A North Dakota State University study showed that some kochia populations in western North Dakota likely have developed resistance to commonly used preplant burndown herbicides.
Resistant Kochia Discovered
For many years, no-till farmers have used Aim (carfentrazone) and Sharpen (saflufenacil) either just prior to planting or after planting to control emerged kochia and other annual weeds.
“Kochia has been difficult to control during the prolonged drought of the past several years,” says Brian Jenks, weed scientist at the NDSU North Central Research Extension Center. “Kochia thrives in dry conditions, and herbicides can be less effective when plants are drought-stressed. However, the NDSU study showed that recent lack of control is not due solely to drought stress, since plants survived these herbicides with little damage in the greenhouse.”
Aim and Sharpen are classified as Group 14 herbicides that control weeds by inhibiting the protoporphyrinogen oxidase (PPO) enzyme, which leads to disruption of plant cell membranes. Susceptible weeds typically die within a few days.
In the NDSU study, a known susceptible kochia population was easily controlled by Aim and Sharpen. However, Aim showed very little activity on four kochia populations from across western North Dakota. Sharpen caused some necrosis on kochia leaves and stunted growth, but most plants survived and had 2- to 8-inches of re-growth 2 weeks after treatment.
“The potential loss of Aim and Sharpen as effective herbicides for kochia control is staggering because affected farmers will have limited control options remaining,” says Jenks.
In addition, Aim is a component of other common herbicides like Spartan Charge (Aim + Spartan) and Anthem Flex (Aim + pyroxasulfone), which rely on Aim to effectively control emerged weeds. Sharpen also is a component of products like Verdict (Sharpen + Outlook) and Zidua Pro (Sharpen + Pursuit + Zidua) that rely on Sharpen to control emerged weeds.
Alternatives to Aim and Sharpen
Other products that can be used as alternatives prior to some crops to control emerged kochia include Gramoxone, Liberty and Dicamba. Gramoxone can be used preplant or preemergence in most crops; however, Liberty and Dicamba can only be used in a limited number of crops. Gramoxone tank mixed with metribuzin also has shown good synergism to control emerged kochia as well as provide some residual control. However, Metribuzin can only be used in a limited number of crops.
“An extremely important question that still needs to be answered is the effectiveness of other Group 14 herbicides like Spartan and Valor that are used for residual kochia control,” says Jenks. “Spartan and Valor are used in many crops to control kochia by root uptake just after seed germination, rather than foliar control after the weed emerges. Based on experience with other weeds, there is a chance that Valor and Spartan may still provide acceptable residual kochia control.”
Some waterhemp and Palmer amaranth populations have been shown to be resistant to foliar-applied Group 14 herbicides used in soybean such as Flexstar and Cobra. However, research has shown that Valor and Spartan still provide effective control or only slightly reduced control of waterhemp or Palmer amaranth populations resistant to foliar-applied PPO-inhibitors. Currently, it is not known how the kochia populations in western North Dakota will respond to the residual Group 14 products, but it will be a focus of future NDSU research, says Jenks.
NDSU recommends monitoring fields 3-5 days after applying preplant burndown herbicides to verify that weeds are being controlled. Kochia should be sprayed when small (less than 3 inches). Where possible, use multiple effective modes of action in a tank mix for burndown or postemergence applications.
For more information on controlling kochia, visit this webpage or contact your local county office of NDSU Extension.