Evolution Of Glyphosate Resistance Research Articles

Evolution of ACCase-inhibitor resistance in Chloris virgata is conferred by a Trp2027Cys mutation in the herbicide target site.

Chloris virgata is a troublesome weed in tropical regions. With the evolution of glyphosate resistance in key grass species, acetyl CoA carboxylase (ACCase) inhibitors have become a commonly used tool in soybean production areas in Brazil. We assessed if suspected resistant populations exhibited cross resistance to the different classes of ACCase inhibitors and investigated the resistance mechanisms in C. virgata. Dose-response experiments revealed resistance to haloxyfop-methyl and pinoxaden, with 432- and 3-fold resistance, respectively, compared to susceptible populations. Due to the lack of genetic resources for C. virgata, we sequenced, assembled, and annotated the genome using short-read Illumina technology. The k-mer analysis estimated a genome size of approximately 336 Mbp, with BUSCO completeness of 97%, and over 36 000 gene models were annotated. We examined if ACCase copy number variation and increased gene expression were involved in the resistance phenotype and found no difference when compared to a susceptible population. A mutation was detected in ACCase that encodes for amino acid position 2027, resulting in a tryptophan-to-cysteine (Trp2027Cys) substitution. We found the resistant population absorbed 11.4% less herbicide and retained 21% more herbicide on the treated leaf compared to the susceptible population. We developed a genotyping assay targeting the resistance-endowing Trp2027Cys substitution for quick resistance diagnosis. A Trp2027Cys amino acid substitution in ACCase confers resistance to haloxyfop and pinoxaden in C. virgata. We provide important insights into the evolutionary history of C. virgata and a draft genome as a useful resource to further our understanding of the biology in the genus Chloris. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Gene Amplification of EPSPS with a Mutation in Conserved Region: The Evolved Glyphosate Resistance Mechanism in Eleusine indica

Goosegrass (Eleusine indica (L.) Gaertn.) is one of the most vicious weeds worldwide, competing with crops and greatly reducing their yields. Glyphosate, a non-selective, broad spectrum, post-emergence herbicide has inevitably induced severe resistance in many weeds owing to its intensive use. Additionally, control strategies rely on the clarity of resistance mechanisms. In this study, we aimed to investigate the resistance levels and potential resistance mechanisms of two goosegrass populations collected from orchards. Results showed that the resistance indexes of LL and SS populations were 3.8 and 1.9, respectively. A single nucleotide change led to a Pro106Leu (P106L) mutation in the LL population and the SS population had a Pro106Ser (P106S) amino acid substitution. The EPSPS expression in both populations was 2.6 times that of the wild-type population. However, the relative copy number and EPSPS protein content in the LL population were higher than those of the SS population, as indicated by immunoblot analysis and enzyme-linked immunosorbent assays. Overall, we confirmed EPSPS amplification with a P106L mutation, resulting in overproduction of this mutated EPSPS protein, which conferred moderate glyphosate resistance. This study details a case of simultaneous evolution of mutation and amplification in EPSPS of glyphosate resistance weeds.

Development of Transgenic Maize Tolerant to Both Glyphosate and Glufosinate

Genetically modified (GM) crops tolerant to glyphosate have delivered significant economic benefits in farm management. However, the evolution of glyphosate resistance in weeds due to prolonged intensive use of glyphosate poses a serious threat to this weed management system. It is highly desirable in China to deploy dual herbicide-tolerant corn at the very beginning of GM corn release to delay the development of weed resistance to herbicides. Here, we report the creation and characterization of a herbicide-tolerant corn event SCB-29 that expresses both cp4 epsps and bar genes. This transgenic maize is tolerant to glyphosate up to 3600 g a.e. ha−1 and glufosinate up to 3600 g a.i. ha−1, which are quadruple the recommended rates for the two herbicides, respectively. SCB-29 is an event with only a single copy of T-DNA inserted into chromosome 10 of the maize genome. An event-specific PCR detection method was established and three generations of SCB-29 were detected by event-specific PCR suggesting that the transgenes are stably integrated into the maize genome. Analysis of the expression levels of the transgenes among plants of multiple generations by enzyme-linked immunosorbent assays suggested that the expressions are stable over different generations. Moreover, the major agronomic performances of SCB-29 appear to be similar to those of non-transgenic maize, suggesting that SCB-29 is not likely to have yield drag. Therefore, SCB-29 is an excellent herbicide-tolerant candidate to be developed into a commercial herbicide tolerance transgenic event.

Improved Method to Characterize Leaf Surfaces, Guide Adjuvant Selection, and Improve Glyphosate Efficacy.

Glyphosate, one of the most widely used herbicides, plays an important role in controlling weeds and ensuring crop production. While using glyphosate, adjuvants are commonly added to improve its deposition on weeds and control efficacy. However, changes in weed leaf surface characteristics may reduce glyphosate penetration and contribute to evolved glyphosate resistance. Therefore, it is significant to introduce an improved method for regularizing leaf surface characterization and guide adjuvant selection to improve glyphosate efficacy. In this work, surface characteristics of typical weed leaves have been systematically investigated by 3D surface analysis and scanning electron microscopy, finally quantified by apparent surface free energy (ASFE) due to its comprehensive and quantitative evaluation of leaf surfaces. Moreover, the relationship between the weed leaf surface characteristics and the retention of glyphosate on weeds was established, further related to the control efficacy against weeds. To maximize the utilization rate of glyphosate, the types and concentrations of adjuvants should be regulated according to the ASFE of weeds. Our findings not only regularize the surface properties of weed leaves but also reveal their influencing mechanism on the deposition and biological activity of glyphosate, which provide effective guidance for the use of glyphosate.

A novel EPSPS Pro-106-His mutation confers the first case of glyphosate resistance in Digitaria sanguinalis.

Digitaria sanguinalis has been identified as a species at high risk of evolving herbicide resistance, but thus far, there are no records of resistance to glyphosate. This weed is one of the most common weeds of summer crops in extensive cropping areas in Argentina. It shows an extended period of seedling emergence with several overlapping cohorts during spring and summer, and is commonly controlled with glyphosate. However, a D.sanguinalis population was implicated as a putative glyphosate-resistant biotype based on poor control at recommended glyphosate doses. The field-collected D.sanguinalis population (Dgs R) from the Rolling Pampas has evolved glyphosate resistance. Differences in plant survival and shikimate levels after field-recommended and higher glyphosate doses were evident between Dgs R and the known susceptible (Dgs S) population; the resistance index was 5.1. No evidence of differential glyphosate absorption, translocation, metabolism or basal EPSPS activity was found between Dgs S and Dgs R populations; however, a novel EPSPS Pro-106-His point substitution is probably the primary glyphosate resistance-endowing mechanism. EPSPS in vitro enzymatic activity demonstrated that an 80-fold higher concentration of glyphosate is required in Dgs R to achieve similar EPSPS activity inhibition to that in the Dgs S population. This study reports the first global case of glyphosate resistance in D.sanguinalis. This unlikely yet novel transversion at the second position of the EPSPS 106 codon demonstrates the intensity of glyphosate pressure in selecting unexpected glyphosate resistance alleles if they retain EPSPS functionality. © 2022 Society of Chemical Industry.

Investigating the origins and evolution of a glyphosate-resistant weed invasion in South America.

The global invasion, and subsequent spread and evolution of weeds provides unique opportunities to address fundamental questions in evolutionary and invasion ecology. Amaranthus palmeri is a widespread glyphosate-resistant (GR) weed in the USA. Since 2015, GR populations of A. palmeri have been confirmed in South America, raising questions about introduction pathways and the importance of pre- vs. post-invasion evolution of GR traits. We used RAD-sequencing genotyping to characterize genetic structure of populations from Brazil, Argentina, Uruguay and the USA. We also quantified gene copy number of the glyphosate target, 5-enolpyruvyl-3-shikimate phosphate synthase (EPSPS), and the presence of an extrachromosomal circular DNA (eccDNA) replicon known to confer glyphosate resistance in USA populations. Populations in Brazil, Argentina and Uruguay were only weakly differentiated (pairwise FST ≤0.043) in comparison to USA populations (mean pairwise FST =0.161, range=0.068-0.258), suggesting a single major invasion event. However, elevated EPSPS copy number and the EPSPS replicon were identified in all populations from Brazil and Uruguay, but only in a single Argentinean population. These observations are consistent with independent in situ evolution of glyphosate resistance in Argentina, followed by some limited recent migration of the eccDNA-based mechanism from Brazil to Argentina. Taken together, our results are consistent with an initial introduction of A. palmeri into South America sometime before the 1980s, and local evolution of GR in Argentina, followed by a secondary invasion of GR A. palmeri with the unique eccDNA-based mechanism from the USA into Brazil and Uruguay during the 2010s.

How Many Ways Can Nature Kill the Goose That Laid the Golden Egg? – The Many Mechanisms of Evolved Glyphosate Resistance

Weeding has been the bane of humanity since the dawn of agriculture. For about 70 years, synthetic herbicides have removed much of the drudgery of this onerous task. Glyphosate was introduced as a non-selective herbicide in 1974. Its ideal properties made it a very popular herbicide, and the introduction of glyphosate-resistant (GR) crops allowed its use as a selective herbicide, greatly expanding its use to become the most used herbicide on earth. For farmers who used glyphosate in GR crops, it was the golden age of weed management, as this technology significantly improved the efficacy and reduced the cost of weed management. Weed management was also simplified, an asset that was particularly valuable to part-time farmers. Furthermore, this technology provided the environmental benefits (reduced soil loss and fossil fuel use) of significantly reducing tillage. Farmers saved billions of US$, and weed management became more effective and simple. Indeed, to many farmers, glyphosate with GR crops became the goose that laid the golden egg. After more than 20 years of use, the first cases of GR weeds were reported in the latter 1990s. After a lag period of less than 10 years after the first GR weed was reported, the number of species reported to have evolved glyphosate resistance began to increase in a linear fashion, reaching 53 species in 2021, third only to atrazine (66 species), a much older herbicide and to ALS inhibitors (168 species), which include several different herbicides used in numerous crops since the 1980s. The long lag phase before any resistance was detected led some to believe by the mid-1990s that evolution of resistance was improbable. By this time, glyphosate use was greatly increasing, especially in GR crops, an ideal situation for the evolution of resistance. After this, the number of glyphosate-resistance cases exploded, and the mechanisms of resistance to many of these cases of resistance were determined. A recent, short commentary detailed these mechanisms after a new mechanism of resistance was reported. The number of mechanisms for resistance to no other herbicide comes close to those of glyphosate. In the present paper, we briefly describe the many evolved mechanisms by which weeds have evolved resistance to glyphosate.

Demographics of glyphosate-resistant and susceptible Italian ryegrass populations from Paraná

Abstract Italian ryegrass ( Lolium multiflorum L.) has become notorious due to glyphosate resistance evolution. To study the demography of susceptible and glyphosate-resistant Italian ryegrass populations, several experiments were conducted in the grower’s field to evaluate seedling emergence, survival and reproductive traits patterns in three cohorts, and the effect of density on seed production. Results showed that seedling emergence is fast, and under suitable environmental conditions, it can happen within 30-days. Lower plant survival was observed in the first two cohorts (2% and 3%, respectively) compared to the last (13%). A larger number of spikes (spk) per plant (pl) was observed in plants of the first two cohorts (50.34 and 47.02 spk pl-1, respectively) in comparison to the last one (26 spk pl-1); however, the third cohort presented a higher potential for flower (flwrs) production per spikelets (spklt) (13.5 flwrs spklt-1) in detriment to the first two (11.9 and 12.12 flwrs spklt-1). Fecundity was 20,300 and 13,830 seeds pl-1 for susceptible and glyphosate-resistant plants, respectively, and presented density-dependent regulation. A knowledge of Italian ryegrass’s demographic parameters provides valuable information for identifying particularly vulnerable life-history processes and developing population dynamics models that can contribute to designing more rational control strategies.

An ABCC-type transporter endowing glyphosate resistance in plants

Glyphosate is the most widely used herbicide in world agriculture and for general vegetation control in a wide range of situations. Global and often intensive glyphosate selection of very large weedy plant populations has resulted in widespread glyphosate resistance evolution in populations of many weed species. Here, working with a glyphosate-resistant (GR) Echinochloa colona population that evolved in a Western Australia agricultural field, we identified an ATP-binding cassette (ABC) transporter (EcABCC8) that is consistently up-regulated in GR plants. When expressed in transgenic rice, this EcABCC8 transporter endowed glyphosate resistance. Equally, rice, maize, and soybean overexpressing the EcABCC8 ortholog genes were made resistant to glyphosate. Conversely, CRISPR/Cas9-mediated knockout of the EcABCC8 ortholog gene OsABCC8 increased rice susceptibility to glyphosate. Subcellular localization analysis and quantification of glyphosate cellular levels in treated ABCC8 transgenic rice plants and isolated leaf protoplasts as well as structural modeling support that EcABCC8 is likely a plasma membrane-localized transporter extruding cytoplasmic glyphosate to the apoplast, lowering the cellular glyphosate level. This is a report of a membrane transporter effluxing glyphosate in a GR plant species, and its function is likely conserved in crop plant species.

Investigating glyphosate resistance in Amaranthus palmeri biotypes from Turkey

Amaranthus palmeri is a troublesome weed which is growing in importance worldwide. It causes serious competition in many crops. A. palmeri was introduced into Turkey 4–5 years ago and appears to adapt to different environmental conditions. Monoculture cropping systems and repeated use of the same herbicides have led to development of herbicide resistance in A. palmeri to several active ingredients including glyphosate, in USA and in South America. This study was conducted to investigate the evolution of glyphosate resistance in A. palmeri in Turkey. Seeds of 21 A. palmeri populations were collected in Turkish citrus fields where control problems with glyphosate were reported. A potentially glyphosate susceptible A. palmeri population was collected from a maize field (GS1 biotype). Seeds of putatively resistant and potentially susceptible (GS) biotypes were germinated and transplanted into large pots, and then allowed to grow in separate greenhouse chambers to obtain F2 generations. After carrying out a preliminary test experiment to exclude the most susceptible populations, a dose-response experiment was conducted in which glyphosate was applied at the 3–4 true leaf stage at 0, 332.5, 665, 1330, 2660, 5320, 10,640 and 21,280 g a.i. ha−1. Plants were harvested 21 days after treatment and dry weight was determined. Glyphosate applied at the recommended rate (1330 g a.i ha−1) controlled GS A. palmeri biotypes by more than 95% while controlling the GR biotypes at about 45%. Among those biotypes, GR1 and GR2 biotypes were confirmed to have an incipient resistance to glyphosate. The effect of glyphosate on shikimic acid accumulation was determined. Results showed that the GS2 biotype accumulated 3.1 and 1.56 times more shikimic acid than GR2 and GR1 biotypes which demonstrates that there is a lower accumulation of shikimic acid in the alleged resistant biotypes than in GS1. These findings demonstrate some increased tolerance of A. palmeri biotypes to glyphosate, which reinforces the need to implement integrated weed management to control this invasive plant in Turkey.

Identification of the first glyphosate-resistant capeweed (Arctotheca calendula) population.

Glyphosate is routinely used in Australia to control the Arctotheca species Arctotheca calendula (L.) Levyns (referred hereinafter as capeweed). This study identifies the first global case of field-evolved glyphosate-resistant capeweed, collected from the grainbelt of Western Australia. In 2020, a capeweed biotype that was collected from Borden in the southern Western Australian grainbelt was confirmed to be glyphosate-resistant (referred hereinafter as Spence population). When compared to the pooled mortality of six field-collected, glyphosate susceptible capeweed populations (S1, S2, S3, S4, S5 and S6), the Spence population was found > 11-fold more resistant to glyphosate than the pooled results of the susceptible populations (S1-S6) at the lethal dose of 50% (LD50 ) level. The growth of the Spence population was also less affected, requiring > 13-fold more glyphosate to reduce growth than the pooled susceptible populations at the growth reduction of 50% (GR50 ) level. Sequencing of the plastidic 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene indicated no known single gene mutation imparting glyphosate resistance. This study, however, did not investigate any other known mechanisms that impart glyphosate resistance. When screened at the field-applied rate, this Spence population was also found to survive an inhibitor of acetolactate synthase (ALS) (metosulam) and an inhibitor of phytoene desaturase (diflufenican). This is the first confirmation of glyphosate resistance evolution in a capeweed population globally. With capeweed resistance already confirmed to photosystem-I inhibiting herbicides (paraquat and diquat), this study emphasizes the importance of using integrated measures that do not depend only on the use of non-selective herbicides for controlling herbicide resistance-prone capeweed populations. © 2021 Society of Chemical Industry.

Evolution of Glyphosate-Resistant Weeds.

Widespread adoption of glyphosate-resistant crops and concomitant reliance on glyphosate for weed control set an unprecedented stage for the evolution of herbicide-resistant weeds. There are now 48 weed species that have evolved glyphosate resistance. Diverse glyphosate-resistance mechanisms have evolved, including single, double, and triple amino acid substitutions in the target-site gene, duplication of the gene encoding the target site, and others that are rare or nonexistent for evolved resistance to other herbicides. This review summarizes these resistance mechanisms, discusses what is known about their evolution, and concludes with some of the impacts glyphosate-resistant weeds have had on weed management.

Evolution of EPSPS double mutation imparting glyphosate resistance in wild poinsettia (Euphorbia heterophyllaL.).

The evolution of glyphosate resistance (GR) in weeds is an increasing problem. Glyphosate has been used intensively on wild poinsettia (Euphorbia heterophylla L.) populations for at least 20 years in GR crops within South America. We investigated the GR mechanisms in a wild poinsettia population from a soybean field in southern Brazil. The GR population required higher glyphosate doses to achieve 50% control (LD50) and 50% dry mass reduction (MR50) compared to a glyphosate susceptible (GS) population. The ratio between the LD50 and MR50 of GR and GS resulted in resistance factors (RF) of 6.9-fold and 6.1-fold, respectively. Shikimate accumulated 6.7 times more in GS than in GR when leaf-discs were incubated with increasing glyphosate concentrations. No differences were found between GR and GS regarding non-target-site mechanisms. Neither population metabolized glyphosate to significant levels following treatment with 850 g ha-1 glyphosate. Similar levels of 14C-glyphosate uptake and translocation were observed between the two populations. No differences in EPSPS expression were found between GS and GR. Two target site mutations were found in all EPSPS alleles of homozygous resistant plants: Thr102Ile + Pro106Thr (TIPT—mutation). Heterozygous individuals harbored both alleles, wild-type and TIPT. Half of GR individuals were heterozygous, suggesting that resistance is still evolving in the population. A genotyping assay was developed based on the Pro106Thr mutation, demonstrating high efficiency to identify homozygous, heterozygous or wild-type EPSPS sequences across different plants. This is the first report of glyphosate-resistant wild-poinsettia harboring an EPSPS double mutation (TIPT) in the same plant.

Transcriptome Analysis Identifies Candidate Target Genes Involved in Glyphosate-Resistance Mechanism in Lolium multiflorum.

Italian ryegrass (Lolium multiflorum; LOLMU) is one of the most troublesome weeds in temperate regions in the world. This weed species interfere with wheat, corn, rye, and oat, causing significant crop yield losses. This species has evolved glyphosate resistance, making it difficult to control. The mechanisms of glyphosate resistance are still unknown, and an understanding thereof will favor the development of new strategies of management. The present study is the first transcriptome study in LOLMU using glyphosate-resistant and -sensitive biotypes, aiming to identify and to provide a list of the candidate target genes related to glyphosate resistance mechanism. The transcriptome was assembled de novo, producing 87,433 contigs with an N50 of 740 bp and an average length of 575 bp. There were 92 and 54 up- and down-regulated genes, respectively, in the resistant biotype, while a total of 1683 were differentially expressed in the sensitive biotype in response to glyphosate treatment. We selected 14 highly induced genes and seven with repressed expression in the resistant biotype in response to glyphosate. Of these genes, a significant proportion were related to the plasma membrane, indicating that there is a barrier making it difficult for glyphosate to enter the cell.

Agnes Rimando, a Pioneer in the Fate of Glyphosate and Its Primary Metabolite in Plants.

Glyphosate is the most used herbicide on the planet because of its excellent efficacy on almost all weed species and due to the large-scale adoption of transgenic, glyphosate-resistant (GR) crops. Agnes Rimando became an expert in glyphosate analysis almost 20 years ago to support research on GR crop safety and on mechanisms of evolved glyphosate resistance by weeds. Her work was the first to show that the amount of glyphosate and its primary metabolite aminomethylphosphonic acid (AMPA) that accumulates in GR soybean seed from plants treated with approved glyphosate doses can approach their legal limits. However, she later found that only trace amounts of these compounds accumulate in the seed of GR maize treated with recommended glyphosate doses. She showed that GR canola, the only transgenic crop with a transgene encoding an enzyme for degradation of glyphosate, metabolizes glyphosate to AMPA very rapidly. Her work was instrumental in providing support for the view that "yellow flash" symptoms sometimes observed in field-grown GR soybeans are due to accumulation of enough AMPA to cause mild phytotoxicity. She did the chemical analyses in the only paper to survey the capacity of an array of plant species to metabolize glyphosate to AMPA. She found a wide range in this capacity, with grasses with little or no metabolism of glyphosate to AMPA and with legumes readily metabolizing glyphosate. Lastly, she found no evidence of enhanced degradation of glyphosate to be a mechanism of evolved resistance to glyphosate by two weed species but that it might be involved in natural tolerance to glyphosate of some weeds.

Target Site-Based Resistance to ALS Inhibitors, Glyphosate, and PPO Inhibitors in an <i>Amaranthus palmeri</i> Accession from Mississippi

Extensive acceptance of glyphosate-resistant (GR) row crops coupled with the simultaneous increase in glyphosate usage has sped the evolution of glyphosate resistance in economically important weeds. GR Amaranthus palmeri populations are widespread across the state with some exhibiting multiple resistance to acetolactate synthase (ALS) inhibiting herbicides such as pyrithiobac. A GR and ALS inhibitor-resistant accession was also resistant to the protoporphyrinogen oxidase (PPO) inhibiting herbicide fomesafen. The PPO inhibitor resistance profile and multiple herbicide resistance mechanisms in this accession were investigated. In addition to fomesafen, resistance to postemergence applications of acifluorfen, lactofen, carfentrazone, and sulfentrazone was confirmed. There was no resistance to preemergence application of fomesafen, flumioxazin, or oxyfluorfen. Molecular analysis of the ALS gene indicated the presence of point mutations leading to single nucleotide substitutions at codons 197, 377, 574, and 653, resulting in proline-to-serine, arginine-to-glutamine, tryptophan-to-leucine, and serine-to-asparagine replacements, respectively. The resistant accession contained up to 87-fold more copies of the EPSPS gene compared to a susceptible accession. A mutation leading to a deletion of glycine at codon 210 (ΔG210) of PPO2 gene was also detected. These results indicate that the mechanism of resistance in the Palmer amaranth accession is target-site based, i.e., altered target site for ALS and PPO inhibitor resistance and gene amplification for glyphosate resistance.

Enhanced Metabolic Degradation: The Last Evolved Glyphosate Resistance Mechanism of Weeds?

For the past two decades, glyphosate has been the most used herbicide worldwide, resulting in prolonged, extreme selection pressure for glyphosate-resistant (GR) weeds. Glyphosate’s only target as a herbicide is 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), an enzyme of the shikimate

Confronting herbicide resistance with cooperative management.

BACKGROUNDResistance of pathogens and pests to antibiotics and pesticides worldwide is rapidly reaching critical levels. The common‐pool‐resource nature of this problem (i.e. whereby the susceptibility to treatment of target organisms is a shared resource) has been largely overlooked. Using herbicide‐resistant weeds as a model system, we developed a discrete‐time landscape‐scale simulation to investigate how aggregating herbicide management strategies at different spatial scales from individual farms to larger cooperative structures affects the evolution of glyphosate resistance in common waterhemp (Amaranthus tuberculatus).RESULTSOur findings indicate that high‐efficacy herbicide management strategies practiced at the farm scale are insufficient to slow resistance evolution in A. tuberculatus. When best practices were aggregated at large spatial scales, resistance evolution was hindered; conversely, when poor management practices were aggregated, resistance was exacerbated. Tank mixture‐based strategies were more effective than rotation‐based strategies in most circumstances, while applying glyphosate alone resulted in the poorest outcomes.CONCLUSIONSOur findings highlight the importance of landscape‐scale cooperative management for confronting common‐pool‐resource resistance problems in weeds and other analogous systems. © 2018 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Optimal intervals differ for double knock application of paraquat after glyphosate or haloxyfop for improved control of Echinochloa colona, Chloris virgata and Chloris truncata

The grasses Echinochloa colona (L.) Link, Chloris virgata Sw. and C. truncata R. Br. are major problems in summer fallows of the sub-tropical grain region of Australia. Traditionally, these weeds were treated with glyphosate alone, but E. colona and C. truncata populations have evolved glyphosate resistance, and the weed flora is also being dominated by the glyphosate-tolerant species C. virgata. For improved control of these populations, sequential application of glyphosate, or recently haloxyfop, followed by paraquat is being used. The optimal interval between sequential applications of these herbicides needs to be defined for these summer-growing grasses. Pot experiments were conducted using glyphosate or haloxyfop followed by paraquat at intervals from 1 to 21 days. In addition, populations of E. colona and C. truncata with resistance to glyphosate were compared. The optimal interval between sequential applications differed for the three grasses and the herbicide used for the first application. For the glyphosate-paraquat sequential treatments, the optimal intervals were 1–14 days for GS and GR E. colona, 7 days for C. virgata, and 7–14 days and 14 days for GS and GR C. truncata populations. For the haloxyfop-paraquat treatments, the optimal intervals were 1–21 days for E. colona, 1–4 days for C. virgata and 1–7 days for C. truncata. This treatment achieved 100% control irrespective of resistance status. Thus, sequential application, particularly haloxyfop followed by paraquat is a highly effective tactic for control of these three weeds.

High Levels of Glyphosate Resistance in Conyza canadensis from Agricultural and Non-Agricultural Sites in Ohio and Iowa

Glyphosate is an important herbicide worldwide, but its efficacy has been compromised where weed species have evolved glyphosate resistance. To better understand evolutionary outcomes of continued and strong selection from glyphosate exposure, we characterized variation in resistance in self-pollinating Conyza canadensis (horseweed) in Ohio and Iowa, where glyphosate resistance was first reported in 2002 and 2011, respectively. In 2015, we collected seeds from a total of 74 maternal plants (biotypes) from no-till soybean fields vs. non-agricultural sites in each state, using one representative plant per site. Young plants from each biotype were sprayed with glyphosate rates of 0x, 1x (840 g ae ha−1), 8x, 20x, or 40x. Resistant biotypes with at least 80% survival at each dosage were designated as R1 (1x), R2 (8x), R3 (20x), or R4 (40x). Nearly all Ohio agricultural biotypes were R4, as were 62% of biotypes from the non-agricultural sites. In Iowa, R4 biotypes were clustered in the southeastern soybean fields, where no-till agriculture is more common, and 45% of non-agricultural biotypes were R1–R4. Our results show that resistance levels to glyphosate can be very high (at least 40x) in both states, and that non-agricultural sites likely serve as a refuge for glyphosate-resistant biotypes.