This field study was conducted during the growing seasons of 2020 and 2021 at the University of Nebraska-Lincoln Panhandle Research and Extension Center in Scottsbluff, Nebraska (41.893 °N, 103.680 °W). Soil type was a Tripp fine sandy loam (coarse-silty, mixed, superactive, mesic Aridic Haplustolls) (NCSS, 2006; USDA-NRCS, 2019) with a pH range from 8.1 to 8.3 and an organic matter content range from 1.1% to 1.7%. Common weed species at the experiment site include bristly foxtail (Setaria verticillata (L.) Beauv.), common lambsquarters (Chenopodium album L.), hairy nightshade (Solanum physalifolium Rusby), kochia (Bassia scoparia (L.) A. J. Scott), and Palmer amaranth. The Palmer amaranth population present at the experiment location is resistant to ALS-inhibiting herbicides. The preceding crop in the experiment site was sugar beet (Beta vulgaris L.) in both years.

This field study was designed as a randomized complete block design with four replications. The dry edible bean cultivar ‘SV6139GR’ (Seminis, 800 N Lindbergh Blvd, St. Louis, MO 63167) was planted on May 26 in 2020 and the cultivar ‘Cowboy’ (ADM Seedwest, 4666 Faries Parkway Decatur, IL 62526) was planted on June 1 in 2021. Both dry edible bean cultivars are upright pinto with indeterminate growth habits. Dry edible beans were planted at a density of 210,000 seeds ha⁻¹, using a 0.56 m row spacing. Each plot consisted of four 9.1 m long rows. Fertilizer in the form of N and P₂O₅ were dry applied at 112 kg ha⁻¹ and 45 kg ha⁻¹ prior to experiment establishment, respectively. Glyphosate was applied at 1,261 g ae ha⁻¹ + ammonium sulfate (AMS) at 3% vol/vol + nonionic surfactant (NIS) at 0.25% vol/vol within one day after dry edible bean planting as a burndown treatment to control emerged weeds. Irrigation was provided throughout the season using an overhead lateral-move irrigation system.

The treatments included a nontreated control (weedy), pendimethalin + dimethenamid-P applied PRE-alone, pendimethalin + S-metolachlor applied PRE-alone, pendimethalin + pyroxasulfone applied PRE-alone, pendimethalin + dimethenamid-P applied PRE fb dimethenamid-P POST at V1 (first trifoliate growth stage), pendimethalin + S-metolachlor applied PRE fb S-metolachlor POST at V1, pendimethalin + pyroxasulfone applied PRE fb pyroxasulfone POST at V1, and pendimethalin + dimethenamid-P PRE fb imazamox + bentazon + fomesafen at V1 ( Table 1 ). The inclusion of pendimetalin + dimethenamid-P PRE fb imazamox + bentazon + fomesafen at V1 was specifically chosen for its high efficacy in weed control, as fomesafen provides excellent control of ALS-inhibitor-resistant Palmer amaranth.

In 2020, PRE applications occurred on May 28, while in 2021, PRE applications occurred on June 2. Herbicides applied PRE were incorporated into the soil with 12.5 mm of irrigation within 24 h after their application. In 2020, POST applications occurred on June 15, while in 2021, POST applications occurred on June 17. Soil-residual herbicides applied POST were incorporated into the soil with 6.3 mm of irrigation within 24 h after their application. AMS (WinField United, 4001 Lexington Ave N, Arden Hills, MN 55126) at 18 g L⁻¹ and methylated seed oil (MSO) (WinField United, 4001 Lexington Ave N, Arden Hills, MN 55126) at 1.5% v/v were used with all POST applications which included imazamox, bentazon, and fomesafen. Herbicides were applied with a CO₂ pressurized backpack sprayer equipped with four TeeJet 11002 AIXR nozzles (TeeJet^® Technologies, Spraying Systems Co., P.O. Box 7900, Wheaton, IL 60187) spaced at 51 cm and calibrated to deliver 140 L hectare^–1 of spray solution at 160 kPa.

Palmer amaranth control was visually estimated every 14 days after POST treatment (DAT) until harvest, using a scale ranging from 0 to 100%, where 0% represented no control and 100% represented complete control. The assessment of injury to dry edible beans was assessed at 0, 14, 28, and 42 DAT. The degree of injury, noting typical responses to PPO-inhibitors and VLCFA-inhibiting herbicides such as bronzing, necrosis, leaf deformation, and stunting, was visually rated using a percentage scale from 0%, indicating no visible injury to the beans, to 100%, indicating complete plant mortality. The evaluation at 0 DAT was conducted within 4–6 hours after POST treatment. Weed density was recorded every 14 DAT until harvest by randomly placing two 0.5 m² quadrants in each plot. At harvest, density was assessed, and above-ground Palmer amaranth biomass was also determined from the same quadrants. The Palmer amaranth aboveground biomass was oven-dried at 60°C for one week before being weighed. To streamline the presentation of results, only Palmer amaranth density measurements and control ratings at harvest, as well as crop injury at 0, 14, and 28 DAT, were reported. Dry edible beans were harvested on September 16, 2020, and September 8, 2021, by hand-pulling all plants from 6.1 m of row in the second and third rows. These hand-pulled dry edible bean plants were threshed using a Zurn 150 plot combine (ZURN USA Inc., 9210 Wyoming Ave N, Brooklyn Park, MN 55445), which served as a static threshing machine. Yield and moisture per plot were collected using a H2 Classic GrainGage (Juniper Systems & HarvestMaster Inc., 1132 W 1700 N Logan, UT, 84321) placed on the plot combine. Yields were adjusted to 15% moisture content.

The data were analyzed using R version 4.0.2 (R Core Team, 2020). Visual estimated Palmer amaranth control and crop injury data were transformed to a beta-distribution (values between 0 and 1) to handle percent data more effectively (Douma and Weedon, 2019). These data were then analyzed using a beta regression with the ‘glmmTMB’ function in the ‘glmmTMB’ package version 1.0.2.1 (Brooks et al., 2017), along with Analysis of Variance (ANOVA) at the 5% level of significance. For the analysis of crop injury, only data from treatments containing pyroxasulfone and fomesafen were included, as no injury was recorded in the remaining treatments (Sarangi and Jhala, 2018). Dry edible bean yield and Palmer amaranth dry biomass data were analyzed using a linear mixed model (LMM) with the ‘lmer’ function in the ‘lme4’ package version 1.1-23 (Bates et al., 2015), along with ANOVA at the 5% level of significance. Palmer amaranth dry biomass residuals were square-root transformed to improve normality and homogeneity. Back-transformed mean values and their standard errors were presented based on transformed data for better interpretation (Onofri et al., 2010). Palmer amaranth density was analyzed using a generalized linear mixed-effect model (GLMM) with a Poisson error distribution, utilizing the ‘lme4’ package version 1.1-23 (Bates et al., 2015), along with ANOVA at the 5% level of significance. The ANOVA was performed using the ‘Anova’ function in the ‘car’ package version 3.0-10 (Fox and Weisberg, 2019). For LMM models, Type III Wald F-tests were conducted in the ANOVA, while Type III Wald chi-square tests were conducted for GLMM and glmmTMB models. Herbicide treatment was considered a fixed effect, and replications were considered random effects. Response variables from each year of the experiment were analyzed separately due to variations in treatments between years. ANOVA assumptions of normality and homogeneity of variance were evaluated by plotting residuals for models assuming normal distributions (Onofri et al., 2010; Kniss and Streibig, 2018). Whenever a significant herbicide treatment effect was found, means were separated using estimated marginal means (also known as least-squares means) with the ‘emmeans’ function in the ‘emmeans’ package version 1.5.1 (Lenth, 2020), and the 5% Fisher’s protected least significant difference (LSD) test was calculated using the ‘cld’ function in the ‘multcomp’ package version 1.4-14 (Hothorn et al., 2008).

In both 2020 and 2021, the average temperatures were higher than the 30-year average by 0.9°C and 1.1°C, respectively ( Table 2 ). During the months of June and July in 2021, average temperatures were higher compared to those in 2020 by 1°C and 0.5°C, respectively. Additionally, rainfall recorded in 2020 and 2021 was lower than the 30-year average by 46% and 33%, respectively. Consequently, the growing seasons of both 2020 and 2021 were notably warmer and drier compared to the long-term average. Irrigation was consistently provided throughout both growing seasons, and despite the variation in rainfall between 2020 and 2021, its impact on the study outcomes in either year would have been minimal.

Pendimethalin + dimethenamid-P applied PRE-alone, along with pendimethalin + S-metolachlor applied PRE-alone, controlled <73% Palmer amaranth in 2020 and 2021 ( Table 3 ). Sequential PRE fb POST programs, including pendimethalin + dimethenamid-P applied PRE fb dimethenamid-P POST, pendimethalin + S-metolachlor applied PRE fb S-metolachlor POST, and pendimethalin + pyroxasulfone applied PRE fb pyroxasulfone POST, exhibited enhanced control of Palmer amaranth compared to PRE-alone programs, surpassing 87% control in both years ( Table 3 ).

In 2020, pendimethalin + dimethenamid-P applied PRE-alone along with pendimethalin + S-metolachlor applied PRE-alone, provided 71% to 72% Palmer amaranth control ( Table 3 ). Pendimethalin + pyroxasulfone applied PRE-alone and pendimethalin + pyroxasulfone applied PRE fb pyroxasulfone POST, provided 87% control of Palmer amaranth. Although pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST did not improve the control compared to using pendimethalin + pyroxasulfone PRE-alone, applying pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST and pendimethalin + S-metolachlor PRE fb S-metolachlor POST significantly improved control compared to their PRE-alone counterparts, achieving 21–23% greater control. These sequential PRE fb POST treatments provided >92% control of Palmer amaranth, similar to the fomesafen-containing treatment.

In 2021, both pendimethalin + S-metolachlor and pendimethalin + pyroxasulfone applied PRE-alone provided 73–80% control of Palmer amaranth, similar to the control achieved by pendimethalin + dimethenamid-P applied PRE-alone (67%) ( Table 3 ). The application of pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST significantly enhanced Palmer amaranth control to 90% compared to its PRE-alone application. Pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST, pendimethalin + S-metolachlor PRE fb S-metolachlor POST, and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST, yielded similar levels of Palmer amaranth control (>90%) as the fomesafen-containing treatment. Interestingly, in 2021, pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST provided similar Palmer amaranth control (92%) as in 2020, whereas pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST showed slightly greater control, 90% in 2021 compared to 87% in 2020. Overall, the sequential PRE fb POST programs provided similar control levels as the fomesafen-containing treatment, with minor variations in performance between years.

The nontreated control had the greatest Palmer amaranth densities and biomass in both 2020 and 2021 ( Table 4 ). Palmer amaranth biomass and densities in the nontreated control were greater in 2020 by 14% and 9% compared to 2021, respectively. The application of herbicides significantly reduced both Palmer amaranth densities and biomass accumulation in the nontreated control. While Palmer amaranth was the primary focus due to its dominance, it is important to note that the treatments also effectively controlled other weeds such as bristly foxtail, common lambsquarters, and hairy nightshade, which were present in the plots. This highlights the broad-spectrum efficacy of the herbicides used.

In 2020, pendimethalin + dimethenamid-P, pendimethalin + S-metolachlor, and pendimethalin + pyroxasulfone applied PRE-alone resulted in greater Palmer amaranth densities compared to pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST, pendimethalin + S-metolachlor PRE fb S-metolachlor POST, and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST ( Table 4 ). Pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST (0.7 plants m⁻²), pendimethalin + S-metolachlor PRE fb S-metolachlor POST (0.5 plants m⁻²), and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST (0.9 plants m⁻²), provided similar Palmer amaranth densities as the fomesafen-containing treatment (0.7 plants m⁻²). Despite statistical differences among treatments in terms of Palmer amaranth densities, Palmer amaranth biomass was similar for all herbicide treatments. The application of pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST, pendimethalin + S-metolachlor PRE fb S-metolachlor POST, and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST reduced Palmer amaranth dry biomass to 78, 34, and 34 g m⁻², respectively.

In 2021, among the herbicide treatments, pendimethalin + dimethenamid-P applied PRE-alone provided the greatest Palmer amaranth densities, with 8.5 plants m⁻² ( Table 4 ). Pendimethalin + S-metolachlor and pendimethalin + pyroxasulfone applied PRE-alone and pendimethalin + S-metolachlor PRE fb S-metolachlor POST and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST resulted in similar Palmer amaranth densities, with fewer than 2.9 plants m⁻². Pendimethalin + S-metolachlor PRE fb S-metolachlor POST, and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST yielded Palmer amaranth densities comparable to the fomesafen-containing treatment, with fewer than 1.5 plants m⁻². Although the pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST treatment improved the reduction of Palmer amaranth densities compared to pendimethalin + dimethenamid-P PRE-alone, it still provided greater densities (3.8 plants m⁻²) than the fomesafen-containing treatment. Despite the greater densities in the pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST treatment, Palmer amaranth biomass was similar to that of the fomesafen-containing treatment. This suggests that although pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST led to greater densities due to late-emerging flushes, the overall biomass remained comparable to the fomesafen-containing treatment. The most effective treatments in terms of Palmer amaranth biomass reduction were pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST (3 g m⁻²), pendimethalin + S-metolachlor PRE fb S-metolachlor POST (79 g m⁻²), and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST (25 g m⁻²), which provided similar biomass reduction as the fomesafen-containing treatment.

Only treatments containing pyroxasulfone and fomesafen caused injury to dry edible beans in both years, despite the differences in cultivars, which can vary significantly in their sensitivity to herbicides ( Figure 1 ) (Soltani et al., 2006, 2014a, 2014b, 2018b). The symptoms observed were characteristic of PPO-inhibitors, including bronzing and necrosis, as well as symptoms typical of VLCFA-inhibiting herbicides such as leaf deformation, stand loss, and stunting. The greatest dry edible bean injury at 0 DAT was caused by pendimethalin + pyroxasulfone applied PRE-alone (47%) and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST (58–62%). The injury caused by pyroxasulfone decreased over time ( Figure 1 ), with dry edible beans recovering faster in the pendimethalin + pyroxasulfone PRE-alone treatment compared to pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST. In 2020, dry edible bean injury caused by pyroxasulfone decreased faster than in 2021, with injury rates at 27% and 48% at 14 DAT, respectively. By 28 DAT, no injury was observed in pendimethalin + pyroxasulfone applied PRE-alone, whereas minimal injury (1%) was detected in the pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST treatment. The decrease in observed injury over time from treatments containing pyroxasulfone is likely due to the surviving plants growing and filling in the canopy. Fomesafen-containing treatment also caused greater injury in 2021 compared to 2020, with injury rates decreasing faster in 2020 than in 2021. Dry edible bean injury decreased from 6% at 0 DAT to 1% at 14 DAT to no injury at 28 DAT when fomesafen was applied POST in 2020. In 2021, dry edible injury decreased from 11% at 0 DAT to 6% at 14 DAT to 0.4% at 28 DAT. The observed recovery of the plants in fomesafen-containing treatments can be attributed to the gradual diminishing of leaf burning and necrosis over time, as new leaf tissue developed. Overall, pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST and pendimethalin + S-metolachlor PRE fb S-metolachlor POST provided excellent crop safety.

The substantial reduction in dry edible bean yield in the nontreated control in both 2020 and 2021 underscores the significant impact of weed interference on crop productivity ( Table 5 ). The greater yield in 2021 compared to 2020 can be attributed to the lower Palmer amaranth density in 2021 ( Table 4 ). Among the herbicide treatments in 2020, pendimethalin + dimethenamid-P applied PRE-alone, pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST, pendimethalin + S-metolachlor PRE fb S-metolachlor POST, and pendimethalin + pyroxasulfone applied PRE-alone exhibited the greatest dry edible bean yields, surpassing 3,740 kg ha⁻¹, comparable to the fomesafen-containing treatment. Pendimethalin + S-metolachlor applied PRE-alone and pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST resulted in the lowest yields, with 2,680 kg ha⁻¹ and 1,930 kg ha⁻¹, respectively. Similarly, in 2021, pendimethalin + pyroxasulfone PRE fb pyroxasulfone POST resulted in the lowest dry edible bean yield (1,860 kg ha⁻¹), demonstrating substantial reductions compared to the top-performing treatments. The application of pendimethalin + dimethenamid-P, pendimethalin + S-metolachlor, and pendimethalin + pyroxasulfone PRE-alone led to notable yield decreases of about 40% when compared to the top-performing treatments. Pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST (4,850 kg ha⁻¹) and pendimethalin + S-metolachlor PRE fb S-metolachlor POST (4,730 kg ha⁻¹) provided yields similar to the fomesafen-containing treatment.

Effective management of herbicide-resistant Palmer amaranth requires an integrated approach aimed at reducing the soil seedbank and implementing diverse control strategies (Harker and O’Donovan, 2013; Jhala et al., 2014; Mahoney et al., 2021). Crop rotation plays a crucial role in long-term management, with tall-stature crops like corn offering competitive advantages against Palmer amaranth (Mahoney et al., 2021). Additionally, the greater diversity of herbicide options available in corn compared to dry edible bean allows for the use of multiple modes of action, enhancing weed control efficacy. Given the rotational restrictions imposed by fomesafen, alternatives such as pendimethalin + dimethenamid-P PRE fb dimethenamid-P POST and pendimethalin + S-metolachlor PRE fb S-metolachlor POST emerge as promising options for integrated Palmer amaranth control. These VLCFA-inhibitor-containing programs offer flexibility in crop rotation while delivering comparable control levels to fomesafen-containing treatments. However, it is essential to exercise caution and implement mixtures with foliar-active herbicides to mitigate the risk of herbicide resistance and maintain the effectiveness of these herbicide programs. By adopting such integrated approaches and expanding the spectrum of controlled weed species, growers can effectively manage herbicide-resistant Palmer amaranth while preserving crop health and productivity in dry edible bean production systems.