Herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) can be mixed with herbicides that generate reactive oxygen species (ROS) to enhance the spectrum, level, speed, and consistency of weed control efficacy; however, mixtures of these herbicides can increase corn injury. A total of five field trials were conducted from 2021 to 2023 in Ridgetown, Ontario, to determine the sensitivity of two corn hybrids (‘DKC39-97’ and ‘B79N56PWE’) to tolpyralate plus ROS-generating herbicides (atrazine, bromoxynil, bentazon, or glufosinate) applied postemergence at the recommended rate (1×) and sequentially to represent a spray overlap (2×) in the field. Tolpyralate plus atrazine, bromoxynil, bentazon, or glufosinate (2× rates) caused greater corn injury to DKC39-97 than B79N56PWE corn at 1, 2, and 4 wk after treatment (WAT). Tolpyralate plus atrazine, bromoxynil, bentazon, or glufosinate (2× rates) caused 38%, 36%, 29%, and 18% injury to DKC39-97 corn, but only 5%, 20%, 9%, and 2% injury to B79N56PWE corn, respectively at 1 WAT. Corn injury to both hybrids decreased over time with ≤2% injury at 8 WAT. Tolpyralate + atrazine, bromoxynil, or bentazon (2× rates) caused a 17%, 16%, and 13% height reduction, respectively, of DKC39-97 corn at 2 WAT; however, tolpyralate + glufosinate did not reduce DKC39-97 corn height. Tolpyralate + bromoxynil or bentazon (2× rates) caused a 12% and 10% height reduction of B79N56PWE corn, respectively, at 2 WAT; however, tolpyralate + atrazine or glufosinate did not reduce B79N56PWE corn height. Tolpyralate + atrazine or glufosinate (2× rates) caused a greater corn height reduction of DKC39-97 corn than B79N56PWE corn at 2 WAT. Grain yield was on average 2% lower from DKC39-97 than B79N56PWE corn. Tolpyralate + bromoxynil or bentazon (2× rates) caused 7% and 6% corn grain yield reduction compared to tolpyralate plus glufosinate (2× rate). Results indicate that tolpyralate + ROS-generating herbicides can cause corn injury, which is influenced by corn hybrid and ROS-generating herbicide. Corn producers need to consider the differential sensitivity of corn hybrids and ROS-generating herbicides when using an HPPD-inhibiting herbicide for weed management.

Multiple herbicide–resistant (MHR) Palmer amaranth is among the most problematic summer annual broadleaf weeds in Nebraska and several other states. A new MHR corn cultivar (resistant to 2,4-D/glufosinate/glyphosate, also known as Enlist corn) has been commercially available in the United States since 2018. Growers are searching for herbicide programs for control and reduce seed production of MHR Palmer amaranth among Enlist corn crops. The objectives of this study were to evaluate herbicides applied preemergence, early postemergence, or preemergence followed by (fb) late postemergence for the management of MHR Palmer amaranth in Enlist corn fields and to assess their effect on Palmer amaranth biomass, density, seed production, and corn yield. Field experiments were conducted near Carleton, NE, in 2020 and 2021, in a grower’s field of Enlist corn infested with acetolactate synthase–inhibitor/atrazine/glyphosate–resistant Palmer amaranth. Herbicides applied preemergence, such as flufenacet/isoxaflutole/thiencarbazone-methyl, acetochlor/clopyralid/flumetsulam, or acetochlor/clopyralid/mesotrione, provided 75% to 99% control of Palmer amaranth 30 d after preemergence. Preemergence fb late postemergence herbicides resulted in 94% Palmer amaranth control 90 d after late postemergence, reduced weed density to 0 to 8 plants m−2 30 d after late postemergence, and reduced biomass to 2 to 14 g m−2 15 d after late postemergence compared to preemergence-only (59% control, 0 to 15 plants m−2, and 4 to 123 g m−2) and early postemergence–only herbicides (78% control, 6 to 30 plants m−2, and 8 to 25 g m−2). Based on contrast analysis, Palmer amaranth seed production was reduced to 14,050 seeds m–2 in preemergence fb late postemergence herbicide programs compared with 325,490 seed m–2 in preemergence-only and 376,750 seed m–2 in early postemergence–only programs. Based on orthogonal contrast, higher corn yield of 12,340 and 11,730 kg ha−1 was obtained with preemergence fb late postemergence herbicide programs compared with preemergence-only (10,840 and 11,510 kg ha−1) and early postemergence–only programs (10,850 and 10,030 kg ha−1) in 2020 and 2021, respectively.

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.

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.

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.

Corn that is resistant to aryloxyphenoxypropionate, known commercially as Enlist™ corn, enables the use of quizalofop-p-ethyl (QPE) as a selective postemergence (POST) herbicide for control of glufosinate/glyphosate-resistant corn volunteers. Growers usually mix QPE with 2,4-D choline or glufosinate or both to achieve broad-spectrum weed control in Enlist corn. The objectives of this study were 1) to evaluate the efficacy of QPE applied alone or mixed with 2,4-D choline and/or glufosinate to control glufosinate/glyphosate-resistant corn volunteers in Enlist corn and 2) to determine the effect of application time (V3 or V6 growth stage of volunteer corn) of QPE-based treatments on volunteer corn control and Enlist corn injury and yield. Field experiments were conducted in Clay Center, NE, in 2021 and 2022. Quizalofop-p-ethyl (46 or 93 g ai ha−1) applied at the V3 or V6 growth stage controlled volunteer corn by ≥88% and ≥95% at 14 and 28 d after treatment (DAT), respectively. QPE (46 g ai ha−1) mixed with 2,4-D choline (800 g ae ha−1) produced 33% less than expected control of V3 volunteer corn in 2021, and 8% less than expected control of V6 volunteer corn in 2022 at 14 DAT. Volunteer corn control was improved by 7% to 9% using the higher rate of QPE (93 g ai ha−1) in a mixture with 2,4-D choline (1,060 g ae ha−1). QPE mixed with glufosinate had an additive effect and interactions in any combinations were additive beyond 28 DAT. Mixing 2,4-D choline can reduce QPE efficacy on glufosinate/glyphosate-resistant corn volunteers up to 14 DAT when applied at the V3 or V6 growth stage; however, the antagonistic interaction did not translate into corn yield loss. Increasing the rate of QPE (93 g ai ha−1) while mixing with 2,4-D choline can reduce antagonism.

More growers across the U.S. Midwest are considering interseeding or overseeding cover crops into corn for soil health purposes. One challenge of this practice is the potential injury from soil residual herbicides applied preemergence (PRE) for weed control in corn to the interseeded and overseeded cover crop species. Field-treated soil was collected in 2021 and 2022 at Janesville, WI, and Lancaster, WI, to investigate the impact of PRE residual herbicides on establishment of interseeded and overseeded cover crops via greenhouse bioassay. Soil samples (0 to 5 cm depth) were collected from field experiments at 0, 10, 20, 30, 40, 50, 60, and 70 days after treatment (DAT). Treatments consisted of 14 single and multiple sites of action (SOAs) PRE herbicides plus a nontreated check (NTC). Four bioindicator cover crop species were used in the greenhouse bioassay: annual ryegrass, cereal rye, radish, and red clover. Cover crop biomass was collected 28 d after bioassay seeding. Cover crop species responded differently across herbicide treatments. Annual ryegrass and cereal rye were sensitive to treatments containing herbicide Group 15, whereas Groups 2, 4, 5, 14, and 27 had minimal impact on their establishment when field soil was collected at 30 DAT (interseeding scenario) and 70 DAT (overseeding scenario) compared to the NTC. Radish and red clover were sensitive to herbicide Groups 2, 4, and 27, whereas Groups 5, 14, and 15 had minimal impact on their establishment. Annual ryegrass, radish, and red clover were more sensitive to PRE herbicides containing two and three SOAs than to herbicides with a single SOA. On the basis of these greenhouse bioassay results, cover crop species should be carefully selected depending on the soil residual herbicide when interseeded and overseeded into corn. Field studies will be conducted to validate these results and support recommendations to growers interested in this system.

Six field experiments were established in southwestern Ontario in 2021 and 2022 to evaluate whether the addition of a grass herbicide (acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor) to tolpyralate + atrazine improves late-season weed control in corn. Tolpyralate + atrazine caused 12% and 5% corn injury at 1 and 4 wk after herbicide application (WAA); corn injury was not increased with the addition of a grass herbicide. Weed interference reduced corn yield 60%. The addition of a grass herbicide to tolpyralate + atrazine did not enhance velvetleaf control. The addition of acetochlor or dimethenamid-p to tolpyralate + atrazine enhanced pigweed species control 4% 4 WAA; the addition of other grass herbicides tested did not increase pigweed species control. The addition of acetochlor enhanced common ragweed control 5% at 4 WAA, and the addition of acetochlor or dimethenamid-p enhanced common ragweed control 8% at 8 WAA; the addition of other grass herbicides did not improve common ragweed control. The addition of acetochlor to tolpyralate + atrazine enhanced common lambsquarters control up to 4%; there was no enhancement in common lambsquarters control with the addition of the other grass herbicides. Tolpyralate + atrazine controlled barnyardgrass 90% and 78% at 4 and 8 WAA, respectively; the addition of a grass herbicide enhanced barnyardgrass control 9% to 10% and 21% at 4 and 8 WAA, respectively. Tolpyralate + atrazine controlled green or giant foxtail 80% and 69% at 4 and 8 WAA, respectively; the addition of a grass herbicide enhanced foxtail species control 15% to 19% and 24% to 29% at 4 and 8 WAA, respectively. This research shows that adding a grass herbicide to tolpyralate + atrazine mixture can improve weed control efficacy, especially increased annual grass control in corn production.

Successful cover crop (CC) establishment in the fall is important to maximize CC production, which is critical for achieving many objectives of CCs. Competition from winter weeds may reduce CC establishment and biomass production. A preplant herbicide, such as paraquat, at the time of CC planting in the fall will reduce winter weed pressure resulting in better establishment and growth. An experiment was conducted between 2019 and 2021 to test this hypothesis by evaluating a no-CC check, cereal rye, hairy vetch, crimson clover, and cereal rye + hairy vetch drilled with and without paraquat applied at planting (mid-October to mid-November) following either a corn or soybean crop. Visible weed suppression ratings were collected in mid-April, and total CC and weed biomass was collected in late April. More CC biomass was accumulated following corn than soybean, regardless of preplant herbicide application, because corn is typically harvested before soybeans. Therefore, CCs should be planted early to accumulate more biomass. Weed suppression varied by weed species from all factors, but in general weed suppression was best from a CC mixture containing cereal rye and paraquat applied at planting. If weed suppression is the main goal of the CC, then a preplant herbicide at CC planting is recommended. However, if CC weed suppression goals can be achieved through biomass accumulation, no preplant herbicide is needed. This information is useful for producers to achieve various CC objectives while managing costs.

Allowing the use of two additional modes of action (MOAs), Enlist™ corn is a novelty in the continuum of herbicide-resistant crop development efforts that have occurred since the 1990s. Knowledge of Enlist corn tolerance to labeled herbicides and other herbicides within the same MOA for various use and/or exposure scenarios is not well established. Four site-year field experiments for preemergence (PRE) and postemergence (POST) applications were conducted at sites in Fayetteville (2021 and 2022) and Tillar (2020 and 2021), Arkansas, to evaluate Enlist corn response following PRE or POST applications of synthetic auxin herbicides or those that inhibit acetyl-CoA carboxylase (ACCase). A non-Enlist and an Enlist corn hybrid were used for each herbicide treatment to establish differential tolerance. Injury response to PRE application varied among site-years; clethodim was the only herbicide that occasionally caused significant (7% to 17%) injury to Enlist corn. None of the PRE treatments affected plant height, stand, or yield of Enlist corn; these responses were generally similar or better for Enlist corn compared to non-Enlist corn. Enlist corn showed significant injury to POST applications of florpyrauxifen-benzyl (>10%), fluazifop-P-butyl and quizalofop-P-ethyl (>5%), and clethodim and sethoxydim (>75%) 1 wk after application (WAA). These initial injury responses to clethodim and sethoxydim were generally reflected in Enlist corn yield; however, the minimal injury from fluazifop-P-butyl and quizalofop-P-ethyl did not affect yield. Injury to non-Enlist corn with POST-applied ACCase-inhibiting herbicides 2 WAA was >80%, resulting in a proportionate yield reduction. Even though florpyrauxifen-benzyl caused more initial injury to non-Enlist corn, yield reduction in non-Enlist corn was occasionally less than of Enlist corn, with both hybrids experiencing >75% yield reduction. In summary, Enlist corn may occasionally show transient injury even to labeled herbicides when applied POST, and even though the injury from florpyrauxifen-benzyl is initially mild, it nonetheless results in substantial yield loss.

Weed control in corn is a major challenge due to increasing problems with highly dominant weed species and herbicide resistance evolution. Common lambsquarters and johnsongrass constitute up to 80% to 90% of the weed population in many spring crops, such as soybean [Glycine max (L.) Merr.], sunflower (Helianthus annuus L.), and corn, in Serbia. Currently, acetolactate synthase–inhibiting herbicides, such as the systemic selective sulfonylurea nicosulfuron, are most commonly used for chemical weed control of those species. A better understanding of the impact of nozzle type and adjuvant use on nicosulfuron efficacy can help to improve control of common lambsquarters and johnsongrass and minimize herbicide resistance development. Field trials were conducted in Serbia from 2020 to 2022 to evaluate the impact of two adjuvants (a non-ionic surfactant [NIS] and a mineral fertilizer ammonium sulfate [AMS]) and two nozzle types (drift-reducing nozzles and flat-fan nozzles) on common lambsquarters and johnsongrass control using nicosulfuron. Satisfactory biomass reduction of common lambsquarters (83% to 87%) and johnsongrass (83% to 97%) was achieved after nicosulfuron application. Adding a NIS adjuvant increased the biomass reduction for common lambsquarters (94% to 98%) and johnsongrass (90% to 100%) independently of the nozzle type used. Selection of nozzle type did not show consistent effects on common lambsquarters and johnsongrass control. Nicosulfuron efficacy was increased with NIS adjuvant for both nozzle types compared to nicosulfuron solo for both species, and Extended Range (XR) TeeJet® nozzles on average resulted in a higher efficacy for common lambsquarters compared to Turbo TeeJet® induction. Adding a mineral AMS adjuvant resulted in lower biomass reduction for both nozzle types and weed species (65% to 78% and 61% to 91% for common lambsquarters and johnsongrass, respectively). Corn grain yield was predominantly influenced by annual meteorological conditions and adjuvant type added to nicosulfuron. This research suggests that addition of the non-ionic adjuvant is an essential factor for successful control of common lambsquarters and johnsongrass in corn and enables use of drift-reducing nozzles.

The herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) are primarily used for weed control in corn, barley, oat, rice, sorghum, sugarcane, and wheat production fields in the United States. The objectives of this review were to summarize 1) the history of HPPD-inhibitor herbicides and their use in the United States; 2) HPPD-inhibitor resistant weeds, their mechanism of resistance, and management; 3) interaction of HPPD-inhibitor herbicides with other herbicides; and 4) the future of HPPD-inhibitor-resistant crops. As of 2022, three broadleaf weeds (Palmer amaranth, waterhemp, and wild radish) have evolved resistance to the HPPD inhibitor. The predominance of metabolic resistance to HPPD inhibitor was found in aforementioned three weed species. Management of HPPD-inhibitor-resistant weeds can be accomplished using alternate herbicides such as glyphosate, glufosinate, 2,4-D, or dicamba; however, metabolic resistance poses a serious challenge, because the weeds may be cross-resistant to other herbicide sites of action, leading to limited herbicide options. An HPPD-inhibitor herbicide is commonly applied with a photosystem II (PS II) inhibitor to increase efficacy and weed control spectrum. The synergism with an HPPD inhibitor arises from depletion of plastoquinones, which allows increased binding of a PS II inhibitor to the D1 protein. New HPPD inhibitors from the azole carboxamides class are in development and expected to be available in the near future. HPPD-inhibitor-resistant crops have been developed through overexpression of a resistant bacterial HPPD enzyme in plants and the overexpression of transgenes for HPPD and a microbial gene that enhances the production of the HPPD substrate. Isoxaflutole-resistant soybean is commercially available, and it is expected that soybean resistant to other HPPD inhibitor herbicides such as mesotrione, stacked with resistance to other herbicides, will be available in the near future.

Injury to quizalofop-resistant rice was reported in some fields following postemergence applications of quizalofop. Glyphosate-resistant (GR) corn, cotton, and soybean, and imidazolinone-resistant rice are grown near quizalofop-resistant rice. Herbicide drift from glyphosate and imazethapyr and the resulting crop injury and potential yield loss is a cause of concern for producers. Field experiments conducted near Colt, and Keiser, AR, in 2021 evaluated whether low rates of glyphosate or imazethapyr interact with sequential quizalofop applications to exacerbate injury to quizalofop-resistant rice compared to quizalofop applications alone. Herbicide treatments consisted of a low rate of glyphosate (90 g ae ha−1) or imazethapyr (10.7 g ai ha−1) applied 10, 7, 4, and 0 d before the 2-leaf growth stage of rice, and glyphosate or imazethapyr, at the same rate and timings, followed by quizalofop at 120 g ai ha−1 applied to 2-leaf rice. All plots treated with quizalofop received a subsequent application of the same herbicide and rate at the 5-leaf rice stage. At 28 d after final treatment (DAFT), glyphosate followed by quizalofop the same day to 2-leaf rice caused 77% injury compared with 58% when glyphosate was applied alone, regardless of location. Glyphosate followed by quizalofop the same day reduced rough rice grain yield by 67% compared with 33% when glyphosate was applied alone to 2-leaf rice at the Colt location. Application of imazethapyr followed by quizalofop the same day to 2-leaf rice caused more injury (63% and 19% injury at the Colt and Keiser locations, respectively) than imazethapyr alone (42% and 7% injury at the Colt and Keiser locations, respectively) at 35 DAFT. Overall, glyphosate and imazethapyr followed by quizalofop applications worsened injury compared to glyphosate, imazethapyr, and quizalofop applications alone. As the interval between exposure to a low rate of glyphosate or imazethapyr and quizalofop decreases, the detrimental effect of herbicide on rice likewise increases.

Tolpyralate is an herbicide that is usually mixed with atrazine for broad-spectrum weed control in corn. Previous research has provided information on the effective dose (ED) of tolpyralate applied alone and in a 1:33.3 mixture with atrazine; however, tolpyralate is commercially applied at a dose of 30 to 40 g ai ha−1 with a minimum of 560 g ai ha−1 of atrazine. Therefore, five field trials were conducted over 3 yr (2019 to 2021) to determine the ED of atrazine to complement 30 g ai ha−1 of tolpyralate to achieve 80%, 90%, and 95% control of seven weed species 2, 4, and 8 wk after application (WAA). Tolpyralate was applied alone and in a mixture with atrazine doses ranging from 50 to 2,000 g ai ha−1. At 8 WAA, the ED of atrazine for 95% control of velvetleaf, common ragweed, common lambsquarters, and wild mustard was below the minimum label dose of atrazine on the commercial tolpyralate label, ranging from 430 to 520 g ai ha−1, which supports the use of the minimum label dose of atrazine. In contrast, redroot pigweed required 1,231 g ai ha−1 of atrazine to complement tolpyralate for 95% control 8 WAA. At 8 WAA, barnyardgrass and a mixture of green foxtail and giant foxtail (Setaria spp.) were not controlled by 80%, 90%, or 95% with tolpyralate applied alone or co-applied with any dose of atrazine evaluated in this study. The results of this study conclude that tolpyralate + atrazine is highly efficacious on several weed species at atrazine doses of 40 to 130 g ai ha−1 below the label dose of 560 g ai ha−1, but the use of the higher dose of tolpyralate or another herbicide may be required to improve control of redroot pigweed and grass weed species.

Nine field experiments were conducted from 2017 to 2019 in Ontario to determine the impact of early weed interference on corn yield based on corn growth stage, days after emergence (DAE), accumulated crop heat units (CHU), and weed size. The predicted weed size at herbicide application that resulted in a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was estimated to be 1, 4, 11, 53, non-estimable (N est.*), and N est.* cm under low weed density and 3, 5, 7, 11, 27, and N est.* cm under high weed density, respectively. The predicted DAE at herbicide application time that resulted in a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was predicted to be 14, 20, 27, 44, N est.*, and N est.* DAE under low weed density and 5, 7, 11, 17, 25, and 59 DAE under high weed density, respectively. The predicted CHU from planting at herbicide application time that led to a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was 468, 636, 821, 1,271, N est.*, and N est.* CHU from planting under low weed density and 207, 283, 385, 551, 972, and 1,748 CHU from planting under high weed density, respectively. The predicted crop stage at herbicide application that led to a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was V5, V6, V7, V11, N est.*, and N est.* under low weed density and V1, V2, V3, V4, V8, and V14 under high weed density, respectively. Results indicate that weeds must be controlled before they reach 7 cm in height, prior to 11 d after crop emergence, prior to 385 accumulated CHU from emergence, or prior to the V3 stage under high weed density to avoid greater than 5% yield loss.

Recent legalization of industrial hemp in the United States has led to increased interest among stakeholders to produce hemp for grain and fiber. However, owing to the lack of herbicides registered for use in hemp, producers are left with limited weed management strategies. Moreover, much of the agricultural land that could be used to cultivate industrial hemp may be prone to carryover of previously applied residual herbicides or physical drift from herbicides sprayed nearby. Industrial hemp sensitivity to herbicides is not well documented. Dose–response studies were conducted under controlled conditions in Madison, WI, screening two industrial hemp grain cultivars for tolerance to 44 preemergence and postemergence herbicides commonly used in corn and soybean. Treatments consisted of herbicides applied at 0×, 0.125×, 0.25×, 0.50×, 0.75×, 1×, 2×, and 4× the recommended maximum labeled rates based on soil type. Preemergence applications were delivered immediately after planting, whereas postemergence applications took place when hemp plants reached 5 to 10 cm in height. Nontreated plants served as the control and were used to estimate percent biomass reduction; dose–response curves were generated. Biomass reduction was >50% for rates under the suggested label rate for 23 preemergence and 21 postemergence herbicides tested. All herbicides tested resulted in >25% biomass reduction at the 0.125× rate, except for clopyralid applied preemergence and postemergence and saflufenacil applied preemergence. This is concerning, as the label rates are determined for effective weed control and the mitigation of herbicide resistance. Overall, these results indicate that industrial hemp is very sensitive to most herbicides tested. Growers should consider herbicide use history and surrounding crops when determining industrial hemp field selection to prevent significant plant injury due to herbicide carryover and drift. Further research into alternative methods of weed control will be vital to establishing hemp as a dominant crop once again.

Residual herbicides applied PRE provide early season weed control, potentially avoid the need for multiple POST herbicides, and can provide additional control of herbicide-resistant weeds. Thus, field studies were conducted in 2017 and 2018 at Concord, NE, to evaluate the influence of PRE herbicides on critical time for postemergence weed removal (CTWR) in corn. The studies were arranged in a split-plot design that consisted of three herbicide regimes as main plot treatments and seven weed removal timings as subplot treatments in four replications. The herbicide regimes included no PRE herbicide, atrazine, and a premix of saflufenacil/dimethenamid-P mixed with pyroxasulfone. The weed removal timings were at V3, V6, V9, V12, and V15 corn growth stages and then plots were kept weed-free until harvest. A weed-free and nontreated control were included for comparison. The relationship between corn growth or yield, and weed removal timings in growing degree days (GDD) was described by a four-parameter log-logistic model. This model was used to estimate the critical time for weed removal based on 5% crop yield loss threshold. A delay in weed removal until the V2 to V3 corn growth stage (91 to 126 GDD) reduced corn biomass by 5% without PRE herbicide application. The CTWR started at V3 without PRE herbicide in both years. Atrazine delayed the CTWR up to V5 in both years, whereas saflufenacil/dimethenamid-P plus pyroxasulfone further delayed the CTWR up to the V10 and V8 corn growth stages in 2017 and 2018, respectively. Herbicide applied PRE particularly with multiple sites of action can delay the CTWR in corn up to a maximum growth stage of V10, and delay or reduce the need for POST weed management.

A stakeholder survey was conducted from April through June of 2018 to understand stakeholders’ perceptions and challenges about cropping systems and weed management in Brazil. The dominant crops managed by survey respondents were soybean (73%) and corn (66%). Approximately 75% of survey respondents have grown or managed annual cropping systems with two to three crops per year cultivated in succession. Eighteen percent of respondents manage only irrigated cropping systems, and over 60% of respondents adopt no-till as a standard practice. According to respondents, the top five troublesome weed species in Brazilian cropping systems are horseweed (asthmaweed, Canadian horseweed, and tall fleabane), sourgrass, morningglory, goosegrass, and dayflower (Asiatic dayflower and Benghal dayflower). Among the nine species documented to have evolved resistance to glyphosate in Brazil, horseweed and sourgrass were reported as the most concerning weeds. Other than glyphosate, 31% and 78% of respondents, respectively, manage weeds resistant to acetyl-CoA carboxylase (ACCase) inhibitors and/or acetolactate synthase (ALS) inhibitors. Besides herbicides, 45% of respondents use mechanical, and 75% use cultural (e.g., no-till, crop rotation/succession) weed control strategies. Sixty-one percent of survey respondents adopt cover crops to some extent to suppress weeds and improve soil chemical and physical properties. Nearly 60% of survey respondents intend to adopt the crops that are resistant to dicamba or 2,4-D when available. Results may help practitioners, academics, industry, and policy makers to better understand the bad and the good of current cropping systems and weed management practices adopted in Brazil, and to adjust research, education, technologies priorities, and needs moving forward.

Herbicide resistance has for decades been an increasing problem of agronomic crops such as corn and soybean. Several weed species have evolved herbicide resistance in turfgrass systems such as golf courses, sports fields, and sod production—particularly biotypes of annual bluegrass and goosegrass. Consequences of herbicide resistance in agronomic cropping systems indicate what could happen in turfgrass if herbicide resistance becomes broader in terms of species, distribution, and mechanisms of action. The turfgrass industry must take action to develop effective resistance management programs while this problem is still relatively small in scope. We propose that lessons learned from a series of national listening sessions conducted by the Herbicide Resistance Education Committee of the Weed Science Society of America to better understand the human dimensions affecting herbicide resistance in crop production provide tremendous insight into what themes to address when developing effective resistance management programs for the turfgrass industry.

Atrazine offers growers a reliable option to control a broad spectrum of weeds in grain sorghum production systems when applied PRE or POST. However, because of the extensive use of atrazine in grain sorghum and corn, it has been found in groundwater in the United States. Given this issue, field experiments were conducted in 2017 and 2018 in Fayetteville and Marianna, Arkansas, to explore the tolerance of grain sorghum to applications of assorted photosystem II (PSII)-inhibiting herbicides in combination with S-metolachlor (PRE and POST) or mesotrione (POST only) as atrazine replacements. All experiments were designed as a factorial, randomized complete block; the two factors were (1) PSII herbicide and (2) the herbicide added to create the mixture. The PSII herbicides were prometryn, ametryn, simazine, fluometuron, metribuzin, linuron, diuron, atrazine, and propazine. The second factor consisted of either no additional herbicide, S-metolachlor, or mesotrione; however, mesotrione was excluded in the PRE experiments. Crop injury estimates, height, and yield data were collected or calculated in both studies. In the PRE study, injury was less than 10% for all treatments except those containing simazine, which caused 11% injury 28 d after application (DAA). Averaged over PSII herbicide, S-metolachlor–containing treatments caused 7% injury at 14 and 28 DAA. Grain sorghum in atrazine-containing treatments yielded 97% of the nontreated. Grain sorghum receiving other herbicide treatments had significant yield loss due to crop injury, compared with atrazine-containing treatments. In the POST study, ametryn- and prometryn-containing treatments were more injurious than all other treatments 14 DAA. Grain sorghum yield in all POST treatments was comparable to atrazine, except prometryn plus mesotrione, which was 65% of the nontreated. More herbicides should be evaluated to find a comparable fit to atrazine when applied PRE in grain sorghum. However, when applied POST, diuron, fluometuron, linuron, metribuzin, propazine, and simazine have some potential to replace atrazine in terms of crop tolerance and should be further tested as part of a weed control program across a greater range of environments.