Be Aware of Evolving Weed Resistance to Herbicides or The Complexity of Controlling Palmer Amaranth with Herbicides

Two recent journal articles objectively portend what has been and is being preached regarding the development of weed resistance to herbicides. The results reported in these articles are from two studies that were conducted independently of each other in the adjoining states of Kansas and Arkansas, and they command the undivided attention of soybean producers.

Results from the Arkansas study are reported in Weed Technology 33:610-615 (2019) (doi:10.1017/wet.2019.37). The article, titled “Field evaluation of preemergence and postemergence herbicides for control of protoporphyrinogen oxidase-resistant Palmer amaranth (Amaranthus palmeri)”, is authored by Houston, Norsworthy, Barber, and Brabham. Highlights of the conduct of and results from the study follow.

•   Field experiments were conducted in 2016 and 2017 at two Arkansas locations.

•   Each site-year contained a PRE-only and a POST-only experiment to determine herbicide efficacy on multi-resistant Palmer amaranth. All experiments were established into a crop-free environment.

•   Herbicides labeled for use on soybean, cotton, and corn were applied at typical rates.

•   Visible control ratings and ratings of weed density reductions were made on a 0% (no control) to 100% (complete weed mortality) scale.

•   In the PRE study, 15 herbicides (4 applied at 2X rate) were applied to weed-free ground where winter weeds had been controlled. At each location, rainfall of 0.5 in. or more was received within 1 to 2 weeks of PRE treatments.

•   In the POST study, 15 herbicides (3 applied at 1.4-2X rate) were applied to Palmer amaranth that was no more than 4 in. tall.

•   Both sites had been in continuous soybean production prior to these studies, and PPO-resistant Palmer amaranth was documented at each location.

•   In the PRE studies, herbicide efficacy was rated at 28 and 35 days after treatment (DAT) since this is typically within the timeframe that POST herbicides should be applied.

•   In the PRE study, only atrazine (91%–Group 5) and Zidua (88%–Group 15) provided greater than 85% control of Palmer amaranth at 28 DAT. Control by both of these herbicides declined at 35 DAT. Metribuzin, a Group 5 herbicide labeled for soybeans, provided only 78% control at 28 DAT.

•   PRE herbicides in Groups 2, 3, 4, 7, 14, and 27 provided <85% control at 28 DAT, and control by all of these herbicides declined precipitously by 35 DAT.

•   All of the ratings for PRE herbicides’ control of Palmer amaranth at 28 and 35 DAT indicate that an effective POST weed control program, in addition to the best PRE program, must be planned to control multi-resistant Palmer amaranth.

•   In the POST study, paraquat (Group 22) provided 91% control at 7 DAT and 75% control at 14 DAT. Of course, this herbicide cannot be applied in over-the-tip POST applications to soybeans.

•   Of the herbicides labeled for POST applications to soybeans, only glufosinate (Liberty–Group 10) at a 1.4X rate provided >80% control at 7 DAT, and that declined to 57% at 14 DAT. At 14 DAT, no treatment provided greater than 75% control.

•   Results from the POST study indicate that reliance on POST herbicides alone for control of multi-resistant Palmer amaranth is not an acceptable weed control program and will result in poor control of multi-resistant Palmer amaranth.

Results from the Kansas study are reported in Agronomy Journal 2020:1-12 (doi:10:1002/agj2.20178). The article, titled “Differential sensitivity of Kansas Palmer amaranth populations to multiple herbicides”, is authored by Kumar, Liu, and Stahlman, Highlights of the conduct of and results from the study follow.

•   Fully mature seeds of Palmer amaranth populations (28 total) from 16 counties in southcentral Kansas were selected during the fall of 2014. Seeds were primarily collected from corn, sorghum, soybean, sunflower, wheat, and fallow fields.

•   Seeds of each Palmer amaranth population were sown in trays in a greenhouse, and field use rates (1X) of 2,4-D (Defy LV-6–Group 4), glyphosate (Roundup PowerMaxx–Group 9), chlorosulfuron (Glean XP–Group 2), dicamba (Clarity–Group 4), atrazine (AAtrex 4L–Group 5), and mesotrione (Callisto–Group 27) were applied to emerged seedlings when they were 2.5-3.5 in. tall.

•   Percent visible injury for each population by herbicide combination was rated on a 0% (no injury) to 100% (plant death) scale at 21 days after treatment (DAT). Populations were rated as resistant (≤49% injury), less sensitive (50-89% injury), and susceptible (90-100% injury). Shoot biomass of plants was weighed immediately after injury ratings were recorded.

•   Across the 28 Palmer amaranth populations used in the study, visual injury ratings ranged from 8-100% for glyphosate (Group 9), and 14-90% for Glean (Group 2). The average shoot biomass reductions followed a similar trend. The majority of tested populations were either less sensitive or moderately sensitive to these herbicides. Thus, these populations are in advanced stages of evolvement toward resistance to these herbicides.

•   Across all 28 populations of Palmer amaranth, average visible injury ratings ranged from 58-100% for 2,4-D and 71-100% for dicamba (both auxinic Group 4 herbicides). Only 11% and 14% were 100% susceptible to 2,4-D and dicamba, respectively. Average shoot biomass reductions followed a similar trend. 54% and 29% of the tested populations were sensitive to 2,4-D and dicamba, respectively. Less than 10% of the tested populations showed less sensitivity to these two Group 4 herbicides. Thus, they are in the early stages of evolvement toward resistance to these herbicides.

•   Among the tested populations, injury estimates at 21 DAT averaged 11-78% for atrazine (Group 5) and 17-100% for Callisto (Group 27). None of the populations were totally susceptible to atrazine. 18% if the populations were susceptible to Callisto.

•   Results from these experiments indicate a variable response among Palmer amaranth populations to each of the tested herbicides, with reduced sensitivity to 2-3 of the herbicide MOA’s tested. Specifically, less sensitivity (≤59% injury) to the Groups 2, 5, 9, and 27 herbicides used in this study was evident.

•   These results corroborate the high genetic diversity within and among field populations of this weed–i.e., not all Palmer amaranth populations are equally sensitive or resistant to the classes of herbicides used in the study. Thus, these results indicate a varying level of resistance development among Palmer amaranth populations to these herbicides. In particular, the results demonstrate that reduced sensitivity to 2,4-D is in the early stage of evolution toward resistance to this herbicide.

•   These results suggest that soybean growers should adopt a diversified approach to management of Palmer amaranth, especially since these results indicate an already-present reduced sensitivity among some of the weed populations to the newly-developed 2,4-D- and HPPD-resistant crop technologies (e.g. Enlist and Balance GT soybeans).

Final Notes.

•   Suppression of multi-resistant Palmer amaranth (and other weeds as well) is possible with either PRE or POST herbicides from multiple MOA Groups, but not as stand-alone options. An effective weed control program that depends solely on herbicides must include both PRE and POST applications of efficacious herbicide premixes and tank mixes to both control multi-resistant weeds and to minimize selection pressure for additional resistance development. Also, overlapping efficacious residual herbicides in both PRE and POST applications is key to providing season-long control of this problematic weed.

•   All of the above results underline the importance of scouting fields and eliminating Palmer amaranth survivors in order to protect the sustainability of present and newly-developed weed control technologies.

•   These results place increasing importance on the potential of HWSC to become an integral part of a complete weed control program in order to reduce the soil weed seedbank of problematic weeds that have evolved or are continuing to evolve toward resistance to current and new HR technologies.

•   Where a Group 27 herbicide such as Balance or Callisto has been applied to corn in a corn-soybean rotation, it is likely that the new Group 27 herbicide Alite 27 that is labeled for Balance GT soybean varieties may not be as effective as hoped if selection against this herbicide MOA is already present or is developing in the weed populations that infest both crops.

Composed by Larry G. Heatherly, May 2020, larryheatherly@bellsouth.net