Palmer amaranth with resistance to dicamba, glufosinate, and protoporphyrinogen oxidase inhibitors has been documented in several southern states. With extensive use of these and other herbicides in South Carolina, a survey was initiated in fall 2020 and repeated in fall 2021 and 2022 to determine the relative response of Palmer amaranth accessions to selected preemergence and postemergence herbicides. A greenhouse screening experiment was conducted in which accessions were treated with three preemergence (atrazine, S-metolachlor, and isoxaflutole) and six postemergence (glyphosate, thifensulfuron-methyl, fomesafen, glufosinate, dicamba, and 2,4-D) herbicides at the 1× and 2× use rates. Herbicides were applied shortly after planting (preemergence) or at the 2- to 4-leaf growth stage (postemergence). Percent survival was evaluated 5 to 14 d after application depending on herbicide activity. Sensitivity to atrazine preemergence was lower for 49 and 33 accessions out of 115 to atrazine applied preemergence at the 1× and 2× rate, respectively. Most of the accessions (90%) were controlled by isoxaflutole applied preemergence at the 1× rate. Response to S-metolachlor applied preemergence indicated that 34% of the Palmer amaranth accessions survived the 1× rate (>60% survival). Eleven accessions exhibited reduced sensitivity to fomesafen applied postemergence; however, these percentages were not different from the 0% survivor group. Glyphosate applied postemergence at the 1× rate did not control most accessions (79%). Palmer amaranth response to thifensulfuron-methyl applied postemergence varied across the accessions, with only 36% and 28% controlled at the 1× rate and 2× rate, respectively. All accessions were controlled by 2,4-D, dicamba, or glufosinate when they were applied postemergence. Palmer amaranth accessions from this survey exhibited reduced susceptibility to several herbicides commonly used in agronomic crops in South Carolina. Therefore, growers should use multiple management tactics to minimize the evolution of herbicide resistance in Palmer amaranth in South Carolina.

Cover crop adoption is increasing among growers with the occurrence of herbicide-resistant weed species. A field study conducted at three sites from autumn 2021 through the crop harvest in 2022 in Alabama aimed to evaluate the combined effect of cover crop residue and herbicides for weed control and improved cotton lint yield. The experiment was conducted in split-plot design with main plots consisting of six cover crop treatments: cereal rye, crimson clover, oat, radish, cover crop mixture, and winter fallow. The subplots included four herbicide treatments: (i) preemergence, pendimethalin + fomesafen, (ii) postemergence, dicamba + glyphosate + S-metolachlor, (iii) preemergence followed by postemergence, and (iv) nontreated (NT) check. Cover crops, excluding radish, exhibited greater weed biomass reduction than winter fallow with corresponding herbicide treatments of either preemergence, postemergence, or preemergence + postemergence as compared to control (winter fallow and NT check). Considering preemergence + postemergence treatment, cereal rye, crimson clover, oat, and cover crop mixture provided >95% weed biomass reduction as compared to control. Looking at the overall effect of cover crop, cereal rye outperformed and showed greater weed biomass reduction than radish relative to control. Preemergence + postemergence herbicide treatment resulted in greater lint yield than other treatments. Cotton in cereal rye plots had a greater lint yield than in winter fallow at one out of three locations. In conclusion, integrating herbicides and incorporating high-residue cover crops such as cereal rye is an effective weed management strategy to control troublesome weeds.

Twelve putative-resistant (R) redroot pigweed populations were collected in sunflower and soybean fields located in northeastern Greece, after repeated exposure to the acetolactate synthase (ALS)–inhibiting herbicides imazamox and tribenuron-methyl. Studies were conducted to determine the resistance status to these two ALS-inhibiting herbicides and evaluate alternative postemergence and preemergence herbicides for effective control. Two susceptible (S) populations were also included for comparison. Among the 12 putative-R populations studied in the whole-plant dose–response pot experiments, 11 were characterized as cross-resistant (R) to the imidazolinone imazamox and the sulfonylurea tribenuron-methyl. In contrast, the putative R5 and the two reference populations (S1, S2) populations were found to be susceptible. Sequencing of the ALS gene revealed that a point mutation (TGG to TTG at position 574) was selected in domain B, where in combination with domain A the majority of point mutations conferring resistance have been detected, resulting in an amino acid substitution from tryptophan (Trp) to leucine (Leu) in the 11 R populations. By contrast, all sequenced plants of the three susceptible populations were found with the wild-type allele encoding Trp574. The labeled rate of the postemergence herbicides tembotrione and dicamba provided fair to excellent control of the populations with ALS cross-resistance. In contrast, at this rate the preemergence herbicides S-metolachlor + terbuthylazine, isoxaflutole, aclonifen, metribuzin, and pendimethalin provided excellent control. These findings strongly suggest that 11 redroot pigweed populations have evolved cross-resistance to ALS-inhibiting herbicides, but viable options for chemical control of this weed still exist.

Benghal dayflower and sicklepod are weeds of economic importance in peanut in the southeastern United States due to their extended emergence pattern and limited effective herbicides for control. Field studies were conducted near Jay, Florida, in 2022 and 2023, to evaluate the effect of planting date and herbicide combinations on Benghal dayflower and sicklepod control in peanut crops. Peanut planted in June was exposed to a higher Benghal dayflower density than peanut planted in May. Sicklepod density was similar between May and June planting dates at 28 d after preemergence and early postemergence herbicide applications, but density was greater in peanut that was planted in June, 28 d after the mid-postemergence application. A preemeergence herbicide application followed by (fb) an early postemergence application of S-metolachlor or diclosulam + S-metolachlor controlled Benghal dayflower 84% to 93% 28 d after early postemergence in peanut that was planted in May, but control was reduced to 58% to 78% in the crop that had been planted in June. Regardless of planting date, a preemeergence application fb S-metolachlor or diclosulam + S-metolachlor applied early postemergence provided <80% sicklepod control 28 d after early postemergence. Imazapic + dimethenamid-P + 2,4-DB applied postemergence improved Benghal dayflower control to at least 94% 28 d after mid-postemergence, but sicklepod control was not >85%. Regardless of the planting date, paraquat + bentazon + S-metolachlor applied early postemergence was required to achieve ≥95% sicklepod control. However, herbicide combinations that included paraquat + bentazon + S-metolachlor reduced peanut yield when planting was delayed to June. In fields that are infested with Benghal dayflower and sicklepod, it is recommended that peanut be planted in early May to minimize the potential impact of these weeds and to increase peanut yield. Late-planted peanut required more intensive herbicide applications to obtain the same peanut yield as the May-planted peanut.

Flumioxazin and S-metolachlor are widely used in conventional sweetpotato production in North Carolina and other states; however, some growers have recently expressed concerns about potential effects of these herbicides on sweetpotato yield and quality. Previous research indicates that activated charcoal has the potential to reduce herbicide injury. Field studies were conducted in 2021 and 2022 to determine whether flumioxazin applied preplant and S-metolachlor applied before and after transplanting negatively affect sweetpotato yield and quality when activated charcoal is applied with transplant water. The studies evaluated five herbicide treatments and two activated charcoal treatments. Herbicide treatments included two flumioxazin rates, one S-metolachlor rate applied immediately before and immediately after transplanting, and no herbicide. Charcoal treatments consisted of activated charcoal applied at 9 kg ha−1, and no charcoal. No visual injury from herbicides or charcoal was observed. Likewise, no effect of herbicide or charcoal treatment on no. 1, marketable (sum of no. 1 and jumbo grades), or total yield (sum of canner, no. 1, and jumbo grades) was observed. Additionally, shape analysis conducted on calculated length-to-width ratio (LWR) for no. 1 sweetpotato roots found no effect from flumioxazin at either rate on sweetpotato root shape. However, both S-metolachlor treatments resulted in lower LWR of no. 1 sweetpotato roots in 2021. Results are consistent with prior research and indicate that flumioxazin and S-metolachlor are safe for continued use on sweetpotato at registered rates.

Sicklepod is one of the most difficult to control weeds in peanut production in the southeastern United States due to its extended emergence pattern and limited effective herbicides for control. Growers rely on preemergence herbicides as the foundation of their weed control programs; however, postemergence herbicides are often needed for season-long weed control. The objectives of this study were to evaluate the effect of planting pattern and herbicide combinations for sicklepod control in peanut crops. Due to rapid canopy closure, twin-row planting improved late-season sicklepod control by 13% and peanut yield by 5% compared with a single-row pattern. A preemergence application of fluridone, flumioxazin, or fluridone + flumioxazin provided 76% to 89% control of sicklepod 28 d after preemergence. Regardless of the herbicide applied preemergence, paraquat + bentazon + S-metolachlor applied early postemergence was required to achieve ≥90% sicklepod control 28 d after early postemergence. All preemergence herbicide treatments followed by (fb) S-metolachlor or diclosulam + S-metolachlor applied early postemergence provided <90% control 28 d after early postemergence. A mid-postemergence application of imazapic + dimethenamid-P + 2,4-DB controlled sicklepod by 67% to 79% prior to peanut harvest, and biomass reduction was unacceptable (<80%), resulting in difficulty in peanut digging. The highest peanut yield was observed when paraquat + bentazon + S-metolachlor was applied early postemergence fb imazapic + dimethenamid-P + 2,4-DB applied mid-postemergence. Based on the results of this study, a herbicide combination of paraquat + bentazon + S-metolachlor is an important early-season tool for controlling sicklepod in peanut crops. The results also showed that a twin-row planting pattern improved late-season sicklepod control but did not reduce herbicide input to protect peanut yield.

In mid-southern, southeastern, and northeastern U.S. soybean production regions, the evolution of herbicide-resistant weeds has become a significant management challenge for growers. The rising herbicide costs for managing herbicide-resistant weeds are also a growing concern, leading to the utilization of cover crops as an integrated weed management strategy for addressing these challenges. Field experiments were conducted at two locations in Alabama in 2022 to evaluate winter cereal cover crops, including a mixture and herbicide system integration in soybean. Treatments included five cover crops: oats, cereal rye, crimson clover, radish, and a cover crop mixture. Cover crops were evaluated for their weed-suppressive characteristics compared to a winter fallow treatment. Additionally, four herbicide treatments were applied: a preemergence (PRE) herbicide, a postemergence (POST) herbicide, PRE plus POST herbicides, and a nontreated (NT) check. The PRE herbicide was S-metolachlor; the POST treatment contained a mixture of dicamba and glyphosate. The PRE plus POST system contained the PRE application followed by POST application. Results show that cereal rye and the cover crop mixture provided weed biomass reduction compared to all cover crop treatments across both locations. Furthermore, we observed greater soybean yield following the cereal rye cover crop than following the winter fallow treatment at one location. POST and PRE plus POST herbicide treatment resulted in greater weed biomass reduction and improved soybean yield than the PRE herbicide treatment alone and the NT check at both locations.

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.

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.

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.

In 2021 and 2022, research was initiated at two locations to evaluate the efficacy and safety of sulfentrazone in transplanted cabbage and broccoli. Treatments included oxyfluorfen at 560 g ha−1 applied pretransplant (PRE-T), sulfentrazone applied at 116 or 233 g ha−1 PRE-T, and S-metolachlor applied at 715 g ha−1 immediately after transplanting (POST-T) followed by (fb) oxyfluorfen applied at 210 g ha−1 postemergence (POST) 14 d after planting (DAP). The weedy cover of nontreated plots averaged between 6% (14 DAP) and 72% (42 DAP); all herbicide-treated plots averaged less than 30% cover at 42 DAP. At 14 and 28 DAP, oxyfluorfen, S–metolachlor fb oxyfluorfen, and the high rate of sulfentrazone reduced total monocotyledonous and dicotyledonous weed densities by 62% and 100%, respectively, relative to the nontreated control. The density of hairy galinsoga (in New Jersey) and combined ladysthumb and prostrate knotweed (in New York) were reduced by 71% to 99%. Except for the low rate of sulfentrazone, all herbicide treatments reduced weed biomass at harvest by ≥88%. Crop injury varied in response to herbicide treatments or weed competition but was also affected by crop and location. Between 14 and 28 DAP, the greatest amount of stunting (22%) was noted in the S-metolachlor fb oxyfluorfen treatments at both locations. Averaged over treatments, greater stunting was observed in broccoli than in cabbage in New York, whereas stunting estimates were higher for cabbage in New Jersey. All treatments in New Jersey resulted in significantly increased cabbage yield and broccoli and cabbage head sizes relative to the nontreated controls. No yield difference was noted between herbicide treatments and the nontreated check in New York. Data derived from these studies will be used to enhance crop safety recommendations in northeastern U.S. production environments for sulfentrazone used PRE in transplanted cabbage and support a potential label for broccoli.

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.

Atrazine and S-metolachlor are the herbicides most relied on by growers to control weeds in sweet corn crops grown in the Everglades Agricultural Area (EAA) in southern Florida. Alternative weed management programs are needed. Field experiments were conducted in 2021 and 2022 to evaluate the efficacy of 1) pyroxasulfone (183 and 237 g ha−1) alone or as a premix with carfentrazone-ethyl (13 and 17 g ha−1) or fluthiacet-methyl (6 and 7 g ha−1), S-metolachlor (1,790 g ha−1) alone or in combination with atrazine (3,360 g ha−1) applied preemergence(PRE); 2) mesotrione (105 g ha−1), topramezone (25 g ha−1), and tembotrione (92 g ha−1) applied postemergence alone or in combination with atrazine (560 and 2,240 g ha−1) or bentazon (1,120 g ha−1); and 3) mechanical cultivation alone at the fourth and the fourth followed by the sixth leaf stages of sweet corn. PRE-applied herbicides did not provide acceptable control of fall panicum, common lambsquarters, or common purslane probably due to a lack of incorporation into the soil because of limited rainfall. POST-applied topramezone alone or in combination with atrazine or bentazon resulted in effective fall panicum control (>91%). Topramezone alone provided 83% and 88% control of common lambsquarters and common purslane, respectively, whereas atrazine added to topramezone resulted in >94% control of both weed species. Mesotrione and tembotrione plus atrazine provided excellent control (>93%) of both broadleaf weed species but poor fall panicum control (<72%). Mechanical cultivation alone did not effectively control any weeds. Overall, treatments that contained topramezone resulted in greater sweet corn yield. These results show that a combination of topramezone, mesotrione, and tembotrione with atrazine resulted in improved broadleaf weed control. Fall panicum control was improved only with the combination of topramezone with atrazine, showing that atrazine is an important mixture component of these herbicides to provide effective POST weed control in sweet corn on organic soils of the EAA.

Cereal rye cover crop (cereal rye) and preemergence (PRE) herbicides are becoming common practices for managing herbicide-resistant weeds in soybean production. Adopting these two practices in combination raises concerns regarding herbicide fate in soil, given that the cereal rye biomass can intercept the herbicide spray solution, preventing it from reaching the soil. Delaying cereal rye termination until soybean planting (planting green) optimizes biomass accumulation but might also increase PRE interception. To better understand the dynamics between cereal rye and PRE herbicides, a field experiment was conducted to evaluate two soil management practices (tillage and no-till) and two cereal rye termination practices in the planting-green system (glyphosate [1,260 g ae ha−1] and roller-crimper) on the spray deposition and fate of PRE herbicides and soybean yield. The spray deposition was assessed by placing water-sensitive paper cards on the soil surface before spraying the PRE herbicides (sulfentrazone [153 g ai ha−1] + S-metolachlor [1,379 g ai ha−1]). Herbicide concentration in soil (0 to 7.6 cm) was quantified 25 d after treatment (DAT). The presence of no-till stubble and cereal rye biomass reduced the spray coverage compared to tillage at PRE application, which reflected in a reduction in the concentration of both herbicides in soil 25 DAT. Soybean yield was reduced in all three years when the cereal rye was terminated with a roller-crimper but only reduced in one year when terminated with glyphosate. Our findings indicate that mainly cereal rye biomass reduced the concentration of PRE herbicides in the soil due to the interception of the spray solution during application. Although higher cereal rye biomass accumulation can provide better weed suppression according to the literature, farmers should be aware that the biomass can lower the concentration of PRE herbicides reaching the soil, thus intensifying field scouting to ensure that weed control is not being negatively affected.

Weed management in cantaloupe and other melon crops is important to maximize fruit yield; however, there are few registered herbicides available in California. Several independent herbicide trials were conducted at University of California field stations in Davis (Yolo County), Five Points (Fresno County), and Holtville (Imperial County) from 2013 to 2019 to evaluate both registered and unregistered herbicides and incorporation methods (sprinklers, cultivation, or none) for crop safety and weed control in melons. Although specific treatments varied among locations depending on local practice and research objectives, ethalfluralin and halosulfuron were used in all experiments, and bensulide and S-metolachlor were evaluated in 4 of 6 site-years. Additional herbicides included clethodim, clomazone, DCPA, napropamide, pendimethalin, sethoxydim, and sulfentrazone. Among registered herbicides, halosulfuron, halosulfuron + ethalfluralin, and ethalfluralin + bensulide combinations provided consistently beneficial weed control across all site-years compared to the nontreated control. S-metolachlor performed as well as the best of the registered herbicides tested at each site-year; although moderate injury was noted at the Davis location, this did not reduce melon yield. The method used to incorporate preplant herbicides had a significant impact on weed control efficacy but varied by location. Mechanical incorporation of preplant herbicides resulted in improved weed control and yield compared to sprinklers. Early-season weed control, whether by herbicides or hand weeding, resulted in significant yield increase in most site-years.

Drill-interseeding cover crops into standing corn (V3 to V5 growth stage) provides opportunities for increasing functional diversity in cropping systems by facilitating use of cover crop mixtures that include grass, legume, and brassica species. Designing herbicide-based weed control programs that negotiate tradeoffs between crop protection and environmental goals when interseeding mixtures remains a major challenge. The objective of this study was to use greenhouse-based dose-response assays to describe the relative sensitivity of 12 cover crop species that differ in traits, including taxonomic group and seed mass, to chloroacetamide (acetochlor, dimethenamid, S-metolachlor; Group 15 herbicides as categorized by the Weed Science Society of America) and pyrazole (pyroxasulfone; Group 15) herbicides. Nonlinear models were fit, and effective doses were estimated (ED50) to compare relative sensitivities of cover crop species to each herbicide. Brassica species (winter canola, Dwarf Essex rape, daikon radish) were less sensitive than small-seeded legumes (medium red clover, crimson clover) and grasses (annual ryegrass) in response to each chloroacetamide, but similar in response to pyroxasulfone. Austrian winter pea, a large-seeded legume, was less sensitive to each herbicide compared to other legumes (crimson clover, medium red clover, hairy vetch). Cereal rye and triticale were less sensitive to dimethenamid, S-metolachlor, and pyroxasulfone compared to annual ryegrass, but similar sensitivities were observed for acetochlor. These results suggest that relative differences in sensitivity to chloroacetamide and pyrazole herbicides could be exploited when designing cover crop mixtures for interseeding. It is imperative, however, to use these findings in conjunction with field-based observations of cover crop injury potential in interseeded systems.

Preemergence (PRE) herbicides are often banded over the entire top of raised beds for broadleaf and grass control in plasticulture vegetable production systems. However, broadleaf and grass weeds may emerge from the planting holes and tears in the plastic mulch. Banded application results in herbicides applied where no holes occur, and therefore, where they are not needed. Our objective is to identify herbicides that do not harm transplanted crops when directed at transplant holes after transplant (POST) with the aim to reduce off-target applications. Therefore, we evaluated tomato and pepper tolerance to PRE herbicides applied to transplant holes 2 wk after transplant and the subsequent effects on crop tolerance and weed density. Halosulfuron, S-metolachlor, metribuzin, and pendimethalin did not injure tomato transplants, reduce height, or reduce yield. Fomesafen caused some tomato injury (7%) but had no effect on other measured parameters in Trial I. All PRE herbicides injured peppers by ≥19%, although no effect on yield was observed. Overall, halosulfuron, S-metolachlor, metribuzin, and pendimethalin can be safely applied to tomato transplant holes 2 wk after transplant with no significant crop injury nor effects on final yield, but none of the evaluated herbicides are safe for use on pepper crops.

A comprehensive, Wisconsin state-wide assessment of waterhemp response to a diverse group of herbicide sites of action has not been conducted. Our objective was to characterize the response of a state-wide collection of waterhemp accessions to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean in Wisconsin. Greenhouse experiments were conducted with more than 80 accessions from 27 counties. POST treatments included 2,4-D, atrazine, dicamba, fomesafen, glufosinate, glyphosate, imazethapyr, and mesotrione at 1× and 3× label rates. PRE treatments included atrazine, fomesafen, mesotrione, metribuzin, and S-metolachlor at 0.5×, 1×, and 3× label rates. Ninety-eight percent and 88% of the accessions exhibited ≥50% plant survival after exposure to imazethapyr and glyphosate POST 3× rate, respectively. Seventeen percent, 16%, and 3% of the accessions exhibited ≥50% plant survival after exposure to 2,4-D, atrazine, and dicamba, respectively, applied POST at the 1× rate. Survival of all accessions was ≤25% after exposure to 2,4-D or dicamba applied POST at the 3× rate, or glufosinate, fomesafen, and mesotrione applied POST at either rate evaluated. No plant of any accession survived exposure to glufosinate at either rate. Forty-five percent and 3% of the accessions exhibited <90% plant density reduction after exposure to atrazine applied PRE at the 3× rate and fomesafen PRE at the 1× rate, respectively. Plant density reduction of all accessions was ≥96% after exposure to fomesafen applied PRE at the 3× rate, or metribuzin, S-metolachlor, and mesotrione applied PRE at the 1× rate. Our results suggest that waterhemp resistance to imazethapyr and glyphosate applied POST is widespread in Wisconsin, whereas resistance to 2,4-D, atrazine, and dicamba applied POST is present to a lower extent. One accession (A75, Fond du Lac County) exhibited multiple resistance to imazethapyr, atrazine, glyphosate, and 2,4-D when applied POST. Overall, atrazine applied PRE was ineffective for waterhemp control in Wisconsin. Proactive resistance management and the use of effective PRE and POST herbicides are fundamental for waterhemp management in Wisconsin.

Hazelnut hectarage is expanding in Oregon. Weed competition in young orchards can severely reduce the growth and survival of plants. New orchards replace crops, including grass seed fields, which often are infested with herbicide-resistant weeds, including Italian ryegrass. This research evaluated hazelnut tolerance to pronamide, pyroxasulfone, and S-metolachlor. Three multi-year field experiments were conducted at newly planted orchards in the Willamette Valley during 2019 and 2020. Treatments compared pyroxasulfone (0.24 kg ai ha–1), pronamide (2.3 kg ai ha–1), and S-metolachlor (1.39 kg ai ha–1) applied at the reference rate, and at 2× and 4× that rate, compared to weed-free check. Treatments were applied within 2 wk after the winter transplant and reapplied the following year. Hazelnuts showed a high tolerance to all herbicides tested, with negligible injury noted (<3%). No changes in leaf chlorophyll were noted, averaging 242, 179, and 225 mg m–2 on each study site. Tree growth was similar among treatments as measured by trunk cross-sectional area, canopy volume, and internode length. A separate study evaluated the control of Italian ryegrass. Pronamide and pyroxasulfone provide 100% control of Italian ryegrass, and weed dry weight was reduced by up to 79 % compared to the grower standard. This study documents that hazelnuts are tolerant to pronamide, pyroxasulfone, and S-metolachlor, and that these herbicides can improve weed management in young orchards.

Small-acreage brassica vegetables need additional herbicide options. Among the vegetables grown in California are a number of niche crops, such as bok choi and brussels sprouts, that have a limited number of registered herbicides, such as DCPA. Sulfentrazone and S-metolachlor have food use tolerances for use on brassica head and stem Group 5-16, which includes crops like bok choi and brussels sprouts, as well as brassica leafy greens Subgroup 4-16B, which includes crops like kale. However, there is a lack of data for S-metolachlor and sulfentrazone on a wide variety of seeded and transplanted brassica vegetables. S-metolachlor applied preemergence (PRE) was evaluated on six direct-seeded brassica vegetables during 2019 and 2020, including bok choi, broccoli rabe, collard, mizuna, radish, and mustard greens. S-metolachlor and sulfentrazone were both evaluated PRE in transplanted brussels sprouts and kale. The results indicate that most of the seeded brassica vegetables were tolerant of S-metolachlor and that transplanted brassica vegetables were tolerant of both S-metolachlor and sulfentrazone. Broccoli rabe was moderately injured in 2020, but yields did not vary among treatments either year.