The WSSA Group 15 (HRAC Group K3) herbicide flufenacet is a key compound in weed resistance management, primarily used for PRE control of grass weeds in winter cereal–based crop rotations in Europe. Although resistance to compounds of its mechanism of action (inhibition of the synthesis of very-long-chain fatty acids) generally evolves slowly, reduced flufenacet efficacy due to enhanced glutathione transferase (GST) activity has been described in several blackgrass (Alopecurus myosuroides Huds.) populations. The present study aimed to better understand of the mechanism of flufenacet detoxification in A. myosuroides. Therefore, we characterized four A. myosuroides populations with different levels of flufenacet sensitivity. Flufenacet degradation was significantly slowed down in a sensitive population and a population with reduced flufenacet sensitivity by the use of the GST inhibitors tridiphane and ethacrynic acid at sublethal rates. Finally, an RNA sequencing (RNA-seq) study with the four A. myosuroides populations was conducted. In total, six differentially expressed GSTs and nine transcription factors as well as a keto-acyl-CoA reductase involved in the biosynthesis of very-long-chain fatty acids were identified as candidate genes among a set of 319 significantly more highly expressed gene-associated contigs. Among a set of 218 contigs with significantly lower expression levels, receptor kinase activity was the most frequent annotation. In summary, the likely GST-mediated reduction in sensitivity evolves in A. myosuroides at a slow rate and can partially be reversed by an interaction between flufenacet and the GST inhibitors tridiphane and ethacrynic acid. This provides further evidence for enhanced GST activity as a key mechanism in flufenacet resistance in A. myosuroides and supports the hypothesis that the six differentially expressed GSTs detected in the present RNA-seq study are potentially involved in flufenacet resistance.

Weed control and corn vigor were evaluated to determine the effects of application rate, timing, and other POST herbicides on primisulfuron activity. Corn injury caused by primisulfuron was very low. Weed control varied with species and stage of growth. Primisulfuron showed high activity on emerged johnsongrass, particularly with early postemergence (EP) and mid postemergence (MP) applications. However, these treatments did not provide full season control of this species due to regrowth from rhizomes and continuous germination of new seeds. Primisulfuron controlled giant foxtail with the EP timing (< 5 cm weed height), but not with MP and late postemergence (LP) applications. Common lambsquarters and redroot pigweed control was most effective with EP applications. Primisulfuron in combination with cyanazine, atrazine, cyanazine plus tridiphane, atrazine plus tridiphane, bentazon, 2,4-D, dicamba, or paraquat controlled common lambsquarters and redroot pigweed better than primisulfuron applied alone. Johnsongrass control decreased when primisulfuron was combined with paraquat or 2,4-D and giant foxtail control decreased when primisulfuron was combined with 2,4-D or dicamba.

Field experiments in 1984 and 1985 near Headland, AL involved the influence of 18 integrated weed management systems incorporated from nine herbicide-tillage systems (H-T) and two row patterns on control of large crabgrass and Texas panicum, crop yield, and net returns in grain sorghum. The conventional-tillage system plus herbicide combinations; alachlor preplant incorporated (PPI), atrazine postemergence (POST), and paraquat POST directed, provided 90% or greater control of large crabgrass throughout the season and high sorghum yields in one of two years; and 90% or greater control of Texas panicum from mid-June through mid-August and high sorghum yields in both years. Results in 1984 and 1985 similar to those above were obtained with strip tillage plus herbicide combinations; alachlor PPI, atrazine POST, and paraquat POST directed; alachlor PPI and tridiphane plus atrazine POST; and tridiphane plus atrazine POST and paraquat POST directed. Row patterns did not consistently influence control of large crabgrass, Texas panicum, or sorghum yield. Economic net returns ranged from – $20 to $210 per hectare for the large crabgrass control systems in 1984; however, low sorghum yields in 1985 resulted largely in negative returns. Similar net returns were obtained in the Texas panicum experiment.

Field research from 1986 to 1988 evaluated control of alfalfa, dandelion, orchardgrass, and annual weeds in corn with sequential herbicide treatments. Glyphosate applied the previous fall controlled 95% or more of the dandelion and orchardgrass before postemergence treatments. Atrazine applied early preplant or with paraquat the day before planting only partially controlled the perennial species. Atrazine applied postemergence usually controlled vegetation escaping initial treatment except orchardgrass following the atrazine treatments. Adding tridiphane increased weed control with atrazine or cyanazine applied postemergence only when fall panicum was present. Herbicides applied postemergence increased corn yield compared to the initial treatments alone, and few differences among postemergence treatments were observed. Sequential treatments which included glyphosate applied in the fall controlled 82% or more of each perennial species before the next growing season. Orchardgrass control was 81% or less the following spring when the initial treatment was atrazine or atrazine plus paraquat.

In laboratory experiments, the safener CGA-92194 {α-[(1,3-dioxolan-2-yl-methoxy)imino] benzeneacetonitrile} increased the in vitro chemical reactivity of the herbicide metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] for glutathione (GSH) when tested at 1, 50, and 100 μM. Pretreatment of sorghum [Sorghum bicolor (L.) Moench. cv. ‘G623’] seeds with the safener CGA-92194 (1.25 g ai/kg of seeds) enhanced significantly the amount of 14C-metolachlor absorbed by the roots and translocated to the shoots and leaves of grain sorghum seedlings grown hydroponically in a nutrient solution. Thin-layer chromatographic analysis of methanol-soluble extracts from roots or shoots and leaves of sorghum seedlings treated with 14C-metolachlor revealed the detection of 8 to 10 metabolites of this herbicide. One of the major metabolites detected was tentatively identified as the conjugate of metolachlor with GSH. A safener-induced stimulation of metolachlor metabolism via conjugation to GSH was observed in sorghum seedlings grown from seeds pretreated with CGA-92194. Addition of tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane], a potent inhibitor of plant GSH-S-transferase enzymes, to the nutrient solution of hydroponically grown sorghum seedlings reduced the formation of GS-metolachlor in both control and safener-treated seedlings and minimized the stimulatory effect of the safener CGA-92194 on the conjugation of metolachlor to GSH. It is suggested that the safener CGA-92194 may protect grain sorghum against injury from the herbicide metolachlor by stimulating the spontaneous and enzymatic conjugation of this herbicide with GSH.

Thioproline (L-thiazolidine-4-carboxylic acid) partially protects sorghum [Sorghum bicolor (L.) Moench. ‘DK 42Y’] seedlings against the herbicides tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane] and alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] in controlled environment chambers. Thioproline alone inhibits plant growth at concentrations above 10−5 M. Its phytotoxicity is intermediate between the two herbicide antidotes OTC (L-2-oxothiazolidine-4-carboxylic acid) and flurazole [phenylmethyl 2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylate]. The two thiazolidine antidotes increased thiol content of excised corn (Zea mays L. ‘DK T 1100’) seedling roots whereas thiol content was decreased by the two herbicides. While thioproline or OTC partially offset tridiphane-induced decreases in thiol content, none of the antidotes effectively circumvented the reduced thiol content resulting from alachlor treatment.

The effects of seed depth on growing point location of wild-proso millet, effects of alachlor placement, and chemical control of wild-proso millet were studied in field and greenhouse experiments. In 1986, 55 to 88% of the seedlings originating in the top 6 cm of soil had their growing point above the soil surface. The respective figures were 11 to 36% in 1987. Shoot exposure to alachlor resulted in significantly greater phytotoxicity than root exposure. Only EPTC or vernolate with a protectant, preplant incorporated, or pendimethalin preemergence, followed by atrazine plus tridiphane postemergence controlled wild-proso millet more than 85% all season.

Greenhouse and field experiments were conducted to identify alternative herbicide treatments for the control of a biotype of American black nightshade which has been shown to tolerate paraquat. In greenhouse experiments the combination of paraquat and the metal chelator diethyldithiocarbamate at either 1% w/v or 3% w/v, and diquat alone significantly lowered I50 values, based on dry weight, when compared with paraquat alone. None of these treatments provided acceptable control (>90%) in field experiments. Treatments that controlled American black nightshade under field conditions were: monocarbamide dihydrogensulfate at 9 kg ai ha-1, monocarbamide dihydrogensulfate at 9 kg ha-1 + paraquat at 0.6 kg ha-1, lactofen at 0.6 kg ha-1, oxyfluorfen at 0.6 kg ha-1, and acifluorfen at 0.6 kg ha-1. Addition of tridiphane at 1.7 kg ha-1 to paraquat at 0.6 kg ha-1 did not improve control of American black nightshade under field conditions.

Field studies with flatfan, twinjet, flood and rotary atomizers were conducted in 1987 and 1988. The results indicated that tridiphane at 0.6 kg ai/ha plus atrazine at 1.7 kg ai/ha with 2.0 L/ha of soybean oil adjuvant was more effective in controlling giant foxtail and common lambsquarters when applied by the flatfan, twinjet, and rotary atomizers. Image analysis indicated that weed control by a postemergence herbicide was generally enhanced by better application coverage. Droplet size measurement by phase droplet particle/droplet analyzer demonstrated that the rotary atomizer produced a narrower range of droplet diameters between the Dv.1 and Dv.9 volumes than any of the other atomizers.

Studies were conducted to determine acifluorfen and lactofen absorption, translocation, and metabolism in protox-inhibiting herbicide-susceptible and -resistant common waterhemp. Acifluorfen and lactofen absorption was similar in both biotypes. Herbicide absorption was 12% in both susceptible and resistant common waterhemp 6 h after treatment (HAT). Absorption increased to 32 and 42% in susceptible and resistant plants, respectively, at 72 HAT. Translocation was similar in both biotypes for both herbicides. Herbicide translocation out of the treated leaf ranged between 5 and 15%. In a separate study, resistant common waterhemp plants were treated with acifluorfen or lactofen, alone or with tridiphane. Acifluorfen or lactofen injury to resistant common waterhemp was not altered with the addition of tridiphane. Treatments of 14C-acifluorfen or -lactofen on susceptible and resistant common waterhemp resulted in similar lactofen metabolism in both biotypes, but acifluorfen was not metabolized in either biotype within 24 HAT. This data indicate that differences in herbicide absorption, translocation, or metabolism are not the mechanism of common waterhemp resistance to protox-inhibiting herbicides.