Pre-plant herbicide applications for kochia control
Now is the time to finalize plans for kochia control. Major flushes of kochia emerge in late February and continue through early April, resulting in dense populations that make adequate herbicide coverage difficult (Figure 1). In addition, glyphosate-resistant kochia is prevalent across western Kansas, making kochia control even more challenging. For these reasons, it is important to apply pre-emergence herbicides in late winter or early spring to control this weed before it emerges.
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.
Note: All graphs in this article were created by Curtis Thompson (retired Extension Weed Science Specialist) based on data from irrigated plots at the K-State Southwest Research-Extension Center at Tribune, and with populations of kochia that are susceptible to triazine herbicides. The kochia at this site is a mixed population of glyphosate-resistant and susceptible plants.
Figure 1. Untreated kochia seedlings amid residue. Photo by Curtis Thompson, K-State Research and Extension.
Components of the herbicide program to effectively manage kochia at germination.
Each herbicide program needs two components. First, a very soluble and effective herbicide that can be incorporated with very little precipitation, such as dicamba. Second, an herbicide that has longer residual activity, which will require perhaps 0.75 inches or more precipitation for adequate incorporation, such as atrazine. Precipitation events during late winter are often too small to activate longer residual herbicides, but dicamba may control kochia for 4 to 6 weeks until the longer residual herbicide is incorporated and. 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. Later applications, for example, at the time of burndown, are more likely to occur after kochia emergence, which increases the risk of control failure (Figure 2). Fall-applied treatments can help ensure timely application. For example, application of one pound of atrazine can be effective through mid-June in fields planted to corn (Figure 3).
Figure 2. EPP/POST herbicides applied March 10, 2015 for kochia control at Tribune, KS. Kochia at cotyledon stage.
Figure 3. December 20th applied herbicide treatments for kochia control at Tribune, KS during 2015-16.
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 oz of dicamba with 1 to 2 pints of atrazine will control existing broadleaf and grass weeds, and will provide extended pre-emergence control of kochia often into May. An application of dicamba alone can control kochia; however, a combination of atrazine and dicamba is better than dicamba alone (Figure 4).
Corvus or Balance Flexx are good residual herbicides, but should be mixed with atrazine. Corvus+atrazine, Scoparia+atrazine, and atrazine+dicamba (Clarity) were among the best treatments in the experiment shown in Figure 5. Scoparia contains isoxaflutole, as do Corvus and Balance Flex; however, Scoparia 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 4-month plant-back restriction to corn and a 6-month plant-back restriction to sorghum.
Figure 4. Early preplant herbicides applied March 16, 2012 for kochia control at Tribune, KS
Figure 5. Herbicide treatments applied February 15 for kochia control at Tribune, KS in 2016.
Fields going to sunflowers
Planting sunflower 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-inhibitor herbicides and cannot be controlled with POST applied herbicides in sunflower.
The best approach to kochia control in sunflower is to start in February/early March with a tankmix of glyphosate (using a minimum of 0.75 lb ae/are) and Spartan (sulfentrazone), Spartan Charge (sulfentrazone+Aim), Broadaxe or Authority Elite (sulfentrazone+Dual Magnum), or Authority Supreme (sulfentrazone+Zidua) before kochia begin to germinate. Figure 6 indicates that 6 oz of Spartan controlled kochia very effectively in the Tribune experiments up to early June. Select pre-emergence products that are effective on kochia and apply 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 glyphosate or Gramoxone SL treatments may be required prior to sunflower planting.
Fields going to soybeans
The best management strategy for controlling kochia in soybeans is similar to the control strategy for sunflower, but there are more herbicide options for soybeans. Start in February or early March with a tankmix of glyphosate (using a minimum of 0.75 lb ae/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. As indicated in the label, Clarity cannot be used as a preplant treatment in soybeans in areas with less than 25 inches of annual rainfall. Paraquat tank-mixed with metribuzin (Dimetric, Metribuzin, Sencor and 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 could also be considered for use prior to kochia emergence to manage an early flush of kochia and may provide control into June. However, it’s important to note the crop rotation restrictions on these products. Zidua also has activity on kochia, although more rain is required for activation. For adequate kochia control with Zidua, use maximum labeled rates for your soil type. Flumioxazin (Valor)-based products have not provided adequate control of kochia (Figure 6).
Figure 6. Early preplant herbicides applied March 16, 2012 for kochia control at Tribune, KS.
Fields going to fall-planted wheat
If kochia is emerging in field 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