Biotypes of Palmer amaranth that are resistant to acetolactate synthase (ALS) inhibitor are becoming widespread in western Nebraska. There are limited effective postemergence (POST) herbicides labeled for ALS-inhibitor-resistant Palmer amaranth control in dry edible bean. The objective of this study was to evaluate the efficacy of dimethenamid-P in a sequential preemergence (PRE) fb followed by (fb) POST program at two POST application timings, the first and third trifoliate stages (V1 and V3, respectively), for controlling ALS-inhibitor-resistant Palmer amaranth in dry edible bean. A field study was conducted in 2019, 2020, and 2021 in Scottsbluff, NE. PRE-alone applications of pendimethalin + dimethenamid-P provided inconsistent Palmer amaranth control. Dimethenamid-P applied POST following a PRE application of pendimethalin + dimethenamid-P provided effective (>90%) Palmer amaranth control at 4 wk after V3 only at the V1 application timing in 2019. In 2020 and 2021 dimethenamid-P applied POST at V1 and V3 following a PRE application of pendimethalin + dimethenamid-P provided 99% and 98% Palmer amaranth control at 4 wk after V3, and 98% and 94% Palmer amaranth control at harvest, respectively. Palmer amaranth biomass was reduced by 95% to 99% and by 96% to 98% compared with the -nontreated control when dimethenamid-P was applied POST at V1 and V3, respectively, following a PRE application of pendimethalin + dimethenamid-P in 2020 and 2021. Application of a mixture of dimethenamid-P with imazamox + bentazon POST provided similar results to those of the fomesafen-containing treatments and dimethenamid-P alone POST. Dimethenamid-P applied POST following a PRE application of pendimethalin + dimethenamid-P resulted in similar yield as the fomesafen-containing treatments. If fomesafen is not an option due to the crop rotation interval restriction, using dimethenamid-P in a sequential PRE fb POST program is the only effective alternative to control ALS-inhibitor–resistant Palmer amaranth in Nebraska. The use of dimethenamid-P in a sequential PRE fb POST program, alone or mixed with foliar-active herbicides should be considered by dry edible bean growers who are dealing with ALS-inhibitor-resistant Palmer amaranth.

Palmer amaranth is the most problematic and troublesome weed in agronomic cropping systems in the United States. Acetolactate synthase (ALS) inhibitor and glyphosate-resistant (GR) Palmer amaranth has been confirmed in Nebraska and it is widespread in several counties. Soybean resistant to isoxaflutole/glufosinate/glyphosate has been developed that provides additional herbicide site of action for control of herbicide-resistant weeds. The objectives of this study were to evaluate herbicide programs for control of ALS inhibitor/GR Palmer amaranth and their effect on Palmer amaranth density and biomass, as well as soybean injury and yield in isoxaflutole/glufosinate/glyphosate–resistant soybean. Field experiments were conducted in a grower’s field infested with ALS inhibitor and GR Palmer amaranth near Carleton, Nebraska, in 2018 and 2019. Isoxaflutole applied alone or mixed with sulfentrazone/pyroxasulfone, flumioxazin/pyroxasulfone, or imazethapyr/saflufenacil/pyroxasulfone provided similar control (86%–99%) of Palmer amaranth 21 d after PRE (DAPRE). At 14 d after early-POST (DAEPOST), isoxaflutole applied PRE and PRE followed by (fb) POST controlled Palmer amaranth by 10% to 63% compared to 75% to 96% control with glufosinate applied EPOST in both years. A PRE herbicide fb glufosinate controlled Palmer amaranth 80% to 99% 21 d after late-POST (DALPOST) in 2018, and reduced density 89% to 100% in 2018 and 58% to 100% in 2019 at 14 DAEPOST. No soybean injury was observed from any of the herbicide programs tested in this study. Soybean yield in 2019 was relatively higher due to higher precipitation compared with 2018 with generally no differences between herbicide programs. This research indicates that herbicide programs are available for effective control of ALS inhibitor/GR Palmer amaranth in isoxaflutole/glufosinate/glyphosate-resistant soybean.

Florpyrauxifen-benzyl and quizalofop were available for POST applications in 2018; however, little is known about the response of acetyl-CoA carboxylase (ACCase)–resistant rice cultivars and advanced lines to POST herbicides. A field study was conducted in 2017 and 2018 at Stoneville, MS, to characterize the response of ACCase-resistant rice cultivars and advanced lines to POST applications of florpyrauxifen-benzyl. The imidazolinone-resistant (IR) rice cultivars ‘CL163’ and ‘CLXL 745’, and ACCase-resistant rice cultivars ‘PVL01’, ‘PVL013’, ‘PVL024-B’, ‘PVL038’, ‘PVL080’, and ‘PVL081’were treated with florpyrauxifen-benzyl at 0 (nontreated control for each cultivar) and 58 g ai ha–1 at the four-leaf to one-tiller (LPOST) growth stage. At 14 d after treatment (DAT), PVL01 was injured 5% to 6% greater than CLXL 745, PVL013, and PVL081; however, injury was ≤10% at that evaluation for all cultivars. Similarly, injury was ≤13% for all cultivars 28 DAT. Mature heights were reduced for all cultivars except PVL013 and PVL081. Rough rice yield was ≥100% of the control for all cultivars except PVL081, PVL013, and CL163. Results suggest that florpyrauxifen-benzyl can safely be applied POST to rice cultivars grown in Mississippi as well as ACCase-resistant cultivars that are currently under development.

Rice with enhanced tolerance to herbicides that inhibit acetyl coA carboxylase (ACCase) allows POST application of quizalofop, an ACCase-inhibiting herbicide. Two concurrent field studies were conducted in 2017 and 2018 near Stoneville, MS, to evaluate control of grass (Grass Study) and broadleaf (Broadleaf Study) weeds with sequential applications of quizalofop alone and in mixtures with auxinic herbicides applied in the first or second application. Sequential treatments of quizalofop were applied at 119 g ai ha−1 alone and in mixtures with labeled rates of auxinic herbicides to rice at the two- to three-leaf (EPOST) or four-leaf to one-tiller (LPOST) growth stages. In the Grass Study, no differences in rice injury or control of volunteer rice (‘CL151’ and ‘Rex’) were detected 14 and 28 d after last application (DA-LPOST). Barnyardgrass control at 14 and 28 DA-LPOST with quizalofop applied alone or with auxinic herbicides EPOST was ≥93% for all auxinic herbicide treatments except penoxsulam plus triclopyr. Barnyardgrass control was ≥96% with quizalofop applied alone and with auxinic herbicides LPOST. In the Broadleaf Study, quizalofop plus florpyrauxifen-benzyl controlled more Palmer amaranth 14 DA-LPOST than other mixtures with auxinic herbicides, and control with this treatment was greater EPOST compared with LPOST. Hemp sesbania control 14 DA-LPOST was ≤90% with quizalofop plus quinclorac LPOST, orthosulfamuron plus quinclorac LPOST, and triclopyr EPOST or LPOST. All mixtures except quinclorac and orthosulfamuron plus quinclorac LPOST controlled ivyleaf morningglory ≥91% 14 DA-LPOST. Florpyrauxifen-benzyl or triclopyr were required for volunteer soybean control >63% 14 DA-LPOST. To optimize barnyardgrass control and rice yield, penoxsulam plus triclopyr and orthosulfamuron plus quinclorac should not be mixed with quizalofop. Quizalofop mixtures with auxinic herbicides are safe and effective for controlling barnyardgrass, volunteer rice, and broadleaf weeds in ACCase-resistant rice, and the choice of herbicide mixture could be adjusted based on weed spectrum in the treated field.

Four field studies were conducted in 2004 to evaluate corn tolerance, weed control, grain yield, and net returns in glufosinate-resistant (GUR), glyphosate-resistant (GYR), imidazolinone-tolerant (IT), and nontransgenic (NT) corn with various herbicide systems. No significant differences between hybrid systems were observed for weed control. Limited corn injury (< 5%) was observed for all herbicide treatments. A single early POST (EPOST) system without S-metolachlor and sequential POST over the top (POT) herbicide systems, averaged over corn hybrids and PRE and late POST-directed (LAYBY) herbicide options, provide 93 and 99% control of goosegrass, respectively, and at least 83 and 97% control of Texas panicum, respectively. A single EPOST system without S-metolachlor, averaged over corn hybrids and LAYBY treatment options, provided at least 88% control of large crabgrass. When averaged over corn hybrid and PRE herbicide options, a sequential POT herbicide system alone provided at least 98, 99, 98, and 100 control of large crabgrass, morningglory species, Palmer amaranth, and common lambsquarters, respectively. The addition of ametryn at LAYBY to a single EPOST system without S-metolachlor was beneficial for improving control of morningglory species, common lambsquarters, and Palmer amaranth, depending on location. However, the observed increases (7 percentage points or less) are likely of limited biological significance. Grain yield was variable between hybrids and locations because of environmental differences. Consequently, net returns for each hybrid system within a location were also variable. Any POT system with or without ametryn at LAYBY, averaged over corn hybrid and PRE herbicide options, provided at least 101, 97, 92, and 92% yield protection at Clayton, Kinston, Lewiston, and Rocky Mount, NC, respectively. Net returns were maximized with treatments that provided excellent weed control with minimal inputs.

Horseweed populations with mixtures of biotypes resistant to glyphosate and acetolactate synthase (ALS)–inhibiting herbicides as well as biotypes with multiple resistance to glyphosate + ALS-inhibiting herbicides have been documented in Indiana and Ohio. These biotypes are particularly problematic because ALS-inhibiting herbicides are commonly tank mixed with glyphosate to improve postemergence horseweed control in soybean. The objective of this research was to characterize the growth and seed production of horseweed populations with resistance to glyphosate or ALS-inhibiting herbicides, and multiple resistance to glyphosate + ALS-inhibiting herbicides. A four-herbicide by four-horseweed population factorial field experiment was conducted in the southeastern region of Indiana in 2007 and repeated in 2008. Four horseweed populations were collected from Indiana or Ohio and confirmed resistant to glyphosate, ALS inhibitors, both, or neither in greenhouse experiments. The four herbicide treatments were untreated, 0.84 kg ae ha−1 glyphosate, 35 g ai ha−1 cloransulam, and 0.84 kg ae ha−1 glyphosate + 35 g ai ha−1 cloransulam. Untreated plants from horseweed populations that were resistant to glyphosate, ALS-inhibiting, or multiple glyphosate + ALS-inhibiting herbicides produced similar amounts of biomass and seed compared to populations that were susceptible to those herbicides or combination of herbicides. Furthermore, aboveground shoot mass and seed production did not differ between treated and untreated plants.