This is the third and final article in a series discussing pre-emergence herbicides for kochia control. This week we will discuss recommendations specific to fields that will be planted to soybean or sunflower this spring and wheat in the fall. Previous articles have discussed general considerations for late-winter kochia control (https://bit.ly/2MWvSrD - Issue 837) and pre-emergence herbicides for kochia control in fields that will be planted to corn or grain sorghum (https://bit.ly/3tKKZp3 - Issue 838).
Fields going to soybeans
Start in February or early March with a tank-mix of glyphosate (using a minimum of 0.75 lb ae/acre) or Gramoxone SL (minimum of 2 pts/acre) and 8 to 16 oz/acre of Clarity prior to kochia emergence. The use of Clarity requires a minimum accumulation of 1 inch of rain and then 28 days prior to planting soybeans, except in case of Roundup Ready 2 Xtend or XtendFlex soybeans. As indicated by the label, Clarity cannot be used as a pre-plant treatment in soybeans in areas with less than 25 inches of annual rainfall. Paraquat tank-mixed with metribuzin (Dimetric, others) will provide extended residual control of kochia, as long as the population of kochia is susceptible to triazine herbicides. Be aware of rate restrictions for metribuzin in western KS, as soil and environmental characteristics influence the potential for soybean injury following metribuzin.
Sulfentrazone-based products (Spartan, others) could also be considered for use prior to kochia emergence to manage an early flush of kochia (Figures 1 and 2). However, it’s important to note the crop rotation restrictions on these products. Pyroxasulfone (Zidua) also has activity on kochia, although more rain is required for activation. Figure 1 illustrates the efficacy of various pre-emergence herbicide programs for controlling glyphosate- and dicamba-resistant kochia in Roundup Ready 2 Xtend soybeans planted in no-till dryland fields at Hays, KS. These treatments were applied to emerged kochia on May 23. All treatments also included Roundup PowerMax.
Figure 1. Kochia control following pre-emergence herbicide application in no-till dryland soybean in Hays, KS (WAA= weeks after application). Data collected by Vipan Kumar, K-State Research and Extension.
Figure 2. Kochia control in non-treated plot (A) and with PRE applied Spartan (B) in Roundup Ready 2 Xtend soybean at 7 weeks after treatment (WAT). Photos by Vipan Kumar, K-State Research and Extension.
Fields going to sunflowers
Planting sunflowers into a clean seedbed is a key step to achieving good season-long control of all broadleaf and grassy weeds. But, it is especially important for getting good control of any weed populations, such as kochia, that are resistant to glyphosate or ALS-inhibiting herbicides and cannot be controlled with post-emergence herbicides-applied herbicides in sunflower.
The best approach to control ALS/glyphosate-resistant kochia in sunflower is to start in February/early March with a tank-mix of Gramoxone (using a minimum of 2 pts/acre) and Spartan, Spartan Charge (sulfentrazone+Aim), Broadaxe or Authority Elite (sulfentrazone+Dual Magnum), or Authority Supreme/Authority Edge (sulfentrazone+Zidua) before kochia begins to germinate. Select pre-emergence products that are effective on kochia and apply additional pre-emergence herbicides at planting to extend control of kochia and other weeds. Dicamba is not an option in these applications, due to label restrictions. Monitor fields closely as additional Gramoxone SL treatments may be required prior to sunflower planting.
Fields going to fall-planted wheat
If kochia is emerging in fields to be planted to wheat this fall, atrazine cannot be used. Metribuzin can substitute for atrazine and has a 4-month plant-back restriction to wheat. Additional products include Scoparia or Authority MTZ and products containing sulfentrazone or isoxaflutole. Zidua also has good activity but requires significant rainfall for activation, so it should be applied with dicamba.
Sarah Lancaster, Weed Management Specialist
Vipan Kumar, Research Weed Scientist