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Dicamba & 2,4-D As Fall Options?

Monday, February 1, 2021
filed under: Weeds

        Controlling wnter annual broadleaf weeds in the spring prior to planting a spring broadleaf crop like sunflower can be a real challenge for many producers.  Could that challenge be reduced by a fall application of dicamba or 2,4-D — without the risk of injury to the next season’s sunflower?
        That has been the premise behind a research project conducted by North Dakota State University weed scientists Caleb Dalley and Brian Jenks.  Dalley is based at NDSU’s Hettinger Research Extension Center in southwestern North Dakota, and Jenks is located at the NDSU North Central Research Extension Center, Minot.
        Their study, initiated in the fall of 2018 and continue into 2021 at the Hettinger location, has looked at not only sunflower, but also dry (field) pea, lentil and chickpea.  “Broadleaf weeds such as horseweed, prickly lettuce, narrow-leaf hawksbeard and mustard species can be difficult or impossible to control in-crop,” they point out in noting the need for an effective, inexpensive herbicide that won’t antagonize glyphosate and won’t carry over to injure the spring-planted crop.
        That brought them to consider the fall application of 2,4-D or dicamba.  “The 2,4-D label states that crops can be planted 30 days after application,” they note, while dicamba labels “are not clear on when specific crops can be planted.”  Another consideration with crop safety:  slower degradation of herbicides when the soil is cold or frozen.  So at present, planting spring broadleaf crops following a fall application of dicamba or 2,4-D “may be off-label, depending on when the herbicide is applied and when the crop is planted,” Dalley and Jenks point out.
        The primary objective of their 2018/19 and 2019/20 study years was to determine the response of these spring crops to fall application of 2,4-D and dicamba.  The NDSU weed scientists set out to evaluate injury to sunflower, dry pea, lentil and chickpea by measuring three components:  (1) crop density, to determine changes in plant stand, (2) crop height and (3) crop yield.
        The herbicide treatments — all of which were tank-mixed with glyphosate — consisted of the following:  dicamba at 4 oz/ac (0.125 lb ae/ac), dicamba at 8 oz/ac (0.25 lb ae/ac), 2,4-D ester LV6 at 10.9 oz/ac (0.475 lb ae/ac) and 2,4-D ester LV6 at 21.7 oz/ac (0.85 lb ae/ac).  The trials also included an untreated check for comparison.. 
        The results presented below are from the 2019/20 research trial period.  Though not reported here, results from 2018/19 were very similar.
        The Minot herbicide treatments were applied on October 8, 2019; the Hettinger treatments not until November 19 due to very wet fall conditions that year.
        At Hettinger, dry pea and lentil were planted on May 5, 2020, followed by chickpea on May 8 and sunflower on May 28.  At Minot, the planting dates were May 12 for dry pea, May 18 for lentil and May 27 for sunflower.  Chickpea was not part of the Minot trial.
        Evaluations for crop injury were carried out in early and late June at Hettinger for sunflower, dry pea and chickpea; the lentil evaluations were done in late May and early June.  Results showed no crop injury across all treatments for sunflower, dry pea and chickpea.  The lentils had no crop injury from the 2,4-D treatments; however, there was some modest injury from the two dicamba treatments.
        The Minot crop injury evaluations — conducted in late June and mid-July  of 2020 — showed no injury in the sunflower or dry pea.  The June lentil evaluations showed light injury from all four fall-applied herbicide treatments; however, no crop injury was detected during the mid-July evaluation.
        The fall-applied herbicides had no significant effect on crop stand or crop height on any of the spring-planted crops at either Hettinger or Minot.  (The sole exception was on sunflower at Hettinger, where the 2020 plant stand was actually higher in the lower-rate dicamba plot compared to the untreated check.)
        There was no significant effect on crop yield of dry pea, chickpea or lentil at Hettinger.  The sunflower portion of the 2020 Hettinger trial was not harvested due to excessive deer and bird damage.  At Minot, there was no significant effect on test weight, oil content or seed yield of sunflower.  The pattern was similar in the dry pea and lentil segments at Minot in terms of test weight, protein and yield.
        Dalley and Jenks suggest their study data to date indicates that “both dicamba and 2,4-D could be used to control winter annual weeds in the fall prior to planting sunflower, dry pea or chickpea,” with the caveat that “some caution is still needed when planting lentil following fall application of dicamba” as there is risk of crop injury.
        “The ability to apply these herbicides in the fall would greatly enhance the management of tough winter broadleaf [weeds] such as narrow-leaf hawksbeard or horseweed,” they reiterate.
        Part of the NDSU study’s objective has been to provide data to companies that could help “open the door” to amending labels should the data show good, consistent crop safety.  “Based on the years we’ve done this study to date, it appears that sunflower, dry pea and chickpea are tolerant to the fall applications,” Jenks states.  He adds, however, that “I would not be comfortable with fall dicamba going to lentil based on our 2018 and 2019 data.  2,4-D on fields going to lentil may be safe at lower rates.”
        The next step is for registrants of these herbicide to review the NDSU data and then determine whether they are willing to modify the labels to allow these applications and make the crop rotation intervals more clear.
          Dalley adds that he is planning to repeat the evaluations this year based on dicamba and 2,4-D treatments that were applied on November 2, 2020. — Don Lilleboe
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