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BACKGROUND Florpyrauxifen‐benzyl (FPB) is an arylpicolinate herbicide (Group IV) for barnyardgrass control in rice. One susceptible (Sus) and three putative FPB‐resistant (R1, R2, and R3) barnyardgrass biotypes were selected based on resistant/susceptible (R/S) ratios obtained from dose–response tests and used to investigate the potential resistance mechanisms. RESULTS Based on visual control results, the R/S ratios of barnyardgrass biotypes R1, R2, and R3 were 60‐, 33‐, and 16‐fold greater than the Sus standard, respectively. Sequencing results of TIR1 and AFB genes in the tested barnyardgrass revealed no difference between Sus and R barnyardgrass biotypes. Absorption of [14C]‐FPB in Sus barnyardgrass increased over time and reached 90%, which was >10 percentage points greater than that in R biotypes. The [14C]‐FPB absorption in all R barnyardgrass equilibrated after 48 h. For both Sus and R barnyardgrass, most [14C]‐FPB absorbed was present in the treated leaf (79.8–88.8%), followed by untreated aboveground (9.5–18.6%) and belowground tissues (1.3–2.2%). No differences in translocation were observed. Differences between Sus and R barnyardgrass biotypes were found for FPB metabolism. Production of the active metabolite, florpyrauxifen‐acid, was greater in Sus barnyardgrass (21.5–52.1%) than in R barnyardgrass (5.5–34.9%). CONCLUSION In conclusion, reductions in FPB absorption and florpyrauxifen‐acid production may contribute to the inability to control barnyardgrass with FPB. © 2021 Society of Chemical Industry.

The ability of weed populations to evolve resistance to herbicides impact management strategies and profitability of crop production. The objective of this research was to screen Palmer amaranth accessions from Arkansas for glufosinate resistance. Additional efforts focused on the effectiveness of various herbicides, across multiple sites of action (SOA), on each putative-resistant accession. The three putative accessions were selected from 60 Palmer amaranth accessions collected in 2019 and 2020 and screened with to 0.5x and 1x rates of glufosinate. A dose-response experiment was conducted for glufosinate on accession A2019, A2020, and B2020. The effectiveness of various preemergence- and postemergence-applied herbicides were evaluated on each accession. Resistance ratios of A2019, A2020, and B2020 to glufosinate ranged from 5.1 to 27.4 when comparing LD50 values to two susceptible accessions, thus all three accessions were resistant to glufosinate. All three accessions (A2019, A2020, and B2020) were found to have a reduction equal to or greater than 20-percentage points in mortality to at least one herbicide from five different sites of action equal to or greater than 5 sites of action. Herbicides from nine different sites of action controlled A2019 at least 20 percentage points less than the susceptible accessions, which points to a need for additional research to characterize the response of this accession.

Rapid crop canopy formation is important to reduce weed emergence and selection for herbicide resistance. Field experiments were conducted in 2017 and 2018 in Fayetteville, AR, to evaluate the impacts of PRE applications of flumioxazin on soybean injury, soybean density, canopy formation, and incidence of soil-borne pathogens. Flumioxazin was applied at 0, 70, and 105 g ai ha−1 to predetermined flumioxazin-tolerant and -sensitive soybean varieties. Flumioxazin at 70 g ha−1 injured the tolerant and sensitive varieties from 0% to 4% and 14% to 15%, respectively. When averaged over flumioxazin rates, density of the sensitive variety was only reduced in 2017 when activation of flumioxazin was delayed 7 d. Compared to the tolerant soybean variety, flumioxazin at 70 g ha−1 delayed the sensitive variety from reaching 20%, 40%, 60%, and 80% groundcover by 15, 16, 11, and 5 d, respectively. No delay in canopy closure (95% groundcover) was observed with either variety. Consequently, no yield loss occurred for either variety following a flumioxazin application. Flumioxazin did not impact root colonization of Didymella, Fusarium, Macrophomina, or Rhizoctonia. Pythium colonization of the soybean stem was increased by flumioxazin in 2017, but not in 2018. Increased injury, delays in percent groundcover, and an increase in Pythium colonization of soybean following a flumioxazin application may warrant the need for other soil-applied herbicides at soybean planting. Alternatively, soybean injury and delays in percent groundcover following flumioxazin applications can be mitigated through appropriate variety selection; however, comprehensive screening is needed to determine which varieties are most tolerant to flumioxazin.

In current and next-generation weed control technologies, sequential applications of contact and systemic herbicides for POST control of troublesome weeds are needed to mitigate the evolution of herbicide resistance. A clear understanding of the impact auxin herbicide symptomology has on Palmer amaranth groundcover will aid optimization of sequential herbicide applications. Field and greenhouse experiments were conducted in Fayetteville, AR and a laboratory experiment was conducted in Lonoke, AR, in 2020 to evaluate changes in Palmer amaranth groundcover following an application of 2,4-D and dicamba with various nozzles, droplet sizes, and velocities. Field experiments utilized three nozzles: Extended Range (XR), Air Induction Extended Range (AIXR), and Turbo TeeJet Induction (TTI), to assess the effect of spray droplet size on changes in Palmer amaranth groundcover. Nozzle did not affect Palmer amaranth groundcover when dicamba was applied. However, nozzle selection did impact groundcover when 2,4-D was applied; the following nozzle order XR>AIXR>TTI reduced Palmer amaranth groundcover the greatest in both site-years of the field experiment. This result (XR>AIXR> TTI) matches percent spray coverage data for 2,4-D and is inversely related to spray droplet size data. Rapid reductions of Palmer amaranth groundcover from 100% at time zero to 39.4 to 64.1% and 60.0 to 85.8% were observed 180 minutes after application in greenhouse and field experiments, respectively, regardless of herbicide or nozzle. In one site-year of the greenhouse and field experiments, regrowth of Palmer amaranth occurred 10080 minutes (14 days) after an application of either 2,4-D or dicamba to larger than labeled weeds. In all experiments, complete reduction of live Palmer amaranth tissue was not observed 21 days after application with any herbicide or nozzle combination. Control of Palmer amaranth escapes with reduced groundcover may potentially lead to increased selection pressure on sequentially applied herbicides due to a reduction in spray solution contact with the targeted pest.