be used in this situation. Metribuzin (Sencor and multiple generics) is a triazine and can a substitute for atrazine and has a four-month plant-back restriction to wheat. Additional products include Scoparia, Authority MTZ and other products containing sulfentrazone. Zidua also has good activity but requires significant rainfall thus applying Zidua with dicamba is critical.
If growers miss this kochia emergence window, be advised that less-than-adequate post emergence control of kochia can be expected.
Fields going to wheat this fall
If kochia is emerging in row crop stubble intended to be planted to wheat this fall, herbicide options exist that provide residual kochia control. Atrazine cannot be used in this situation, as this treatment is off-label. The following herbicides could provide effective residual control of kochia for fields to be planted to wheat this fall: dicamba, metribuzin or Dimetric, Corvus, Balance Flexx, Scoparia (equal to Balance Pro) and Lumax EZ. These products allow wheat to be planted four months following application.
These treatments can be effective when made prior to kochia emergence. A November application of one pound of atrazine was effective through June 12. However, this treatment is labeled only if corn or sorghum will be planted the following year. The November application of Corvus was not adequate. The addition of metribuzin to Corvus would have improved kochia control. HPPD inhibitors always should be applied with a triazine. Only metribuzin, which is a triazine, can be applied in the late fall or early spring when wheat will be planted in the fall. February and March applications of Corvus and metribuzin were very similar and effective. This suggests if weather cooperates and a window for application is available in February, getting these early treatments applied at that time could be beneficial.
• Information provided by Curtis Thompson and Dallas Peterson, K-State Extension weed control specialists.
Stacy Campbell is a Kansas State Research and Extension agent in Hays for the Cottonwood Extension District Office.