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Glyphosate‐resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) is widespread in the Central Great Plains. Introduction of newly developed dicamba/glufosinate/glyphosate (DGG)‐resistant soybean varieties allows postemergence (POST) applications of dicamba and glufosinate for in‐season control of GR Palmer amaranth. Limited information exists on the effectiveness of glufosinate applied late‐POST for tall (70–90 cm) GR Palmer amaranth control in DGG‐resistant soybean. The objectives of this study were to (1) determine the effectiveness of late‐POST glufosinate‐based programs for GR Palmer amaranth control, and (2) determine the impact of those programs on soybeans grain yields. Ten glufosinate‐based programs were tested in a field study at Kansas State University Agricultural Research Center near Hays, Kansas. Results indicated that single (655 or 737 g ha−1) and all sequential (594 followed by [fb] 594, 655 fb 594, and 737 fb 594 g ha−1) applications (7‐days apart) of glufosinate provided 87%–93% control of GR Palmer amaranth 28 days after last POST (DALPOST). Palmer amaranth control with single late‐POST application of glufosinate (594 g ha−1) or glufosinate plus S‐metolachlor did not exceed 84% at 28 DALPOST. Majority of the evaluated programs reduced shoot dry weights of GR Palmer amaranth by 83%–91%. The least control (11%) and shoot dry weight reduction (33%) of GR Palmer amaranth were observed with glyphosate fb glyphosate. Glufosinate‐based programs resulted in soybean grain yield of 626–701 kg ha−1. These results conclude that glufosinate applied late‐POST may provide effective control of tall GR Palmer amaranth in DGG‐resistant soybeans. Core Ideas: Late‐POST glufosinate‐based herbicide programs were evaluated in dicamba/glufosinate/glyphosate (DGG)‐resistant soybeans.Single (655 or 737 g ha−1) and all sequential late‐POST glufosinate programs provided 87%–93% control of GR Palmer amaranth.The majority of glufosinate‐based programs reduced shoot dry weights of GR Palmer amaranth by 83%–91%.Late‐POST glufosinate programs had 56%–61% higher soybean yield compared to glyphosate followed by glyphosate. [ABSTRACT FROM AUTHOR]

Increasing reports of herbicide‐resistant Palmer amaranth (Amaranthus palmeri S. Watson) pose a serious management concern for Kansas corn (Zea mays L.) producers. The objectives of this study were to evaluate various herbicide programs, including preemergence (PRE) alone, PRE followed by (fb) early postemergence (EPOST), and PRE fb late POST (LPOST) for control of glyphosate‐ and mesotrione‐resistant (GMR) Palmer amaranth in glyphosate‐ and glufosinate‐resistant corn, and their effect on crop injury, yield, and net economic return. Field experiments were conducted in 2018 and 2019 in grower fields with natural infestations of GMR Palmer amaranth near Seward in Stafford County, KS. Preemergence programs, including dicamba + thiencarbazone‐methyl + atrazine and dicamba + [atrazine, bicyclopyrone, mesotrione, S‐metolachlor (ABMS)] or dicamba + ABMS in mixtures with atrazine, mesotrione, S‐metolachlor, and metribuzin provided 89–93% control of GMR Palmer amaranth 3 wk after PRE. Preemergence fb EPOST programs provided 87–96% control of GMR Palmer amaranth at 2 wk after EPOST through 7 wk after LPOST. In comparison, PRE‐only programs provided 62–82% and PRE fb LPOST programs provided 75–86% control. The various herbicide programs resulted in greater corn grain yield ranging from 9,207 to 10,508 kg ha–1 compared with 6,056 kg ha–1 in the nontreated control. The highest net return ($1,300 ha–1) was achieved from dicamba + ABMS + atrazine applied PRE fb EPOST application of ABMS + atrazine. These results suggest that effective PRE fb EPOST herbicide programs consisting of multiple sites of actions are available and provide effective control of GMR Palmer amaranth in glyphosate‐ and glufosinate‐resistant corn. Core Ideas: Herbicide programs were evaluated for controlling multiple herbicide‐resistant Palmer amaranth.PRE followed by early POST programs provided 87–96% control of Palmer amaranth.The majority of programs protected against corn yield losses of 34–41% and improved net return. [ABSTRACT FROM AUTHOR]

BACKGROUND Evolution and spread of resistance to glyphosate in kochia [Bassia scoparia (L.) A.J. Scott] is a major challenge for the sustainability of glyphosate‐resistant crop technology in this region. Dicamba offers a viable option to manage glyphosate‐resistant kochia. However, the recent and rapid evolution of dicamba resistance in glyphosate‐resistant kochia populations in Kansas (KS), and other states in the USA is a threat to the management of this weed. Our previous research suggests that two distinct mechanisms confer dicamba resistance in KS (KSUR) and NE (CSUR) kochia. CSUR kochia is dicamba‐resistant due to a double mutation in an auxin and dicamba coreceptor gene (Aux/IAA16), and CSUR kochia plants show reduced dicamba translocation. However, the mechanism of dicamba resistance in KSUR is not known. The objective of this research was to determine if dicamba resistance in KSUR is due to a different mechanism and therefore evolved independently from CSUR by measuring whether the resistance traits are chromosomally linked. RESULTS: The F1 and F2 progenies from KSUR × CSUR were generated. Single dicamba rate tests were conducted using the F1 and F2 progeny. The results indicate that two different genes confer dicamba resistance in KSUR and CSUR; importantly, these two genes are not linked. CONCLUSION: This research provides evidence that different populations of kochia have independently evolved resistance to dicamba by different mechanisms, and we confirmed that the genes conferring resistance to the same herbicide in different populations are not chromosomally linked. [ABSTRACT FROM AUTHOR]

A field survey was conducted in 2014 to determine the response of Palmer amaranth (Amaranthus palmeri S. Wats.) populations from 16 counties in southcentral Kansas to commonly used herbicides. The main objective was to evaluate the sensitivity of 28 randomly collected populations to field‐use rates of glyphosate, chlorsulfuron, 2,4‐D, dicamba, atrazine, and mesotrione herbicides; and further characterize the sensitivity levels in two populations. The response of all tested populations was categorized as less sensitive (≤59% injury), moderately sensitive (60–89% injury), and susceptible (90–100% injury) based on visible injury at 21 days after treatment (DAT). Results from herbicide screening experiments indicated that 47, 36, 7, 75, and 32% of tested populations were less sensitive (putative resistant) to glyphosate, chlorsulfuron, 2,4‐D, atrazine, and mesotrione herbicide, respectively. None of the tested populations showed less sensitivity (≤59% injury) to dicamba herbicide. Whole plant dose‐response assays on two populations collected from Kiowa (KW2) and Pratt (PR8) counties indicated that the KW2 population had 3.0‐, 2.3‐, 5.8‐, and 8.4‐fold less sensitivity to 2,4‐D, chlorsulfuron, atrazine, and mesotrione, respectively, compared to a known susceptible (MHS) population. In contrast, the PR8 population was 2.0‐, 8.6‐, 10.6‐, 3.7‐, and 2.8‐fold less sensitive to 2,4‐D, glyphosate, chlorsulfuron, atrazine, and mesotrione, respectively, compared to MHS population. Overall, these results suggest that reduced sensitivity to glyphosate, chlorsulfuron, atrazine, and/or mesotrione is highly prevalent in southcentral Kansas. Growers should adopt diversified weed control methods to manage these difficult‐to‐control populations on their production fields. [ABSTRACT FROM AUTHOR]

BACKGROUND Amaranthus palmeri S. Wats is among the most problematic annual broadleaf weed species in the USA, including in Kansas. In late summer 2015, seeds of an A. palmeri population (MHR) that had survived field‐use rates of 2,4‐D were collected from Barton County, KS, USA. The main objectives were to: (i) confirm and characterize 2,4‐D resistance in a MHR population; (ii) characterize the resistance profile of the MHR population in relation to a multiple herbicide‐susceptible (MHS) population to glyphosate, chlorsulfuron, atrazine, mesotrione, fomesafen; and (iii) determine the effectiveness of alternative POST burndown herbicides for controlling MHR population. RESULTS: The MHR population had 3.2‐fold resistance to 2,4‐D. In addition, the MHR population also exhibited multiple resistance to glyphosate (11.8‐fold), chlorsulfuron (5.0‐fold), atrazine (14.4‐fold), and mesotrione (13.4‐fold). Furthermore, the MHR population also showed reduced sensitivity to fomesafen (2.3‐fold). In a separate study, dicamba with glyphosate, atrazine or fluroxypyr + 2,4‐D, and paraquat alone or with atrazine, metribuzin, saflufenacil or 2,4‐D provided ≥ 99% injury to the MHR population. Similarly, saflufenacil alone or with atrazine, metribuzin or 2,4‐D, and glufosinate alone or with glyphosate + 2,4‐D, and glyphosate + dicamba, and a premix of bicyclopyrone + atrazine + mesotrione + S‐metolachlor also effectively controlled the MHR population. CONCLUSION: This research confirms the first global case of an A. palmeri population from Kansas with multiple resistance to 2,4‐D, glyphosate, chlorsulfuron, atrazine and mesotrione, and reduced sensitivity to fomesafen. Dicamba, glufosinate, paraquat, and saflufenacil alone or in tank‐mixtures with PRE herbicides effectively controlled this MHR population. © 2019 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Background: Evolution of multiple herbicide resistance in weeds is a serious threat to weed management in crop production. Kochia is an economically important broadleaf weed in the U.S. Great Plains. This study aimed to confirm resistance to four sites of action of herbicides in a single kochia (Kochia scoparia L. Schrad.) population from a crop field near Garden City (GC), Kansas, and further determine the underlying mechanisms of resistance., Results: One-fourth of the GC plants survived the labeled rate or higher of atrazine [photosystem II (PSII) inhibitor], and the surviving plants had the Ser-264 to Gly mutation in the psbA gene, the target site of atrazine. Results showed that 90% of GC plants survived the labeled rate of dicamba, a synthetic auxin. At least 87% of the plants survived up to 72 g a.i. ha(-1) of chlorsulfuron [acetolactate synthase (ALS) inhibitor], and analysis of the ALS gene revealed the presence of Pro-197 to Thr and/or Trp-574 to Lue mutation(s). Most GC plants also survived the labeled rate of glyphosate [5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitor), and the resistant plants had 5-9 EPSPS gene copies (relative to the ALS gene)., Conclusion: We confirm the first case of evolution of resistance to four herbicide sites of action (PSII, ALS and EPSPS inhibitors and synthetic auxins) in a single kochia population, and target-site-based mechanisms confer resistance to atrazine, glyphosate and chlorsulfuron., (© 2015 Society of Chemical Industry.)