Glufosinate serves as both a primary herbicide option and a complement to glyphosate and other postemergence herbicides for managing herbicide-resistant weed species. Enhancing broadleaf weed control with glufosinate through effective mixtures may mitigate further herbicide resistance evolution in soybean and other glufosinate-resistant cropping systems. Two field experiments were conducted in 2020 and 2021 at four locations in Wisconsin (Arlington, Brooklyn, Janesville, and Lancaster) and one in Illinois (Macomb) to evaluate the effects of postemergence-applied glufosinate mixed with inhibitors of protoporphyrinogen oxidase (PPO) (flumiclorac-pentyl, fluthiacet-methyl, fomesafen, and lactofen; Group 14 herbicides), bentazon (a Group 6 herbicide), and 2,4-D (a Group 4 herbicide) on waterhemp control, soybean phytotoxicity, and yield. The experiments were established in a randomized, complete block design with four replications. The first experiment focused on soybean phytotoxicity 14 d after treatment (DAT) and yield in the absence of weed competition. All treatments received a preemergence herbicide, with postemergence herbicide applications occurring between the V3 and V6 soybean growth stages, depending on the site-year. The second experiment evaluated the effect of herbicide treatments on waterhemp control 14 DAT and on soybean yield. Lactofen, applied alone or with glufosinate, produced the greatest phytotoxicity to soybean at 14 DAT, but this injury did not translate into yield loss. Mixing glufosinate with 2,4-D, bentazon, and PPO-inhibitor herbicides did not increase waterhemp control, nor did it affect soybean yield compared to when glufosinate was applied alone, but it may be an effective practice to reduce selection pressure for glufosinate-resistant waterhemp.

Palmer amaranth, a competitive weed in cotton and soybeans, poses challenges due to its rapid growth, high fertility, and herbicide resistance. Effective management strategies targeting sex ratios could reduce seed production by female plants. Protoporphyrinogen oxidase (PPO-) inhibiting herbicides play a role in the evolving resistance of Amaranthus spp. in the US Midwest. These herbicides may also affect the male-to-female ratio of Palmer amaranth. A 2-yr field experiment (2015 and 2016) was conducted in a soybean field in Collinsville, IL, evaluating various preemergence and postemergence PPO-inhibiting herbicide treatments. Untreated Palmer amaranth populations exhibited a bias toward females. Preemergence application of sulfentrazone and flumioxazin effectively reduced Palmer amaranth density (1.66 plants m–2) throughout the season, whereas postemergence applications of fomesafen and lactofen provided limited control (27 and 31 plants m–2, respectively). Early-season mortality was high (96%) among Palmer amaranth seedlings, especially with pyroxasulfone + fluthiacet-methyl treatment. Fomesafen increased female biomass (28.8%) while reducing male biomass compared to the nontreated control. In 2015, pyroxasulfone + fluthiacet-methyl and acetochlor altered the male-to-female sex ratio compared to the nontreated control, with pyroxasulfone + fluthiacet-methyl reducing the proportion of females (–0.11 M/F) and acetochlor slightly increasing the proportion of males (0.03 M/F), though not different from a 1:1 ratio. In 2016, pendimethalin and flumioxazin (71 g ai ha–1) resulted in a strong female-biased sex ratio, with an almost exclusively female population. In both years, the nontreated control plots (–0.58 and –0.55 M/F) maintained a naturally female-biased sex ratio, deviating significantly from a 1:1 ratio. These findings suggest that specific herbicide treatments can alter the sex ratio. Understanding sex determination in Palmer amaranth holds promise for developing more effective control strategies in the future.

Palmer amaranth is one of the most problematic weeds in the midsouthern United States, and the evolution of resistance to protoporphyrinogen oxidase (PPO) inhibitors in biotypes already resistant to glyphosate and acetolactate synthase (ALS) inhibitors is a major cause of concern to soybean and cotton growers in these states. A late-season weed-escape survey was conducted in the major row crop–producing counties (29 counties) to determine the severity of PPO-inhibitor resistance in Arkansas. A total of 227 Palmer amaranth accessions were sprayed with fomesafen at 395 g ha−1 to identify putative resistant plants. A TaqMan qPCR assay was used to confirm the presence of the ΔG210 codon deletion or the R128G/M (homologous to R98 mutation in common ragweed) target-site resistance mechanisms in the PPX2 gene. Out of the 227 accessions screened, 44 were completely controlled with fomesafen, and 16 had only one or two severely injured plants (≥98% mortality) when compared with the 1986 susceptible check (100% mortality). The remaining 167 accessions were genotypically screened, and 82 (49%) accessions were found to harbor the ΔG210 deletion in the PPX2 gene. The R128G was observed in 47 (28%) out of the 167 accessions screened. The mutation R128M, on the other hand was rare, found in only three accessions. About 13% of the accessions were segregating for both the ΔG210 and R128G mutations. Sixteen percent of the tested accessions had mortality ratings <90% and did not test positive for the ΔG210 or the R128G/M resistance mechanisms, indicating that a novel target or non–target site resistance mechanism is likely. Overall, PPO inhibitor–resistant Palmer amaranth is widespread in Arkansas, and the ΔG210 resistance mechanism is especially dominant in the northeast corridor, while the R128G mutation is more prevalent in counties near Memphis, TN.