Genetic similarities between johnsongrass and grain sorghum leave producers with limited herbicide options for postemergence johnsongrass control. TamArk^TM grain sorghum with resistance to acetyl-CoA carboxylase-inhibiting herbicides was developed through a collaboration between the University of Arkansas System Division of Agriculture and Texas A&M AgriLife Research. Two field experiments were conducted in 2021 in two locations each: Keiser and Marianna, AR, or Fayetteville and Marianna, AR. The objective of the first was to determine the optimal rate and application timing of fluazifop-butyl for control of natural johnsongrass populations in a noncrop setting, and the objective of the second was to evaluate johnsongrass control and TamArk^TM grain sorghum tolerance in response to fluazifop-butyl applied at different timings and rates based on crop growth stage. The highest levels of johnsongrass control occurred when sequential applications of fluazifop-butyl were utilized. All sequential treatments provided at least 80% johnsongrass control at any rate or application timing tested. A single application of fluazifop-butyl provided greater than 90% johnsongrass control when applied at 210 g ai ha–¹ to johnsongrass with fewer than 6 leaves. Weed size played a role in achieving high levels of johnsongrass control. Greater than 90% control was achieved when johnsongrass had 6 leaves or fewer at the initial application for the sequential application treatments. A single application of fluazifop-butyl at 105 g ai ha–¹ resulted in no more than 82% johnsongrass mortality at any application timing. TamArk™ grain sorghum injury did not exceed 6% at any application timing or rate. It was therefore considered to be safe even if the initial application was made before the 6-leaf crop stage. Because no unacceptable levels of injury were observed with TamArk™ grain sorghum for fluazifop-butyl, johnsongrass size at the time of application should be the most critical aspect for control with this herbicide.

Nomenclature: fluazifop-butyl; johnsongrass, Sorghum halepense (L.) Pers.; grain sorghum, Sorghum bicolor (L.) Moench

The herbicides that inhibit very-long-chain fatty acid (VLCFA) elongases are primarily used for residual weed control in corn, barley, oat, sorghum, soybean, sugarcane, certain vegetable crops, and wheat production fields in the United States. They act primarily by inhibiting shoot development of susceptible species, preventing weed emergence and growth. The objectives of this review were to summarize 1) the chemical family of VLCFA-inhibiting herbicides and their use in the United States, 2) the VLCFA biosynthesis in plants and their site of action, 3) VLCFA-inhibitor resistant weeds and their mechanism of resistance, and 4) the future of VLCFA-inhibiting herbicides. After their reclassification as Group 15 herbicides to include shoot growth-inhibiting herbicides (Group 8), the VLCFA-inhibiting herbicides are currently represented by eight chemical families (benzofurans, thiocarbamates, α-chloroacetamides, α-oxyacetamides, azolyl-carboxamides, isoxazolines, α-thioacetamides, and oxiranes). On average, VLCFA-inhibiting herbicides are applied once a year to both corn and soybean crops in the United States with acetochlor and S-metolachlor being the most used VLCFA-inhibiting herbicides in corn and soybean production, respectively. The site of action of Group 15 herbicides results from inhibition of the VLCFA synthase, which is encoded by several fatty acid elongase (FAE1)-like genes in VLCFA elongase complex in an endoplasmic reticulum. The VLCFA synthase is a condensing enzyme, and relies on a conserved, reactive cysteinyl sulfur in its active site that performs a nucleophilic attack on either the natural substrate (fatty acyl-CoA) or the herbicide. As of August 2023, 13 weed species have been documented to be resistant to VLCFA inhibitors, including 11 monocot weeds and two dicot weeds (Palmer amaranth and waterhemp). The isoxazolines (pyroxasulfone and fenoxasulfone) are the most recently (2014) discovered VLCFA-inhibiting herbicides. Although the intensity of VLCFA-inhibitor-directed discovery efforts has decreased over the past decade, this biochemical pathway remains a viable mechanistic target for the discovery of herbicide premixes and a valuable component of them.

Nomenclature: Acetochlor; α-chloroacetamides; α-oxyacetamides; α-thioacetamides; azolyl-carboxamides; benzofurans; fenoxasulfone; isoxazolines; thiocarbamates; oxiranes; S-metolachlor; pyroxasulfone; Palmer amaranth, Amaranthus palmeri S. Watson; waterhemp, Amaranthus tuberculatus (Moq) J.D. Sauer; barley, Hordeum vulgare L.; corn, Zea mays L.; oat, Avena sativa L.; sorghum, Sorghum bicolor (L.) Moench; soybean, Glycine max L.; sugarcane, Saccharum officinarum L.; wheat, Triticum aestivum L.

Knotroot foxtail has become more prevalent and problematic in pastures and hayfields in the southeastern United States. Gaps exist in our knowledge of which herbicide practices are best for managing this species in bermudagrass forage production. This study was conducted to determine the efficacy of various ways to control knotroot foxtail in bermudagrass with herbicide applications in autumn, postemergence (POST), with and without also applying a herbicide in preemergence (PRE), in spring. The study was a randomized complete block with a factorial arrangement of treatments and included a nontreated control for both fall and spring timings. Glyphosate at two rates (0.35 or 0.7 kg ae ha–¹), nicosulfuron (0.07 kg ai ha–¹) + metsulfuron (0.012 kg ai ha–¹), and hexazinone (1.3 kg ai ha–¹) were applied alone in the fall or followed by indaziflam (0.067 kg ai ha–¹) or pendimethalin (4.46 kg ai ha–¹) in the spring. Three harvests were conducted throughout the growing season to evaluate weed species (knotroot foxtail, large crabgrass, and horsenettle) and bermudagrass biomass as well as overall species composition. The combination of fall and spring treatments did not affect weed species or bermudagrass biomass. Therefore, treatment main effects were analyzed by fall or spring application timing. A spring application of either pendimethalin or indaziflam increased bermudagrass biomass compared with that of the nontreated control. However, neither PRE herbicide effectively reduced knotroot foxtail biomass compared with the nontreated control, although pendimethalin did reduce season-long knotroot foxtail composition. Spring PRE herbicides are an effective tool for forage producers, but further research is needed to identify effective herbicides and additional approaches for the control of knotroot foxtail.

Nomenclature: Glyphosate; hexazinone; indaziflam; metsulfuron-methyl; nicosulfuron; pendimethalin; Carolina horsenettle; Solanum carolinense L. SOLCA; knotroot foxtail; Setaria parviflora (Poir.) Kerguélen] SETGE; large crabgrass; Digitaria sanguinalis (L.) Scop. DIGSA; bermudagrass; Cynodon dactylon (L.) Pers. CYNDA

Preemergence applications of mesotrione, an herbicide that inhibits 4-hydroxyphenolpyruvate dioxygenase (HPPD), have recently gained regulatory approval in soybean varieties with appropriate traits. Giant ragweed is an extremely competitive broadleaf weed, and biotypes resistant to acetolactate synthase inhibitors (ALS-R) can be particularly difficult to manage with soil-residual herbicides in soybean production. This study investigated control of giant ragweed from preemergence applications of cloransulam (32 g ai ha–¹), metribuzin (315 g ai ha–¹), and S-metolachlor (1,600 g ai ha–¹) in a factorial design with and without mesotrione (177 g ai ha–¹) at two different sites over 2 yr. Treatments with mesotrione were also compared with two commercial premix products: sulfentrazone (283 g ai ha–¹) and cloransulam (37 g ai ha–¹), and chlorimuron (19 g ai ha–¹), flumioxazin (69 g ai ha–¹), and pyroxasulfone (87 g ai ha–¹). At 42 d after planting, control and biomass reduction of giant ragweed were greater in treatments with mesotrione than any treatment without mesotrione. Giant ragweed biomass was reduced by 84% in treatments with mesotrione, whereas treatments without mesotrione did not reduce biomass relative to the nontreated. Following these preemergence applications, sequential herbicide treatments utilizing postemergence applications of glufosinate (655 g ai ha–¹) plus fomesafen (266 g ai ha–¹) and S-metolachlor (1,217 g ai ha–¹) resulted in at least 97% control of giant ragweed at 42 d after planting, which was greater than sequential applications of glufosinate alone in 3 of 4 site-years. Preemergence applications of mesotrione can be an impactful addition to soybean herbicide programs designed to manage giant ragweed, with the potential to improve weed control and delay the onset of herbicide resistance by providing an additional effective herbicide site of action.

Nomenclature: Chlorimuron; cloransulam; flumioxazin; fomesafen; glufosinate; mesotrione; metribuzin; pyroxasulfone; S-metolachlor; sulfentrazone; giant ragweed; Ambrosia trifida L. AMBTR; soybean; Glycine max (L.) Merr.

Palmer amaranth, which is resistant to glyphosate and protoporphyrinogen oxidase inhibitors, remains a threat to cotton and soybean production in Tennessee. This is partly due to the recent evolution of dicamba-resistant Palmer amaranth in western Tennessee, which further complicates weed management. Experiments were conducted in 2021 and 2022 to determine the best timing between sequential applications and the order in which 2,4-D or dicamba should be used with glufosinate to control resistant Palmer amaranth. Palmer amaranth control increased when the interval between postemergence herbicide applications decreased from 21 to 7 d. At the 7-d interval in a dicamba-based system, the order of herbicides did not affect Palmer amaranth control. However, in a 2,4-D-based system, the greatest control was achieved when 2,4-D was applied first, followed by either 2,4-D or glufosinate. While weed height at the time of application had a significant effect on Palmer amaranth control with auxin herbicides, control was still unacceptable in the field at the labeled rates of dicamba or 2,4-D when applied to weeds that were <10 cm tall (48% and 53%, respectively). Neither dicamba nor 2,4-D provided acceptable control of the Palmer amaranth populations evaluated in this study. Sequential applications separated by 7 d provided better weed control than those separated by 21 d. Given that the better 7-d sequential treatments provided less than 90% control and resulted in more than 64,000 surviving Palmer amaranth plants per hectare suggests that relying solely on these herbicides for Palmer amaranth control is not a sustainable weed management strategy.

Nomenclature:

2,4-D; dicamba; glufosinate; Palmer amaranth; Amaranthus palmeri S. Wats; cotton; Gossypium hirsutum L.; soybean; Glycine max L. Merr.

Herbicides that inhibit protoporphyrinogen oxidase (PPO) are used in more than 40 agronomic and specialty crops across Georgia to manage weeds through residual and postemergence (POST) control. In 2017, a population of Palmer amaranth exhibiting reduced sensitivity to POST applications of PPO-inhibiting herbicides was identified by the University of Georgia. Seed were collected from the site along with a known sensitive population; distance between the samples was 200 m, increasing the likelihood of similar environmental and genetic characteristics. To quantify sensitivity for both preemergence (PRE) and POST uses, 21 greenhouse dose-response assessments were conducted from 2017 to 2022. After conducting initial rate-response studies, 13 doses per herbicide were chosen for the POST experiment; field use rates of fomesafen (420 g ai ha–¹), lactofen (219 g ai ha–¹), acifluorfen (420 g ai ha–¹), and trifludimoxazin (25 g ai ha–¹) ranging from 0× to 4× the field use rate for the susceptible population, and 0× to 40× for the suspect population were applied. Herbicide treatments included adjuvants and were applied to plants 8 to 10 cm in height. Relative resistance factors (RRFs) were calculated for control ratings, mortality, and biomass, and ranged from 105 to 318, 36 to 1,477, 215 to 316, and 9 to 49 for fomesafen, lactofen, acifluorfen, and trifludimoxazin, respectively. In the PRE experiment, herbicide applications included five to nine doses of fomesafen (1× = 210 g ai ha–¹), flumioxazin (1× = 57 g ai ha–¹), oxyfluorfen (1× = 561 g ai ha–¹), and trifludimoxazin (1× = 38 g ai ha–¹); doses ranged from 0× to 6× for the suspect population and 0× to 2× for the susceptible population. Visual control, mortality, and biomass RRFs ranged from 3 to 5 for fomesafen, 21 to 31 for flumioxazin, 6 to 22 for oxyfluorfen, and 8 to 38 for trifludimoxazin. Results confirm that a Georgia Palmer amaranth population is resistant to PPO-inhibiting herbicides applied both PRE and POST.

Nomenclature: Aciflurofen; flumioxazin; fomesafen; lactofen; oxyfluorfen; trifludimoxazin; Palmer amaranth; Amaranthus palmeri S. Watson; cotton; Gossypium hirsutum L.; peanut, Arachis hypogaea L.; soybean, Glycine max (L.)

Cotton and soybean growers were offered new technologies in 2016, expanding in-crop herbicide options to include dicamba or 2,4-D. Within 3 yr of commercialization, dicamba use in these crops increased 10-fold, and growers began to report Palmer amaranth escapes in dicamba-tolerant production systems in western Tennessee. In 2020, Palmer amaranth seed was collected from eight Tennessee locations where growers witnessed poor control following dicamba. Greenhouse experiments were conducted to evaluate the response of these Palmer amaranth populations to dicamba. In 2021, field experiments were conducted on two tentative dicamba-susceptible populations in Georgia, on three confirmed dicamba-resistant populations in Tennessee, and on a tentative dicamba-susceptible population in Texas to evaluate cotton response following dicamba and to examine if malathion insecticide (a cytochrome P450 inhibitor) would improve weed control and not reduce cotton yield when applied in conjunction with dicamba. Palmer amaranth populations collected in 2020 survived dicamba in the greenhouse at 1, 2, and 4 times the labeled rate. Five Palmer amaranth populations exhibited 15% to 26% survival to the labeled dicamba rate (560 g ha^–1) in the greenhouse. These findings were reinforced in the field when research on three of those populations in 2021 showed 55% control with the labeled dicamba rate and 69% control with 2 times the labeled rate. This demonstrates that the dicamba resistance allele or alleles were passed between generations. This result was not consistent in the Macon County, GA, or Worth County, GA, locations, where malathion improved dicamba control of 15- to 38-cm-tall Palmer amaranth. Cotton injury was observed when malathion was applied in combination with dicamba. These results further document the evolution of dicamba-resistant Palmer amaranth in Tennessee. Moreover, the nonreversal of resistance phenotype by malathion may suggest that the resistance mechanism is something other than metabolism.

Nomenclature: Dicamba; malathion; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.; Palmer amaranth, Amaranthus palmeri S. Watson

This article overviews the earliest weed management book published in the United States. The most problematic weeds of that era are named, along with suggestions for their control.

Control of barnyardgrass is becoming increasingly difficult as plants evolve resistance to herbicides. ROXY oxyfluorfen-resistant rice (ROXY^® Rice Production System) has been developed to provide an alternative mode of action for controlling barnyardgrass and other weeds. In 2021 and 2022, field trials were conducted at the Pine Tree Research Station near Colt, AR; the Northeast Research and Extension Center in Keiser, AR; and the University of Arkansas Pine Bluff Small Farm Research Center near Lonoke, AR, to determine the level of weed control and crop tolerance following oxyfluorfen applied preemergence (PRE) or postemergence (POST) relative to herbicides currently labeled for use in rice crops. When applied post-plant PRE on silt loam soil, oxyfluorfen alone at 1,120 and 1,680 g ai ha^–1 resulted in barnyardgrass control comparable to that of clomazone applied alone at 336 g ha^–1. Still, injury to rice was often greater than with clomazone, ranging from 20% to 45%. On clay soil, oxyfluorfen applied at 1,680 g ha^–1 resulted in barnyardgrass control that was comparable to that of clomazone alone in both site-years at 3 wk after emergence but caused up to 18% injury to rice. When oxyfluorfen was applied at 560 to 1,680 g ha^–1 at the 2-leaf rice growth stage, barnyardgrass control was ≥85% in three of four site-years 1 wk after treatment. However, injury to rice ranged from 38% to 73% for the rates evaluated. Propanil caused the greatest injury by a herbicide currently labeled for use in rice at 34%. Oxyfluorfen should be used as a post-plant PRE herbicide rather than a POST herbicide due to the injury that occurred after a POST application. The data indicate that if used as a PRE herbicide, oxyfluorfen should be applied at 560 g ha^–1 to reduce the injury that occurred on silt loam and clay soils.

Nomenclature: Clomazone; oxyfluorfen; barnyardgrass; Echinochloa crus-galli (L.) P. Beauv.; rice; Oryza sativa L.

Soil-applied herbicides are important for controlling weeds in many crops but risk damage to susceptible rotational crops if they persist. Field studies were conducted in Powell, WY, from 2015 through 2017 to evaluate the effect of reduced water availability on soil-applied herbicide dissipation. Eight soil-applied herbicides, applied to dry bean or corn, were exposed to three season-long irrigation treatments (100%, 85%, and 70% of estimated crop evapotranspiration [ETc]) by overhead sprinkler. Soil samples were collected to a depth of 10 cm from 0 to 140 d after application, and soil herbicide concentrations were quantified using gas or liquid chromatography and mass spectrometry. Herbicide concentrations were regressed over time to produce a soil half-life estimate for each herbicide and irrigation treatment. Reduced irrigation decreased dry bean yield by up to 77% and corn yield by up to 50%. After adjusting for precipitation, the lowest irrigation treatment received 78% and 76% as much water as the full irrigation treatment in 2015 and 2016, respectively. This significantly increased the soil half-life of imazethapyr but did not increase the soil half-life of atrazine, pyroxasulfone, saflufenacil, ethalfluralin, trifluralin, or pendimethalin. Reduced irrigation did not increase carryover injury to rotational crops from these herbicides 1 yr after application. Instead, carryover response was determined by the inherent persistence of individual herbicides. Imazethapyr (0.1 kg ai ha^–1) injured rotational sugar beet, and isoxaflutole (0.1 kg ai ha^–1) injured rotational dry bean. Pyroxasulfone (0.2 kg ai ha^–1), atrazine (2.0 kg ai ha^–1), saflufenacil (0.1 kg ai ha^–1) + dimethenamid-P (0.6 kg ai ha^–1), ethalfluralin (0.8 kg ai ha^–1), trifluralin (0.6 kg ai ha^–1), and pendimethalin (1.1 kg ai ha^–1) did not injure rotational crops regardless of irrigation treatment. Drought stress sufficient to cause up to 77% crop yield loss did not increase soil-applied herbicide carryover.

Nomenclature: Atrazine; dimethenamid-P; ethalfluralin; imazethapyr; isoxaflutole; pendimethalin; pyroxasulfone; saflufenacil; trifluralin; corn, Zea mays L.; dry bean, Phaseolus vulgaris L.; sugar beet, Beta vulgaris L.

Florpyrauxifen-benzyl (FPB) is an important postemergence rice herbicide. This study tested the potential for seed production in an FPB-resistant barnyardgrass population. A barnyardgrass population (NL) collected from a rice field in eastern China was highly resistant to FPB with a GR₅₀ dose (the FPB dose causing a 50% reduction in fresh weight of aboveground parts) of 50.2 g ai ha^–1. No significant differences in the percentages of surviving seedlings after treatment with different doses of the herbicide were found between F₁ lines collected from F₀ plants surviving a 36 g ai ha^–1 FPB treatment and those collected from nontreated control F₀ plants. Additionally, no significant differences were found in the rate of surviving seedlings after treatment with varying doses of FPB among the F₂ lines collected from F₁ plants that survived varying doses of FPB. At a constant temperature of 30 C, seeds from different F₁ and F₂ lines showed germination percentages of 85% to 92.0% and 68.3% to 89.0%, respectively. In the absence of competition, plants from the NL population surviving 0–144 g ai ha^–1 FPB showed no significant differences in plant height, dry weight of aboveground parts, effective accumulated temperature (EAT) from sowing to seed maturation, seed production per plant, or 1,000-seed weight. In the susceptible population (HJ), plants surviving 18 g ai ha^–1 FPB showed no significant differences compared to the nontreated control plants of the same population for the above variables. This is the first report of FPB-resistant barnyardgrass in China. Barnyardgrass seedlings that survived FPB application showed a higher potential for accumulating in the soil seedbank and negatively affecting rice.

Nomenclature: Florpyrauxifen-benzyl (FPB); barnyardgrass, Echinochloa crus-galli (L.) P. Beauv.; rice, Oryza sativa L.

Managing winter annual grass weeds has long been a challenge in the dryland regions of the Pacific Northwest (PNW) where soft white winter wheat is grown. The recent development of quizalofop-resistant (CoAXium) wheat varieties allows growers to use quizalofop (QP), a herbicide that inhibits acetyl-CoA carboxylase (ACCase) for postemergence grass control. Field experiments were conducted over two winter wheat growing seasons in 2021–2022 and 2022–2023 near Adams, OR, to evaluate QP efficacy on feral rye and for crop safety. Downy brome and jointed goatgrass control with QP were assessed in 2021–2022 and 2022–2023, respectively. QP treatments provided effective control of feral rye (≥95%), downy brome (≥87%), and jointed goatgrass (99%) regardless of rate, adjuvant, and spray volume tested. Spring-applied QP caused no injury to winter wheat. Results indicate that the QP-resistant wheat technology can help PNW wheat growers selectively control winter annual grasses.

Nomenclature: Quizalofop; downy brome, Bromus tectorum L.; feral rye, Secale cereale L.; jointed goatgrass, Aegilops cylindrica Host.; wheat, Triticum aestivum L.

In New Mexico chile pepper production, pendimethalin is traditionally applied shortly after crop thinning, which is 9 to 10 wk after crop seeding. Pendimethalin applications before crop thinning may be a method for controlling early-season weeds in chile pepper; however, chile pepper tolerance to early-season applications of pendimethalin is poorly understood. We conducted a greenhouse study to evaluate young chile pepper responses to pendimethalin. We also conducted a field study to determine weed and chile pepper responses to early-season, postemergence-directed pendimethalin in combination with herbicides registered for preemergence applications. The greenhouse study included three treatments administered when chile pepper was at the four-leaf stage: (i) pendimethalin applied to foliage and soil, (ii) pendimethalin applied to soil only, and (iii) a nontreated control. The field study included four treatments: (i) preemergence applications of napropamide followed by postemergence-directed pendimethalin at 5 wk after crop seeding, (ii) preemergence applications of clomazone followed by postemergence-directed pendimethalin at 5 wk after crop seeding, (iii) postemergence-directed pendimethalin without preemergence herbicides, and (iv) nontreated, weed-free control. We conducted the field study at two sites that differed in soil texture. Pendimethalin application rates were maximum labeled rates for the specific soil. Results from the greenhouse study indicated that pendimethalin applied to foliage and soil stunted two of five cultivars, whereas pendimethalin applied to soil did not affect chile pepper height, fresh weight, dry weight, or root area. Results from the field study indicated that postemergence-directed pendimethalin did not affect chile pepper height or fruit yield, or cause visible symptoms of herbicide injury. Postemergence-directed pendimethalin reduced the densities of weeds, including junglerice. The results of this study indicate that postemergence-directed applications of pendimethalin at 5 wk after crop seeding do not cause crop injury or yield loss in chile pepper, while providing some weed control benefits.

Nomenclature: Clomazone; napropamide; pendimethalin; junglerice, Echinochloa colona (L.) Link ECHCO; chile pepper, Capsicum annuum L.

Palmer amaranth can grow 4.2 mm in height per degree day; hence, delays of a few days in weed control deployment can result in applications of herbicides to weeds that are larger than those for which the herbicide label recommends. Therefore, it is critically necessary to understand the effect of plant size at the time of herbicide application in conjunction with herbicide spray solution and nozzle type pairings on the effectiveness of weed management programs in the Enlist E3 and XtendFlex production systems. Field experiments were conducted in 2020, in no-crop conditions, at two locations in Arkansas, to evaluate the influence of Palmer amaranth size on its control with glufosinate, dicamba, and 2,4-D applied alone and in mixture with specific nozzle pairings as mandated by label requirements. Also, a laboratory experiment was conducted to evaluate the droplet size and velocity of the spray solutions and nozzles used in the field experiments. A 5- and 10-percentage point reduction in control was observed when dicamba (66%) and 2,4-D (63%) were applied alone, respectively, compared with those herbicides mixed with glufosinate (71% and 73%, respectively). Palmer amaranth density increased to 55, 73, 100, 115, and 140 plants m^–2 when plants were sprayed at heights of 15, 25, 41, 61, and 76 cm, respectively, compared with plants that were sprayed when they were 5 cm tall (9 plants m^–2). Nozzle type did not affect weed control or density. The percentage of driftable fines increased when a mixture of glufosinate and 2,4-D were used compared with 2,4-D alone. Effective short-term and long-term chemical control of Palmer amaranth will require growers to correctly time their weed management practices and overlay residuals, and expect the need for sequential applications.

Nomenclature: Dicamba; 2,4-D; glufosinate-ammonium; Palmer amaranth; Amaranthus palmeri S. Wats

A significant proportion of the forested production area in South Carolina is managed using aerial applications of imazapyr. Cotton injury from off-target movement of imazapyr has been observed in South Carolina. Field experiments were conducted twice at the Edisto Research and Education Center (EREC) in 2021 and 2022, and once at the Pee Dee Research and Education Center (PDREC) in 2022, to evaluate the response of cotton at two growth stages to imazapyr at 0.1×, 0.05×, 0.025×, 0.0125×, and 0.00625× of the normal use rate of 0.84 kg ae ha^–1. Injury to cotton at the vegetative stage was 86% and 74% at 0.1× and 0.05× imazapyr rates 28 d after application (DAA). Cotton height ranged from 23 to 93 cm at all three locations. Yield at the EREC location in 2021 was reduced by 79%, 48%, and 31% at the 0.1×, 0.05×, and 0.025× rates of imazapyr, respectively. Similar reductions from imazapyr were observed at both EREC and PDREC in 2022. Injury to cotton at the reproductive stage based on visual estimates at 28 DAA ranged from 95% to 64% for the 0.1× to 0.0125× rates, respectively. Cotton height at the reproductive stage was reduced to 59% of the untreated control 28 DAA when the 0.1× rate of imazapyr was applied. Seed cotton (which included both seed and lint) yield ranged from 0 to 2,880 kg ha^–1 at the three locations in both years. Seed cotton yield was lowest when imazapyr was applied at the 0.1× to 0.025× rates. Cotton exposure to imazapyr at the vegetative and reproductive growth stages resulted in plant injury, height, and yield reductions, especially at the higher rates of imazapyr. The greatest reduction in cotton growth and yield was observed after exposure at the reproductive growth stage regardless of imazapyr rate. In summary, the magnitude of cotton response to imazapyr depends on crop growth stage and imazapyr concentration at the time of exposure with the greatest impact occurring at the reproductive growth stage.

Nomenclature: Imazapyr; cotton, Gossypium hirsutum L. ‘Deltapine 2038 B3XF’

Carrier water quality is an important consideration for herbicide efficacy. Field and greenhouse studies were conducted from 2021 to 2023 to evaluate the effect of carrier water pH and hardness on imazapic efficacy for sicklepod control in peanut crops. In separate field experiments imazapic was applied postemergence at 0.071 kg ai ha^–1 with carrier water pH levels of 5, 6, 7, 8, or 9; and hardness levels of 0 (deionized water), 100, 200, 400, or 500 mg L^–1 of CaCO₃ equivalent. In greenhouse experiments, imazapic was applied to sicklepod that was either 10 cm, 15 cm, or 20 cm tall at similar carrier water pH levels and hardness levels of 0, 100, 200, 400, or 800 mg L^–1 of CaCO₃. In the field study, sicklepod control, density, and biomass reductions were lower with carrier water pH 5 or 9 compared with pH 7. In the greenhouse study, control was not different among carrier water pH levels when imazapic was applied to 10-cm-tall sicklepod; however, when applied to 15- or 20-cm-tall sicklepod, control was at least 25% greater with acidic (pH 5) compared to alkaline (pH 9) carrier water. Results from the field study showed that carrier water hardness ≤500 ppm did not reduce the efficacy of imazapic to control sicklepod. In the greenhouse study, regardless of sicklepod height, carrier water hardness of 800 mg L^–1 reduced sicklepod control by 15% and biomass reduction by 17% compared with deionized water (pH 7). The effects of carrier water pH and hardness on imazapic efficacy did not compromise peanut yield in the field study. However, this study indicates that both acidic and alkaline carrier water pH and hardness (800 mg L^–1 CaCO₃ L^–1) have the potential to reduce imazapic efficacy on sicklepod, and appropriate spray solution amendments maybe be needed to maintain optimum efficacy.

Nomenclature: Imazapic; sicklepod; Senna obtusifolia L.; peanut; Arachis hypogaea L.

Methiozolin is labeled for goosegrass and smooth crabgrass control in golf course putting greens, but no peer-reviewed literature exists regarding this use. Greenhouse experiments were conducted evaluating goosegrass and smooth crabgrass response to increasing rates of methiozolin as affected by weed growth stage. In general, as weed growth stage increased, the methiozolin rate required to reduce weed biomass 90% (WR₉₀) increased. Goosegrass was more sensitive to preemergence-applied methiozolin than smooth crabgrass, and the WR₉₀ was 30.4 and 118 g ai ha^–1 for goosegrass and smooth crabgrass, respectively. However, smooth crabgrass was generally more sensitive to postemergence-applied methiozolin than goosegrass. Subsequent field studies were conducted to evaluate goosegrass and smooth crabgrass control with methiozolin applied singularly or sequentially at standard preemergence timings. Results indicated methiozolin applied singularly or sequentially at the label-recommended rate (500 g ha^–1) is not persistent enough to provide season-long control of goosegrass and smooth crabgrass. Ten field studies were conducted in Alabama, California, Florida, and Virginia to evaluate frequent methiozolin application programs with the objective of providing selective, season-long goosegrass and smooth crabgrass control. Results from these studies indicate methiozolin can be safely applied to hybrid bermudagrass and creeping bentgrass putting greens despite exceeding the yearly maximum use rate for putting greens (2,500 g ha^–1) with some treatments. Methiozolin effectively controlled smooth crabgrass throughout the growing season in California and Virginia when 10 biweekly applications were applied at 250 g ha^–1 or higher. In Florida, methiozolin did not acceptably (80%) control goosegrass regardless of application rate. In Virginia, methiozolin acceptably controlled goosegrass only when applied at rates and frequencies that exceeded the maximum yearly methiozolin usage rate. These data indicate that methiozolin has the potential to control smooth crabgrass preemergence when applied frequently, but does not provide acceptable goosegrass control at labeled rates.

Nomenclature: Methiozolin; goosegrass, Eleusine indica L. Gaertn.; smooth crabgrass, Digitaria ischaemum Schreb.; creeping bentgrass, Agrostis stolonifera L.; hybrid bermudagrass Cynodon transvaalensis Burtt Davy x dactylon (L.) Pers

Targeted spraying application technologies have the capacity to drastically reduce herbicide inputs, but to be successful, the performance of both machine vision–based weed detection and actuator efficiency needs to be optimized. This study assessed (1) the performance of spotted spurge recognition in ‘Latitude 36’ bermudagrass turf canopy using the You Only Look Once (YOLOv3) real-time multiobject detection algorithm and (2) the impact of various nozzle densities on model efficiency and projected herbicide reduction under simulated conditions. The YOLOv3 model was trained and validated with a data set of 1,191 images. The simulation design consisted of four grid matrix regimes (3 × 3, 6 × 6, 12 × 12, and 24 × 24), which would then correspond to 3, 6, 12, and 24 nonoverlapping nozzles, respectively, covering a 50-cm-wide band. Simulated efficiency testing was conducted using 50 images containing predictions (labels) generated with the trained YOLO model and by applying each of the grid matrixes to individual images. The model resulted in prediction accuracy of an F1 score of 0.62, precision of 0.65, and a recall value of 0.60. Increased nozzle density (from 3 to 12) improved actuator precision and predicted herbicide-use efficiency with a reduction in the false hits ratio from ∼30% to 5%. The area required to ensure herbicide deposition to all spotted spurge detected within images was reduced to 18%, resulting in ∼80% herbicide savings compared to broadcast application. Slightly greater precision was predicted with 24 nozzles but was not statistically different from the 12-nozzle scenario. Using this turf/weed model as a basis, optimal actuator efficacy and herbicide savings would occur by increasing nozzle density from 1 to 12 nozzles within the context of a single band.

Nomenclature: Spotted spurge, Chamaesyce maculata (L.) Small; bermudagrass, Cynodon spp.

Herbicide-resistant barnyardgrass and weedy rice control, without crop injury, is a challenge for rice producers in the United States. Herbicides not initially labeled for rice, such as oxyfluorfen, are now being evaluated as new tools for weed control. The ROXY^® trait allows for the use of oxyfluorfen in rice for weed control preemergence and postemergence. Experiments were initiated in 2021 and 2022 to evaluate (1) the effectiveness of preemergence- and postemergence-applied oxyfluorfen on barnyardgrass and weedy rice, (2) the sensitivity of oxyfluorfen-resistant rice to oxyfluorfen as a function of application timing, and (3) the influence of soil moisture on oxyfluorfen-resistant rice sensitivity to oxyfluorfen. In the field, a rate response was observed for oxyfluorfen applied to weedy rice when averaged over application timings of 1-leaf, 2-leaf, 3-leaf, and tillering, with oxyfluorfen at 1,680 g ai ha–1 resulting in 81% and 72% control 7 d after application (DAA) in 2021 and 2022, respectively. Under greenhouse conditions, barnyardgrass and weedy rice control averaged by the rate of oxyfluorfen was ≥85 and ≥70%, respectfully, 7 DAA for the 1-, 2-, and 3-leaf rice growth stage timings. Preemergence applications of oxyfluorfen under 100% soil saturation resulted in 75% injury to oxyfluorfen-resistant rice, greater than all other soil moisture at 7 DAA. All postemergence applications of oxyfluorfen resulted in 63% to 70% injury to oxyfluorfen-resistant rice at 7 DAA, regardless of soil moisture. Barnyardgrass and weedy rice control with oxyfluorfen is achieved with timely applications; however, injury to oxyfluorfen-resistant rice is likely.

Nomenclature: Oxyfluorfen; barnyardgrass, Echinochloa crus-galli (L.) P. Beauv.; weedy rice, Oryza sativa L.; rice, Oryza sativa L.

Industrial hemp is a multipurpose crop cultivated for fiber, seed, human food, and animal feed. Hemp legalization in Texas creates a considerable potential to increase its acreage in semi-arid conditions; however, knowledge is limited on growing hemp optimally in Texas. Best management practices, including weed control, require evaluation for profitable hemp production. As little is known about the herbicide tolerance of hemp, field studies were conducted to test several soil-residual herbicides with different modes of action for phytotoxicity to two hemp cultivars, ‘Yuma’ and ‘Jinma’. The experimental units were randomized three times in a blocked split-plot design with hemp cultivars in the main plots and soil-residual herbicides in the subplots. Ethalfluralin, the mixture of sulfentrazone and S-metolachlor, prometryn, and S-metolachlor, resulted in 60% to 90% and 73% to 100% weed control as compared to the nontreated control in 2021 and 2022, respectively. The highest hemp germination, stand count, and plant height were observed with ethalfluralin and S-metolachlor herbicides; however, no significant differences were observed for hemp germination and plant height compared to the nontreated control. S-metolachlor, ethalfluralin, fomesafen, and prometryn resulted in similar hemp biomass compared to the nontreated control. Overall, the results indicate that hemp is tolerant to ethalfluralin, prometryn, and S-metolachlor, and these soil-residual herbicides were effective for weed control in hemp. The mixture of bicyclopyrone plus S-metolachlor, metribuzin plus S-metolachlor, and mesotrione should be avoided, as they caused significant injury to hemp plants. Future research is needed to test the efficacy of different preemergence and postemergence herbicides that can be used in industrial hemp grown under different environments, making sure the delta-9-tetrahydrocannabinol content of the hemp is below the legal content restrictions.

Nomenclature: Bicyclopyrone; ethalfluralin; fomesafen; mesotrione; metribuzin; prometryn; S-metolachlor; sulfentrazone; industrial hemp, Cannabis sativa L.

Common ragweed is a troublesome weed in many crops. Farmers and crop advisors in the coastal Mid-Atlantic region have reported inadequate control of common ragweed in soybean fields with glyphosate and other herbicide modes of action. To determine whether herbicide resistance was one of the causes of poor herbicide performance, 29 accessions from four states (Delaware, Maryland, New Jersey, and Virginia) where common ragweed plants survived herbicide applications and produced viable seeds were used for greenhouse screening. Common ragweed seedlings from those accessions were treated with multiple rates of cloransulam, fomesafen, or glyphosate, applied individually postemergence (POST). All accessions except one demonstrated resistance to at least one of the herbicides applied at twice the effective rate (2×), 17 accessions were two-way resistant (to glyphosate and cloransulam, or to glyphosate and fomesafen), and three-way resistance was present in eight accessions collected from three different states. Based on the POST study, five accessions were treated preemergence (PRE) with herbicides that inhibit acetolactate synthase (ALS), and two accessions were treated with herbicides that inhibit protoporphyrinogen oxidase (PPO). All accessions treated PRE with the ALS inhibitors chlorimuron or cloransulam demonstrated resistance at the 2× rates. Both accessions treated PRE with the PPO inhibitor sulfentrazone had survivors at the 2× rate. When the same accessions were treated PRE with fomesafen, one had survivors at the 2× rate, and one had survivors at the 1× rate. Results from these tests confirmed common ragweed with three-way resistance to POST herbicides is widespread in the region. In addition, this is the first confirmation that common ragweed accessions in the region are also resistant to ALS- or PPO-inhibiting herbicides when applied PRE.

Nomenclature: Chlorimuron; cloransulam; fomesafen; glyphosate; sulfentrazone; common ragweed, Ambrosia artemisiifolia L.; soybean, Glycine max L. Merr.

Waterhemp has evolved resistance to Group 2, 5, 9, 14, and 27 herbicides in Ontario, Canada, making control of this challenging weed even more difficult. Acetochlor is a Group 15, chloroacetanilide herbicide that has activity on many small-seeded annual grasses and some small-seeded annual broadleaf weeds, including waterhemp. The objective of this study was to ascertain if acetochlor mixtures with broadleaf herbicides (dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin), applied preemergence (PRE), increase multiple-herbicide-resistant (MHR) waterhemp control in soybean. Five trials were conducted over 2 yr (2021 and 2022). The acetochlor mixtures caused ≤7% soybean injury, except acetochlor + flumioxazin, which caused 19% soybean injury. Acetochlor applied PRE controlled MHR waterhemp 89% at 4 wk after application (WAA). Dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin controlled MHR waterhemp 59%, 67%, 58%, 64%, and 86%, respectively, at 4 WAA. Acetochlor applied in a mixture with dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin provided good to excellent control of MHR waterhemp; control ranged from 91% to 98% but was similar to acetochlor applied alone. Acetochlor alone reduced MHR waterhemp density and biomass 98% and 93%; acetochlor + flumioxazin reduced waterhemp density and biomass by an additional 2% and 7%, respectively. This research concludes that acetochlor applied in a mixture with flumioxazin was the most efficacious mixture evaluated for MHR waterhemp control.

Nomenclature: Acetochlor; dicamba; diflufenican; flumioxazin; metribuzin; sulfentrazone; waterhemp, Amaranthus tuberculatus (Moq.) Sauer.; soybean, Glycine max (L.) Merr.

A Palmer amaranth biotype (CT-Res) with resistance to glyphosate was recently confirmed in a pumpkin field in Connecticut. However, the underlying mechanisms conferring glyphosate resistance in this biotype is not known. The main objectives of this research were 1) to determine the effect of plant height (10, 20, and 30 cm) on glyphosate resistance levels in CT-Res Palmer amaranth biotype, and 2) to investigate whether the target site–based mechanisms confer glyphosate resistance. To achieve these objectives, progeny seeds of the CT-Res biotype after two generations of recurrent selection with glyphosate (6,720 g ae ha–¹) were used. Similarly, known glyphosate-susceptible Palmer amaranth biotypes from Kansas (KS-Sus) and Alabama (AL-Sus) were included. Results from greenhouse dose-response studies revealed that CT-Res Palmer amaranth biotype had 69-, 64-, and 54-fold resistance to glyphosate as compared with the KS-Sus biotype when treated at heights of 10, 20, and 30 cm, respectively. Sequence analysis of the EPSPS gene revealed no point mutations at the Pro₁₀₆ and Thr₁₀₂ residues in the CT-Res Palmer amaranth biotype. Quantitative polymerase chain reaction analysis revealed that the CT-Res biotype had 33 to 111 relative copies of the EPSPS gene compared with the AL-Sus biotype. All these results suggest that the EPSPS gene amplification endows a high level of glyphosate resistance in the GR Palmer amaranth biotype from Connecticut. Because of the lack of control with glyphosate, growers should adopt the use of effective alternative preemergence and postemergence herbicides in conjunction with other cultural and mechanical tactics to mitigate the further spread of GR Palmer amaranth in Connecticut.

Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Watson; pumpkin (Cucurbita pepo L.)

Currently, a limited number of herbicides is available to treat water-seeded rice in California, with widespread resistance to most of those herbicides. Because no resistant grasses showed resistance to pendimethalin, a series of studies were conducted to evaluate water-seeded rice response to pendimethalin. In a field study conducted at the Rice Experiment Station at Biggs, California, in 2020 and 2021, three pendimethalin formulations, a granule (GR), emulsifiable concentrate (EC), and capsule suspension (CS), were applied at 1.1, 2.3, and 3.4 kg ai ha^–1 rates, and at 5, 10, and 15 d after seeding onto water-seeded rice. In addition, a greenhouse study was conducted to examine the response of five common California rice cultivars to GR and CS formulation applications. Echinochloa control levels were reduced at 15 d after seeding after use of EC and CS formulations compared with earlier timings. In both years, rice grain yields were increased by 3,014 kg ha^–1 after application of pendimethalin at 3.4 kg ai ha^–1 when applied at 15 d after seeding compared with 5 and 10 d after seeding, and similar to 1.1 kg ai ha^–1 applications. The GR and CS were safer formulations based on a reduction in injury and an increase in grain yields compared to the EC formulation. Differences in seedling vigor across cultivars appeared to incur an advantage after a pendimethalin application. However, most cultivars evaluated for stand reduction and dry biomass demonstrated tolerance to GR and CS formulation applications only after rice reached the 3-leaf stage. In contrast, an application at 1-leaf stage rice reduced stand up to 68%. Application rate, timing, and formulation are important factors to consider if the use of pendimethalin in water-seeded rice is to be pursued.

Nomenclature: Pendimethalin; rice, Oryza sativa L.

Waterhemp is a summer annual, broadleaf weed with high fecundity, short seed longevity in the soil, and wide genetic diversity. Populations have evolved resistance to five herbicide modes of action (Groups 2, 5, 9, 14, and 27), which are present across southern Ontario; this has increased the challenge of controlling this competitive weed species in corn, the most important grain crop produced worldwide and the highest-value agronomic crop in Ontario. Acetochlor is a Group 15 soil-applied residual herbicide that has activity on many grass and broadleaf weeds but has yet to be registered in Canada. The objective of this study was to ascertain whether mixtures of acetochlor with flumetsulam, dicamba, atrazine, isoxaflutole/diflufenican, or mesotrione + atrazine applied preemergence would increase the control of multiple-herbicide-resistant (MHR) waterhemp in corn. Five field trials were conducted between 2022 and 2023. No corn injury was observed. Acetochlor applied alone controlled MHR waterhemp 97% 12 wk after application (WAA). All herbicide mixtures controlled MHR waterhemp similarly at ≥98% 12 WAA; there were no differences among herbicide mixtures. Flumetsulam, dicamba, and atrazine provided lower MHR waterhemp control than all other herbicide treatments and did not reduce density or biomass. Acetochlor reduced waterhemp density 98%, while the acetochlor mixtures reduced density similarly at 99% to 100%. This study concludes that the acetochlor mixtures evaluated provide excellent waterhemp control; however, control was not greater than acetochlor alone. Herbicide mixtures should be used as a best management practice to mitigate the evolution of herbicide resistance.

Nomenclature: Acetochlor; flumetsulam; dicamba; atrazine; isoxaflutole/diflufenican; mesotrione; waterhemp, Amaranthus tuberculatus (Moq.) Sauer; corn, Zea mays L.

Fall-sown cereal rye has gained popularity as a cover crop in vegetable production due to its weed-suppressive capabilities. However, previous research has shown that replacing preemergence and/or postemergence herbicide applications with roller-crimped rye has variable success at controlling weeds and maintaining vegetable cash crop yields. The objective of this research was to determine whether roller-crimped rye can provide season-long weed control and maintain sweet corn yield. Two rye cultivars (early vs. standard maturity) were compared at three seeding rates (150, 300, and 600 seeds m^–2) for their effect on weed control and sweet corn yield. The trial was conducted at three locations: Harrow, ON, and St. Jean-sur-Richelieu, QC, from 2019 to 2021; and Agassiz, BC, in 2019 and 2021. Results suggest that although the early-maturing cultivar allowed for earlier roller crimping in some locations, it was inferior at weed control and resulted in lower sweet corn yield than local standard cultivars. The average rye biomass was lower than the current literature recommendations, and the resulting level of weed control was not high enough to prevent sweet corn yield loss in cover crop treatments. Weed control provided by roller-crimped rye peaked between crimping and 8 wk after crimping and was highest in the standard cultivars sown at 300 and 600 seeds m^–2. Preliminary testing of supplemental postemergence weed control showed evidence for sweet corn yields comparable to the weed-free no-cover crop check. However, more research is needed. Overall, with the cultivars and seeding rates tested, roller-crimped rye is not a suitable stand-alone weed control option in sweet corn production. Given the benefits of cover crops, further research should evaluate its potential as a component of an integrated weed management program.

Nomenclature: Sweet corn; Zea mays L.; cereal rye; Secalecereale L.

Downy brome is a cleistogamous facultative winter-annual grass weed that invades cropland, pastureland, and ruderal areas in western North America. Glyphosate-resistant downy brome, the first known glyphosate-resistant grass weed in Canada, was confirmed in a glyphosate-resistant canola field in southern Alberta in 2021. A controlled-environment study was conducted to determine the impact of preemergence soil-applied residual herbicides on glyphosate-resistant and susceptible downy brome in two field soils. Flumioxazin/pyroxasulfone (70/89 g ai ha^–1), carfentrazone/pyroxasulfone (18/150 g ai ha^–1), sulfentrazone/ pyroxasulfone (100/100 or 150/150 g ai ha^–1), and saflufenacil/pyroxasulfone (36/120 g ai ha^–1) resulted in excellent (≥90%) visible control and downy brome biomass reduction 8 wk after treatment (WAT). The low rate of carfentrazone/pyroxasulfone (12/100 g ai ha^–1) resulted in good (≥80%) visible control and biomass reduction 8 WAT, while the low and medium rates of saflufenacil/pyroxasulfone (18/60 or 25/84 g ai ha^–1) resulted in ≥80% biomass reduction but suppression only (66% to 75%) based on visible control. Flumioxazin alone (105 g ai ha^–1) resulted in good visible control (81%) 8 WAT in a sandy loam soil, but poor (13%) control in a clay loam soil. Soil type affected plant growth as evidenced by reduced growth in the untreated sandy loam soil compared to clay loam soil. The glyphosate-resistant population emerged and grew more vigorously than the glyphosate-susceptible population resulting in greater plant densities in the untreated control and some less-effective herbicide treatments. These results suggest that mixtures of a protoporphyrinogen oxidase-inhibiting herbicide with the very-long-chain fatty acid elongase inhibitor pyroxasulfone applied preemergence at ≥89 g ai ha^–1 could be effective components of an herbicide layering strategy targeting glyphosate-resistant and glyphosate-susceptible downy brome.

Nomenclature: Carfentrazone; flumioxazin; glyphosate; pyroxasulfone; saflufenacil; sulfentrazone; downy brome, Bromus tectorum L.; canola, Brassica napus L.