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New Twist in Kochia Battle

Wednesday, February 1, 2023
filed under: Weeds

kochia weeds in field
Photo: Phil Westra / Colorado State University

        Each year, North Dakota State University weed scientists designate one weed species as the “Weed of the Year.”  It’s not a term of endearment; rather, it speaks to that weed’s ongoing — or emerging — impact on the state’s agricultural community.
        In 2022, for the second time since 2016, NDSU’s “Weed of the Year” was kochia.  Not a big surprise there, considering that kochia has ranked among the state’s most troublesome — and expensive — weeds in NDSU surveys since 1978.  Kochia biotypes with resistance to ALS (Group 2) herbicides were identified by the late 1980s, and kochia with resistance to glyphosate is now quite common.  Adapted to drought conditions and saline soils, kochia grows prolifically in field locations where other plant species (crops) often provide little or no competition.
        Sunflower is no exception.  In a 1990 sunflower grower survey conducted by NDSU crop specialists, kochia was ranked the #1 weed problem by 42% of survey respondents and as the #1, #2 or #3 top weed problem by 66% of responding growers.  The crop survey (now biennial) has been conducted under the auspices of the National Sunflower Association since 2002, with kochia consistently ranking in the top tier of yield-limiting weeds in each survey, including the most recent one in 2021.
        Kochia’s notoriety flared even higher in North Dakota in 2022, as it became apparent that some kochia populations in western North Dakota have developed resistance to the commonly used preplant burndown herbicides Aim® (carfentrazone) and Sharpen® (saflufenacil).  With kochia having developed resistance previously to glyphosate, these other herbicides have played a very significant role in preplant and pre-emergence burndowns in a number of crops — sunflower prominently among them.
        In the past, “it used to be we could spray straight glyphosate to start out with a clean field,” notes Brian Jenks, weed scientist with the NDSU North Central Research Extension Center at Minot.  Now, though, “it’s probably more common than not to have kochia that’s resistance to glyphosate.”
        With glyphosate’s burndown reliability compromised, many North Dakota sunflower growers — no-till ones especially — turned to using Aim (FMC) and Sharpen (BASF) either just prior to planting or after planting to control kochia and other annual weeds.  Spartan®Charge (Spartan + Aim) is a primary herbicide mix in sunflower.  “The Spartan component with Aim burns down kochia,” Jenks affirms.
        Ryan Hunt, Northern Plains technical services manager for FMC, recounts that warning signs of resistance to Aim were popping up by early summer 2022 in western North Dakota.  “We started to notice a lot of kochia in certain fields, and we had more calls, more concerns, than in the past.  In every situation, it seemed it could be traced to the burndown having failed, for whatever reason. 
        “Right away, the ‘resistance’ word began circulating,” Hunt continues.  “We were initially skeptical, but agreed it was possible.”
        The heavy snows and resulting late spring planting in the region in 2022 tossed in an element of doubt — at least initially.  “The weeds were pretty big in a lot of those fields when they were getting burned down, so we thought that could be part of the problem, too,” he says.
        Circling back in the fall, Hunt visited four sites that were “head-scratchers.”  Everything had been done right management-wise; the kochia should have been dead.  “But we saw kochia there.” 

        Samples from three of the western North Dakota sites — Berthold, Mandan and Mott — were delivered to Brian Jenks in Minot.  He conducted greenhouse tests with those samples, as well as with another one from Minot and a fifth that was known to be susceptible to Aim. 
        Jenks’ growouts, replicated three times, were sprayed with Aim at the 1x (1 fl oz) and 2x (2 oz) rates with an AMS + MSO.  The kochia plants were 2.0 to 2.5 inches tall at the time.  The accompanying two photos show the five populations at six and 13 days after treatment, respectively.
These western North Dakota kochia populations were treated with Aim herbicide, with AMS + MSO, at the 1x and 2x rates (1 fl oz and 2 fl oz, respectively) in 2022 in the greenhouse at the NDSU North Central Research Extension Center, Minot.  The sample at left is a kochia population known to be susceptible to Aim.  The test was replicated three times.  This photo shows the populations at six (6) days after treatment.
The same samples at 13 days after treatment.

        Aim showed very little activity on the four suspect kochia populations.  A parallel greenhouse study found that Sharpen caused some necrosis on kochia leaves and stunted growth; however, most of those plants survived and had two to eight inches of regrowth within two weeks following treatment.
        The implications are huge.  “The potential loss of Aim and Sharpen as effective herbicides for kochia control is staggering,” Jenks affirms, “because affected farmers will have limited control options remaining.”
        A parallel concern is that Aim and Sharpen are components of other common herbicides.  In addition to Spartan Charge (Aim + Spartan), there is Anthem Flex (Aim + pyroasulfone).  Sharpen is a component of such herbicides as Verdict (Sharpen + Outlook) and Zidua Pro (Sharpen + Pursuit + Zidua).
        One of the most effective remaining burndown options is Gramoxone, which can be used preplant or pre-emergence in most crops, including sunflower.  The problem with using Gramoxone for burndown this year, though, may be one of supply.  “Gramoxone is used quite a bit,” FMC’s Hunt observes, “but it’s used in August or September for desiccation.  So a lot of companies don’t have a lot of it produced yet (for 2023).  If growers are wanting to use Gramoxone, they’ll need to get in touch with their dealer as soon as possible.”
        As to this coming spring, can sunflower growers in those areas not yet seeing Aim- or Sharpen-resistant kochia still use these products with confidence?  “Yes,” says Hunt.  “But I would definitely recommend you add something else with it, depending on the crop.  Gramoxone is going to be the most common, if you can procure it.”
        Meanwhile, the search is on for other options for sunflower field weed management.  NDSU’s Jenks has begun investigating sunflower’s tolerance to several non-registered preplant herbicides. Most of the herbicides tested in 2022 caused no visible injury to sunflower.
        Jenks will be repeating the sunflower tolerance study in 2023 to confirm crop safety.  He also will be continuing to evaluate kochia control among this group of preplant products as well as the current group of preplant/pre- herbicides.
        “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,” Jenks observes.  “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.”  The hope is that Valor and Spartan will provide acceptable residual kochia control.  That will be closely monitored in ongoing winter greenhouse testing and in the field this upcoming season.
        For now, NDSU recommends monitoring fields three to five days after applying preplant burndown herbicides to verify that weeds are being controlled.  Kochia should be sprayed when still small (less than three inches tall). 
        Where possible, utilize multiple effective modes of action in a tank mix for burndown or postemergence applications, Jenks advises.
Photo: Ryan Hunt, FMC
       FMC’s Ryan Hunt echoes that advice.  “Going into ’flowers, you need to have multiple modes of action to make sure you cover your bases,” he emphasizes.  “Obviously you’re going to want the glyphosate in there for the majority of other weed species.  And you need some Gramoxone in there — if you can get it.
        “Otherwise, there’s not much else labeled besides Aim,” Hunt continues.  “That’s still going to work great on some fields.  But even if you don’t have the [kochia resistance] problem yet, you’d better get another mode of action or two in there just to prevent it from developing — and to give that residual the best chance to work.”
        One final point with sunflower is the huge importance of good weed control in other rotational crops, thus starting the sunflower year out with as clean a field as possible.  “Sunflower growers definitely need to stay on top of it in their small grains, corn, soybeans — whatever else they’re producing,” Hunt stresses. — Don Lilleboe
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