Multiple Herbicide Research Articles

Changing dominance of invasive Phragmites australis and native plant colonization with variation in management, wildfires, and soils in a desert wetland

Abstract Among the most widely distributed species globally, common reed (Phragmites australis (Cav.) Trin. ex Steud.) has generated extensive interest in invasive plant science and management because its introduced strains are highly invasive and often form monocultures that alter ecosystem properties. In desert wetlands in Las Vegas, Nevada, USA, where management goals included reducing hazardous P. australis fuels and increasing native plant diversity, we assessed variation in P. australis cover, the degree of native plant colonization, and soil seed banks after P. australis management treatments (cutting, glyphosate-imazapyr herbicide) and wildfires across gradients in soil properties. Based on change in P. australis cover during six measurement events over 24 months, 24 study sites formed three groups: 1) decreasing cover, where initially high P. australis cover (60-85%) decreased to < 5% following multiple cutting or herbicide treatments; 2) sustaining low cover, where wildfire or clearing was associated with initially low P. australis cover which remained low (< 30%) after multiple herbicide applications; and 3) sustaining high cover (45-100% initially and remaining 30-100%), including sites unmanaged or treated/burned only once. High soil salinity correlated with low post-management P. australis cover. No native plants were detected in the sustaining high P. australis cover group, despite natives occurring in the seed bank. Where management reduced P. australis cover, minimal native plant colonization did occur. Secondary invasion by other non-native plants was nearly absent. Our results suggest that if P. australis can be initially cleared, multiple herbicide applications can persistently keep cover low, especially on drier, saline soils. Slow native plant colonization suggests that a phased approach may be useful to initially reduce P. australis cover, keep it low via repeated treatments, and actively revegetate sites with native species tailored to the moisture-salinity gradient across P. australis-invaded habitats.

Multiple Herbicide Resistance in Annual Ryegrass (Lolium rigidum Gaudin) in the Southeastern Cropping Region of Australia

Annual ryegrass (Lolium rigidum) is a problematic weed in winter crops and fallows in the southeastern cropping region (SCR) of Australia. This weed has evolved resistance to multiple herbicide groups, globally. In Australia, L. rigidum is more prevalent in the western and southern regions than in SCR. To assess the herbicide resistance status of L. rigidum, the response of five L. rigidum populations (collected from the SCR) to glyphosate, glufosinate, paraquat, haloxyfop-P-ethyl, and clethodim is determined using dose–response curves. Three parametric logistic models are used to determine the herbicide dose required to achieve 50% survival (LD50) and 50% growth reduction (GR50). The LD50 values for 50% survival at 28 days after treatment range from 1702 g a.e. ha−1 to 8225 g a.e. ha−1 for glyphosate, 1637 g a.i. ha−1 to 1828 g a.i. ha−1 for glufosinate, 141 g a.i. ha−1 to 307 g a.i. ha−1 for paraquat, 11 g a.i. ha−1 to 107 g a.i. ha−1 for haloxyfop-P-ethyl, and 17 g a.i. ha−1 to 48 g a.i. ha−1 for clethodim. The resistance factor, based on GR50 value, is highest in the S7 population (2.2 times) for glyphosate, the S11 population (2.3 times) for glufosinate, the S11 population (2.0 time) for paraquat, the S7 population (3.9 times) for haloxyfop-P-ethyl, and the S3 population (3.1 times) for clethodim, compared with the susceptible or less tolerant population. The S11 population is found to be resistant to five tested herbicides, based on resistance factors. Similarly, the S3 population is highly resistant to glyphosate, haloxyfop-P-ethyl, and clethodim compared with the W4 population. These results suggest that L. rigidum populations in the SCR exhibit resistance to multiple herbicide groups at labelled field rates. The findings highlight the necessity of adopting an integrated management approach, including the use of residual herbicides, tank mixing herbicides with different modes of action, and rotating herbicides in conjunction with cultural and mechanical control methods.

Degradation of Three Herbicides and Effect on Bacterial Communities under Combined Pollution.

Pesticide residues in soil, especially multiple herbicide residues, cause a series of adverse effects on soil properties and microorganisms. In this work, the degradation of three herbicides and the effect on bacterial communities under combined pollution was investigated. The experimental results showed that the half-lives of acetochlor and prometryn significantly altered under combined exposure (5.02-11.17 d) as compared with those of individual exposure (4.70-6.87 d) in soil, suggesting that there was an antagonistic effect between the degradation of acetochlor and prometryn in soil. No remarkable variation in the degradation rate of atrazine with half-lives of 6.21-6.85 d was observed in different treatments, indicating that the degradation of atrazine was stable. 16S rRNA high-throughput sequencing results showed that the antagonistic effect of acetochlor and prometryn on the degradation rate under combined pollution was related to variation of the Sphingomonas and Nocardioide. Furthermore, the potential metabolic pathways of the three herbicides in soil were proposed and a new metabolite of acetochlor was preliminarily identified. The results of this work provide a guideline for the risk evaluation of combined pollution of the three herbicides with respect to their ecological effects in soil.

Efficient Biodegradation of Multiple Aryloxyphenoxypropionate Herbicides by Corynebacterium sp. Z-1 and the Proposed Degradation Mechanism.

Esterases are crucial for aryloxyphenoxypropionate herbicide (AOPP) biodegradation. However, the underlying molecular mechanisms of AOPP biodegradation by esterases are poorly understood. In the current work, Corynebacterium sp. Z-1 was isolated and found to degrade multiple AOPPs, including quizalofop-p-ethyl (QPE), haloxyfop-p-methyl (HPM), fenoxaprop-p-ethyl (FPE), cyhalofop-butyl (CYB), and clodinafop-propargyl (CFP). A novel esterase, QfeH, which catalyzes the cleavage of ester bonds in AOPPs to form AOPP acids, was identified from strain Z-1. The catalytic activities of QfeH toward AOPPs decreased in the following order: CFP > FPE > CYB > QPE > HPM. Molecular docking, computational analyses, and site-directed mutagenesis indicated the catalytic mechanisms of QfeH-mediated degradation of different AOPPs. Notably, the key residue S159 is essential for the activity of QfeH. Moreover, V222Y, T227M, T227A, A271R, and M275K mutants, exhibiting 2.9-5.0 times greater activity than QfeH, were constructed. This study facilitates the mechanistic understanding of AOPPs bioremediation by esterases.

Multiple antibiotic resistance and herbicide catabolic profiles of bacteria isolated from Lake Villarrica surface sediments (Chile)

Antibiotics and herbicides are contaminants of emerging concern in aquatic environments. Lake Villarrica is a relevant freshwater body in Chile and was recently designated a ‘saturated nutrient zone’. Here, we investigated the occurrence of multiple antibiotic resistance (MAR) and herbicide catabolic profiles among bacteria present in the surface sediments of Lake Villarrica. The occurrence of antibiotic-resistant genes (ARGs; blaTEM, catA and tetM) and herbicide-catabolic genes (HCGs; phnJ and atzA) was investigated by qPCR. Subsequently, the presence of culturable bacteria with multiple resistance to amoxicillin (AMX), chloramphenicol (CHL) and oxytetracycline (OXT) was studied. Forty-six culturable MAR (AMX + CHL + OXT) strains were isolated and characterized with respect to their resistance to 11 antibiotics by using a disc diffusion assay and testing their ability to use herbicides as a nutrient source. qPCR analyses revealed that ARGs and HCGs were present in all sediment samples (101 to 103 gene copies g−1), with significant (P ≤ 0.05) higher values in sites near Villarrica city and cattle pastures. The plate method was used to recover MAR isolates from sediment (103–106 CFU g−1), and most of the 46 isolates also showed resistance to oxacillin (100%), cefotaxime (83%), erythromycin (96%) and vancomycin (93%). Additionally, 54 and 57% of the MAR isolates were able to grow on agar supplemented (50 mg L−1) with atrazine and glyphosate as nutrient sources, respectively. Most of the MAR isolates were taxonomically close to Pseudomonas (76.1%) and Pantoea (17.4%), particularly those isolated from urbanized sites (Pucón city). This study shows the presence of MAR bacteria with herbicide catabolic activity in sediments, which is valuable for conservation strategies and risk assessments of Lake Villarrica. However, major integrative studies on sediments as reservoirs or on the fate of MAR strains and traces of antibiotics and herbicides as a result of anthropic pressure are still needed.

Early detection and management of herbicide resistant weeds

Small weed patches may be noticed in fields after herbicide application, but they typically do not have a significant impact on the season’s crop yield. As a result, they are usually not treated as a threat to future yields. However, if these patches harbour weed biotypes resistant to one or multiple herbicides, resistance alleles can spread both spatially (via various dispersal pathways, including seed transport by machinery and commodity contamination) and temporally (through seed persistence). This poses a significant threat to herbicide-based weed management. Once these populations spread and cover a large enough area, eradication becomes improbable despite all the resistance management efforts. Therefore, a proactive and collaborative endeavour is needed to detect and manage small and patchy resistant weed populations. In this paper we review the current potential of weed resistance detection using imagery and molecular markers as well as possible weed management approaches. Finally, we advocate for the use of a combination of these techniques to manage herbicide resistant weeds when populations are small. This multifaceted approach is presently not applicable to all resistance mechanisms, and all weed species located in any crop, but could initially focus on biotypes and species that are easy to detect and represent the greatest threat.

Two Cytochrome P450s, CYP709B1 and CYP704C1, Play Essential Roles in Metabolism-Based Multiple Herbicide Resistance in American Sloughgrass (Beckmannia syzigachne (Steud.) Fernald).

This study confirmed a field population of American sloughgrass (Beckmannia syzigachne (Steud.) Fernald) that developed simultaneously high levels of resistance (resistance index >10) to three divergent modes of action herbicides: fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon. The resistance phenotype observed in this population was not attributed to target-site alterations; rather, the resistant plants exhibited a significant increase in the activity of cytochrome P450s (P450s) and enhanced metabolism rates for all three herbicides. RNA sequencing revealed significant upregulation of two P450s, CYP709B1 and CYP704C1, in the resistant plants both before and after herbicide treatments. Molecular docking predicted that the homology models of these P450s should exhibit a binding affinity for a range of herbicides. The heterologous expression of the identified P450s in yeast cells indicated improved growth in the presence of all three of the aforementioned herbicides. Collectively, the increased expression of CYP709B1 and CYP704C1 likely contributed to the P450s-mediated enhanced metabolism, thereby conferring multiple herbicide resistance in B. syzigachne.

Current Status of Auxin‐Mimic Herbicides

The challenge of managing pesticide resistance in modern agriculture has become increasingly daunting. Herbicide resistance presents a complex scenario due to the continuous selection pressure from synthetic herbicides in global agricultural systems. Auxin‐mimic herbicides (AMHs) have traditionally served as a dependable tool for managing resistant weed populations in crop systems. However, overreliance on AMHs because of the lower incidence of resistance compared to other herbicide classes has intensified selection pressure on weed populations. Resistance to AMHs primarily arises from generalist mechanisms such as non-target site resistance (NTSR), involving reduced translocation, absorption, and enhanced metabolism. Nevertheless, specific mechanisms like target site resistance have also emerged in certain problematic weed species. These resistance mechanisms are complex to manage because some of them can confer resistance to single, cross, or multiple herbicide groups with diverse chemical and action mechanisms.

Metabolism-Based Nontarget-Site Mechanism Is the Main Cause of a Four-Way Resistance in Shortawn Foxtail (Alopecurus aequalis Sobol.).

Alopecurus aequalis Sobol. is a predominant grass weed in Chinese winter wheat fields, posing a substantial threat to crop production owing to its escalating herbicide resistance. This study documented the initial instance of an A. aequalis population (AHFT-3) manifesting resistance to multiple herbicides targeting four distinct sites: acetyl-CoA carboxylase (ACCase), acetolactate synthase, photosystem II, and 1-deoxy-d-xylulose-5-phosphate synthase. AHFT-3 carried an Asp-to-Gly mutation at codon 2078 of ACCase, with no mutations in the remaining three herbicide target genes, and exhibited no overexpression of any target gene. Compared with the susceptible population AHFY-3, AHFT-3 metabolized mesosulfuron-methyl, isoproturon, and bixlozone faster. The inhibition and comparison of herbicide-detoxifying enzyme activities indicated the participation of cytochrome P450s in the resistance to all four herbicides, with glutathione S-transferases specifically linked to mesosulfuron-methyl. Three CYP72As and a Tau class glutathione S-transferase, markedly upregulated in resistant plants, potentially played pivotal roles in the multiple-herbicide-resistance phenotype.

Efficacy of sequential application of herbicide mixture for management of multiple herbicide resistant in Phalaris minor in wheat (Triticum aestivum)

The emergence of multiple herbicide resistance in Phalaris minor is a major threat to wheat (Triticum aestivum L.) production in north-western Indo-Gangetic Plains of India. The present study was carried out during winter (rabi) seasons of 2020–21 and 2021–22 at research farm of Chaudhary Charan Singh Haryana Agricultural University, Hisar, Haryana for management of herbicide resistance in Phalaris minor Retz. in wheat using pre and post-emergence herbicide mixtures with different mode of action. The experiment was laid out in randomized block design (RBD) with 15 treatments, replicated thrice. Sequential application of flumioxazin 125 g/ha with pendimethalin (tank mix) 1500 g/ha fb clodinafop + metribuzin 174 g/ha resulted in significantly lowest weed density, dry matter of P. minor and broad-leaf weeds, and it was statistically comparable to application of flumioxazin alone or as mixture with pendimethalin as pre-emergence; pyraxosulfone + pendimethalin 127.5 + 1500 g/ha fb PoE (post-emergence) application of either mesosulfuron + iodosulfuron 14.4 g/ha; sulfosulfuron + metsulfuron, pinoxaden + metsulfuron 50 + 4 g/ha, clodinafop + metribuzin 174 g/ha, respectively. Phytotoxicity of flumioxazin was observed on wheat after 2–3 days of 1st irrigation i.e. 25 DAS (days after sowing). Application of pyroxasulfone + pendimethalin pre fb PoE mesosulfuron + iodosulfuron resulted in significantly higher grain yield (5847 and 5369 kg/ha, during 2021 and 2022, respectively) as compared to other herbicides.

Enhanced Herbicide Metabolism and Target Site Mutation Enabled the Multiple Resistance to Cyhalofop-butyl, Florpyrauxifen-benzyl, and Penoxsulam in Echinochloa crus-galli.

This study investigated the multiple herbicide resistance (MHR) mechanism of one Echinochloa crus-galli population that was resistant to florpyrauxifen-benzyl (FPB), cyhalofop-butyl (CHB), and penoxsulam (PEX). This population carried an Ala-122-Asn mutation in the acetolactate synthase (ALS) gene but no mutation in acetyl-CoA carboxylase (ACCase) and transport inhibitor response1 (TIR1) genes. The metabolism rate of PEX was 2-fold higher, and the production of florpyrauxifen-acid and cyhalofop-acid was lower in the resistant population. Malathion and 4-chloro-7-nitrobenzoxadiazole (NBD-Cl) could reverse the resistance, suggesting that cytochrome P450 (CYP450) and glutathione S-transferase (GST) contribute to the enhanced metabolism. According to RNA-seq and qRT-PCR validation, two CYP450 genes (CYP71C42 and CYP71D55), one GST gene (GSTT2), two glycosyltransferase genes (rhamnosyltransferase 1 and IAAGLU), and two ABC transporter genes (ABCG1 and ABCG25) were induced by CHB, FPB, and PEX in the resistant population. This study revealed that the target mutant and enhanced metabolism were involved in the MHR mechanism in E. crus-galli.

Two-Pass Weed Management Programs for Identity-Preserved Soybean

Weed management is a challenge in Identity-Preserved (IP) soybean in Ontario, Canada. Six experiments were established in southwestern Ontario, Canada during 2021 and 2022 to evaluate weed control and soybean yield with preemergence (PRE), early postemergence (EPOST), and PRE followed by (fb) late POST (LPOST) herbicide programs. At 8 weeks after LPOST herbicide applications, S-metolachlor/metribuzin, pyroxasulfone/sulfentrazone, and flumioxazin/metribuzin/imazethapyr applied PRE provided 7, 49, and 99% control of velvetleaf; 65, 98, and 100% control of green pigweed; 7, 8, and 82% control of common ragweed; 25, 68, and 98% control of common lambsquarters; 91, 77, and 89% control of barnyardgrass; and 62, 68, and 93% control of green foxtail, respectively. Imazethapyr + bentazon applied EPOST provided 91% control of velvetleaf; 91% control of green pigweed; 78% control of common ragweed; 95% control of common lambsquarters; 76% control of barnyardgrass; and 79% control of green foxtail. S-metolachlor/metribuzin, pyroxasulfone/sulfentrazone, and flumioxazin/metribuzin/imazethapyr applied PRE fb bentazon + fomesafen + quizalofop applied LPOST provided 61, 86, and 100% control of velvetleaf; 97, 99, and 100% control of green pigweed; 94, 88, and 99% control of common ragweed; 96, 98, and 100% control of common lambsquarters; 97, 95, and 97% control of barnyardgrass; and 97, 96, and 99% control of green foxtail, respectively. There was minimal and transient soybean injury (6% or less) with all PRE or EPOST herbicide treatments, however, S-metolachlor/metribuzin, pyroxasulfone/sulfentrazone, or flumioxazin/metribuzin/imazethapyr applied PRE fb bentazon + fomesafen + quizalofop LPOST caused up to 22% injury in soybean. Weed interference reduced soybean yield 40%. Weed interference with S-metolachlor/metribuzin and pyroxasulfone/sulfentrazone applied PRE reduced soybean yield 25 and 31%, respectively. Reduced weed interference with flumioxazin/metribuzin/imazethapyr applied PRE, imazethapyr + bentazon applied EPOST, and the two-pass programs of a PRE fb LPOST herbicide resulted in soybean yield that was similar to the weed-free control. This study concludes that there are many effective weed management programs in IP soybean; however, the two-pass weed control programs are recommended since they provide good to excellent weed control, minimize soybean yield loss from weed interference, and ensure the use of multiple herbicide modes of action which reduces the selection intensity for the evolution of herbicide-resistant weeds.

Herbicide leakage into seawater impacts primary productivity and zooplankton globally.

Predicting the magnitude of herbicide impacts on marine primary productivity remains challenging because the extent of worldwide herbicide pollution in coastal waters and the concentration-response relationships of phytoplankton communities to multiple herbicides are unclear. By analyzing the spatiotemporal distribution of herbicides at 661 bay and gulf stations worldwide from 1990 to 2022, we determined median, third quartile and maximum concentrations of 12 triazine herbicides of 0.18 nmol L-1, 1.27 nmol L-1 and 29.50 nmol L-1 (95%Confidence Interval: CI 1.06, 1.47), respectively. Under current herbicide stress, phytoplankton primary productivity was inhibited by more than 5% at 25% of the sites and by more than 10% at 10% of the sites (95%CI 3.67, 4.34), due to the inhibition of highly abundant sensitive species, community structure/particle size succession (from Bacillariophyta to Dinophyceae and from nano-phytoplankton to micro-phytoplankton), and resulting growth rate reduction. Concurrently, due to food chain cascade effects, the dominant micro-zooplankton population shifted from larger copepod larvae to smaller unicellular ciliates, which might prolong the transmission process in marine food chain and reduce the primary productivity transmission efficiency. As herbicide application rates on farmlands worldwide are correlated with residues in their adjacent seas, a continued future increase in herbicide input may seriously affect the stability of coastal waters.

Genome-wide study of glutathione transferases and their regulation in flufenacet susceptible and resistant black-grass (Alopecurus myosuroides Huds.).

Glutathione transferases (GSTs) are enzymes with a wide range of functions, including herbicide detoxification. Up-regulation of GSTs and their detoxification activity enables the grass weed black-grass (Alopecurus myosuroides Huds.) to metabolize the very-long-chain fatty acid synthesis inhibitor flufenacet and other herbicides leading to multiple herbicide resistance. However, the genomic organization and regulation of GSTs genes is still poorly understood. In this genome-wide study the location and expression of 115 GSTs were investigated using a recently published black-grass genome. Particularly, the most abundant GSTs of class tau and phi were typically clustered and often followed similar expression patterns but possessed divergent upstream regulatory regions. Similarities were found in the promoters of the most up-regulated GSTs, which are located next to each other in a cluster. The binding motif of the E2F/DP transcription factor complex in the promoter of an up-regulated GST was identical in susceptible and resistant plants, however, adjacent sequences differed. This led to a stronger binding of proteins to the motif of the susceptible plant, indicating repressor activity. This study constitutes the first analysis dealing with the genomic investigation of GST genes found in black-grass and their transcriptional regulation. It highlights the complexity of the evolution of GSTs in black-grass, their duplication and divergence over time. The large number of GSTs allows weeds to detoxify a broad spectrum of herbicides. Ultimately, more research is needed to fully elucidate the regulatory mechanisms of GST expression. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Management of Herbicide resistant Phalaris minor: A Review

Littleseed canarygrass (Phalaris minor Retz), an annual species of the family Poaceae, is a major weed problem in many wheat-producing countries. The excessive dependence on herbicides for its control led to the evolution of multiple herbicides resistance and yield losses. The first case of resistant P. minor in India was to the Photosystem-II inhibitor isoproturon in the early 1990’s. P. minor has been found resistant to Acetolactate Synthase (ALS) inhibitors, Acetyl-CoA Carboxylase (ACCase) inhibitors, Photosystem II inhibitors. It is highly competitive and in severe cases can cause complete crop failure, forcing farmers to harvest their crop as fodder or ploughing the standing crop along with weeds. For management of these multiple herbicide resistant P. minor, pyroxasulfone, pendimethalin, flumioxazin, trifluralin and flufenacet can be used as alternative herbicides. Moreover, in-depth biological and ecological studies on P. minor should be carried out for devising integrated weed management strategies. The long-term strategies of efficient weed management and sustainability of wheat production should include the use of alternative herbicides, herbicide mixture, crop rotation along with other non-chemical methods like manual removal of weeds, mulching, Summer ploughing etc providing the competitive advantages to the wheat crop over P. minor, at the same time increasing the profitability of the farmer by controlling the weed effectively and reducing the selection pressure on the weeds which will inturn reduce the resistance development in weeds

Effect of herbicide programs on control and seed production of multiple herbicide–resistant Palmer amaranth (Amaranthus palmeri) in corn resistant to 2,4-D/glufosinate/glyphosate

Abstract 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.

Identification, mapping, and chemical control of fleabane resistant to glyphosate, chlorimuron, paraquat, and 2,4-D

Abstract Monitoring herbicide-resistant weeds makes it possible to study the evolution and spread of resistance, which provides important information for their management. The objective of this study was to map fleabane accessions in the states of Paraná (PR) and Mato Grosso do Sul (MS), Brazil, to identify herbicide-resistant accessions and their response to soybean preplant chemical burndown management strategies. Fleabane seeds were collected in agricultural areas in PR and MS in 2018, 2019, and 2020. Initial screening was performed for glyphosate, chlorimuron, paraquat, 2,4-D, saflufenacil, and glufosinate efficacy. Subsequently, dose-response experiments were conducted. Field experiments were carried out in three locations, where accessions of multiple herbicide–resistant Sumatran fleabane were identified. Herbicides were used in single or sequential applications at three plant heights (<5 cm, 5 to 10 cm, and >10 cm). After preliminary screening, accessions were classified as putative resistant (<80% control for all four replicates), segregated (<80% control for one to three replicates), or susceptible (>80% control for all four replicates). There was no evidence of resistance to glufosinate or saflufenacil in any of the 461 accessions, while 65 demonstrated possible resistance or segregation to glyphosate only, 235 to glyphosate + chlorimuron, 79 to glyphosate + chlorimuron + paraquat, 59 to glyphosate + chlorimuron + 2,4-D, and 23 with four-way resistance (glyphosate, chlorimuron, paraquat, and 2,4-D). Of these 23 accessions, seven were analyzed using dose-response curves (F2 generation), all from PR, confirming four-way resistance to glyphosate, chlorimuron, paraquat, and 2,4-D. To control resistant Sumatran fleabane, an application should prioritize smaller plants. Despite resistance to 2,4-D, double mixtures containing this herbicide were among the most effective treatments in plants <5 cm in height. If a sequential application is needed for plants >5 cm in height, we recommend glyphosate + synthetic auxin followed by glufosinate or glyphosate + saflufenacil.

Demographics of Palmer amaranth (Amaranthus palmeri) in annual and perennial cover crops

AbstractPalmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed of cotton (Gossypium hirsutum L.)-cropping systems in the U.S. Southeast. Heavy reliance on herbicides has selected for resistance to multiple herbicide mechanisms of action. Effective management of this weed may require the integration of cultural practices that limit germination, establishment, and growth. Cover crops have been promoted as a cultural practice that targets these processes. We conducted a 2-yr study in Georgia, USA, to measure the effects of two annual cover crops (cereal rye [Secale cereale L.] and crimson clover [Trifolium incarnatum L.]), a perennial living mulch (‘Durana®’ white clover [Trifolium repens L.]), and a bare ground control on A. palmeri population dynamics. The study was conducted in the absence of herbicides. Growth stages were integrated into a basic demographic model to evaluate differences in population trajectories. Cereal rye and living mulch treatments suppressed weed seedling recruitment (seedlings seed−1) 19.2 and 13 times and 12 and 25 times more than the bare ground control, respectively. Low recruitment was correlated positively with low light transmission (photosynthetic active radiation: above canopy photosynthetically active radiation [PAR]/below cover crop PAR) at the soil surface. Low recruitment rates were also negatively correlated with high survival rates. Greater survival rates and reduced adult plant densities resulted in greater biomass (g plant−1) and fecundity (seeds plant−1) in cereal rye and living mulch treatments in both years. The annual rate of population change (seeds seed−1) was equivalent across all treatments in the first year but was greater in the living mulch treatment in the second year. Our results highlight the potential of annual cover crops and living mulches for suppressing A. palmeri seedling recruitment and would be valuable tools as part of an integrated weed management strategy.

Spatial distribution and ecological-health risks associated with herbicides in soils and crop kernels of the black soil region in China

Herbicides are vital inputs for food production; however, their associated risks and hazards are pressing concerns. In black soil, the cumulative toxic effects of compound herbicides and potential risks to humans are not yet fully understood. Thus, this study conducted a comprehensive investigation to assess herbicide residue characteristics and the associated ecological health risks in representative black soil regions where major food crops (maize, soybean, and rice) are cultivated. Findings revealed that the soil harbored a collective presence of 29 herbicides, exhibiting total concentrations ranging from 111.92 to 996.14 μg/kg dry weight (dw). This can be attributed to the extensive use of herbicides over the years and their long half-lives, which results in the accumulation of multiple herbicide residues in the soil. Similarly, the total herbicide levels in maize, soybean, and rice kernels were 1173–61,564, 1721–9342, and 3775–8094 ng/kg dw, respectively. Multiple herbicide residues at all monitored sites were attributed to continuous crop barriers in soybean fields and the adoption of soybean and maize crop rotations. Notably, herbicides pose ecological risks in the black soil region, exhibiting high-risk levels of 79 %, 24 %, and 14 % at the sites monitored for oxyfluorfen, clomazone, and butachlor, respectively. Carcinogenic atrazine exhibited low- and medium-risk levels in 34 % and 63 % of soil samples, respectively. These results can serve as a scientific basis for establishing herbicide residue thresholds in agricultural soils within black soil areas and for implementing effective control measures to prevent herbicide contamination in agricultural ecosystems.

Investigation of inquiries on weed control efficacy of XtendiMax® herbicide with VaporGrip® technology

AbstractHerbicide resistance in weeds significantly threatens crop production in the United States. The introduction of dicamba-resistant soybean and cotton stacked with other herbicide tolerance traits has provided farmers with the flexibility of having multiple herbicide options to diversify their weed management practices and delay resistance evolution. XtendiMax® herbicide with VaporGrip® Technology is a dicamba formulation registered for use on dicamba-resistant soybean and cotton crops by the U.S. Environmental Protection Agency (EPA). One of the terms of its registration includes an evaluation of inquiries on reduced weed control efficacy by growers or users of XtendiMax for suspected weed resistance. A total of 3,555 product performance inquiries (PPIs) were received from 2018 to 2021 regarding reduced weed control efficacy by dicamba. Following the criteria recommended by EPA for screening of suspected resistance in the field, a total of 103 weed accessions from 63 counties in 13 states were collected for greenhouse testing over those 4 yr. Weed accessions for greenhouse testing were collected only in states where resistance to dicamba was not yet confirmed in the weed species under investigation. The accessions, which consisted primarily of waterhemp and Palmer amaranth, were treated with dicamba at rates of 560 g ae ha−1 and 1,120 g ae ha−1. All weed accessions, except for one accession each of Palmer amaranth and waterhemp, were controlled by ≥90% with dicamba at 21 d after treatment in the greenhouse.