Glyphosate-resistant Biotypes Research Articles

Influence of gender and glyphosate resistance on Palmer amaranth growth and interference with cotton

AbstractManagement of herbicide‐resistant weeds can be improved by understanding the biology of resistant biotypes. While the majority of research has focused on female plants and seed production of Palmer amaranth (Amaranthus palmeri S. Watson) that are resistant to glyphosate, growth of male plants that are resistant to this herbicide has not been studied in detail. Additionally, interference of male versus female Palmer amaranth plants on cotton (Gossypium hirsutum) yield has not been reported. Plant height and biomass of male and female plants from a mixed population of glyphosate‐resistant (GR) and glyphosate‐susceptible (GS) plants was studied in North Carolina when grown season‐long with cotton. Palmer amaranth height was less for GR male plants compared with GS males and both GR and GS females. Biomass of Palmer amaranth female plants was twice that of male plants irrespective of glyphosate resistance. Cotton yield was affected similarly by Palmer amaranth regardless of either gender or glyphosate resistance status. The implications of shorter GR male plants on pollen dispersal and ramifications on management of glyphosate resistance are not known. Results from these trials did not address implications of the height of male plants on fitness of GR resistance. Nonetheless, the finding that GR male plants were shorter in the field than GS male plants warrants a new look at this topic. Similar reductions for cotton yield in presence of both GR and GS biotypes and genders suggest that current yield loss assessments and management decisions do not need to consider these variables in Palmer amaranth populations.

Smooth pigweed (Amaranthus hybridus) and unresolved Amaranthus spp. from Brazil resistant to glyphosate exhibit the EPSPS TAP-IVS substitution

AbstractThe presence of glyphosate-resistant smooth pigweed (Amaranthus hybridus L.) biotypes has increased in southern Brazil in recent years, presenting the triple amino acid substitution TAP-IVS in 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS) previously found in Argentina. Some of these biotypes have morphological characteristics of A. hybridus and redroot amaranth (Amaranthus retroflexus L.). The present study aimed to identify, through molecular markers, the herbicide-resistant species of Amaranthus from Brazil that have the TAP-IVS substitution and to analyze the occurrence of pollen-mediated gene flow (PMGF) as the source of the TAP-IVS substitution in these biotypes. Six biotypes were evaluated using internal transcribed spacer (ITS) sequences, of which two (AMACHY-S and CAMAQ-R) were molecularly classified as A. hybridus and four (AMACRET-S, AMACVI-S, ARRGR-R, and SAOJER-R) were unclassified. Interestingly, all the glyphosate-resistant biotypes (ARRGR-R, SAOJER-R, and CAMAQ-R) had the TAP-IVS substitution and an increase in EPSPS relative copy number; however, only CAMAQ-R was confirmed as A. hybridus. Although the biotypes ARRGR-R and SAOJER-R are closely related to A. hybridus and green pigweed (Amaranthus powellii S. Watson), their species identity could not be resolved. The biotype SAOJER-R also was resistant to acetolactate synthase (ALS)-inhibiting herbicides due to a tryptophan to leucine substitution at position 574 in ALS. The evaluation of 119,746 seedlings in an intraspecific hybridization study of A. hybridus indicated an outcrossing frequency of 0.09%. In contrast, an absence of interspecific hybridization (A. hybridus × unclassified biotype, AMACVI-S) was found after screening 111,429 offspring. Unclassified biotypes might be derived from one or more ancient hybridization events and subsequently evolved the triple mutation independently. Alternatively, such biotypes could have evolved from recent hybridization events, which occur at a frequency below the level of detection in our study.

Management of Eleusine indica (L.) Gaertn Resistance to Glyphosate Herbicide in Indonesia

Eleusine indica (L.) Gaertn, commonly known as goosegrass or wiregrass, is a type of grass that is widespread in many parts of the world. The broad-spectrum herbicide glyphosate is most frequently used in Indonesian oil palm plantations to get rid of weeds and other undesirable plants. However, improper rotation of herbicide types by farmers has led to an increased risk of resistant weed emergence. This investigation tries to validate E. indica’s glyphosate resistance, investigate mutations in the EPSPS gene of the resistant biotype, and determine the type of herbicides that can control E. indica glyphosate-resistant biotypes. The whole plant pot test method was used to measure the resistance level, while DNA sequencing using the PCR method was conducted on all samples to identify mutations in the EPSPS gene of the resistant biotype. The study results showed that all biotypes of E. indica were identified as resistant to glyphosate but susceptible to propaquizafop, ametryn, and sulfentrazone herbicides. Several biotypes, such as the North Sumatra biotype, were identified as having multiple resistances to glyphosate, paraquat, and ammonium glufosinate. Thr102Iso and Pro106Ser amino acid substitutions were found in the EPSPS gene of E. indica-resistant biotypes. The findings of this study showed that E. indica was resistant to paraquat and ammonium glufosinate; further research is required to determine the mechanism.

The Molecular, Morphological and Genetic Characterization of Glyphosate Resistance in Conyza bonariensis from South Africa.

Six Conyza bonariensis (L.) Cronquist populations were screened in a pot experiment at the University of Pretoria's Hatfield experimental farm to evaluate and confirm the degree of glyphosate response. Resistance factors ranged from 2.7- to 24.8-fold compared to the most susceptible biotype. Partial sequencing of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene found no mutation at the Thr102, Ala103 or Pro106 positions. EPSPS mRNA expression levels in glyphosate-resistant biotypes (Swellendam and Piketberg seed sampling sites) were comparable or lower than those in susceptible biotypes (George and Fauresmith sites). Additionally, the highest expression level was reported in the susceptible Fauresmith biotype. These results indicate that glyphosate resistance in the tested resistant biotypes is not caused by target-site mutations and EPSPS gene amplification. Leaf surface characteristics can influence the spread and subsequent absorption of glyphosate. The study established non-significant results in the amount of leaf wax and insufficient mean separations in cuticle thickness and trichome density data. Therefore, the observed differences in response of biotypes to glyphosate treatment could not be attributed conclusively to differences in the leaf morphological characteristics investigated. Results from the inheritance study were consistent with glyphosate resistance being inherited in an incompletely dominant manner when plants were treated with glyphosate herbicide at 900 g ae ha-1.

Monitoring and baseline of glyphosate-resistant sourgrass in the main soybean growing regions of Brazil

Frequent application of glyphosate for consecutive years has enhanced the selection pressure on sourgrass (Digitaria insularis) populations, which resulted in the development of glyphosate-resistant biotypes. Therefore, this work was developed with the objective of monitoring sourgrass resistance to glyphosate, develop a baseline of sourgrass susceptibility to this molecule and, consequently, identify the discriminatory dose between resistant and susceptible populations. This work was divided into three steps. The first step consisted of identifying and sorting sourgrass resistant and susceptible biotypes among 30 samples. In the second step, glyphosate baseline was elaborated considering exclusively the glyphosate-susceptible biotypes, which allowed the definition of a discriminatory dose. At the end, the third step, monitoring of glyphosate-resistant biotypes was achieved, considering five growing seasons (2016 – 2020) and 809 samples of sourgrass populations, collected throughout 12 states of Brazil. Glyphosate baseline was elaborated to sourgrass and ideal discriminatory rate was identified as 960 g ha-1. Glyphosate-resistant populations of sourgrass were found in all soybean growing regions sampled. Among 809 populations, 25.96% were considered resistant to glyphosate. The states with the highest frequency of glyphosate-resistant populations were Rio Grande do Sul, Mato Grosso do Sul, Bahia, Mato Grosso and Paraná.

Economic impact of glyphosate-resistant weeds on major field crops grown in Ontario

AbstractLimited information exists on the global economic impact of glyphosate-resistant (GR) weeds. The objective of this manuscript was to estimate the potential yield and economic loss from uncontrolled GR weeds in the major field crops grown in Ontario, Canada. The impact of GR weed interference on field crop yield was determined using an extensive database of field trials completed on commercial farms in southwestern Ontario between 2010 and 2021. Crop yield loss was estimated by expert opinion (weed scientists and Ontario government crop specialists) when research data were unavailable. This manuscript assumes that crop producers adjust their weed management programs to control GR weeds, which increases weed management costs but reduces crop yield loss from GR weed interference by 95%. GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed would cause an annual monetary loss of (in millions of Can$) $172, $104, $11, $3, and $0.3, respectively, for a total annual loss of $290 million if Ontario farmers did not adjust their weed management programs to control GR biotypes. The increased herbicide cost to control GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed in the major field crops in Ontario is estimated to be (in millions of Can$) $17, $9, $2, $0.1, and $0.02, respectively, for a total increase in herbicide expenditures of $28 million annually. Reduced GR weed interference with the adjusted weed management programs would reduce farm-gate monetary crop loss by 95% from $290 million to $15 million. This study estimates that GR weeds would reduce the farm-gate value of the major field crops produced in Ontario by Can$290 million annually if Ontario farmers did not adjust their weed management programs, but with increased herbicide costs of Can$28 million and reduced crop yield loss of 95% the actual annual monetary loss in Ontario is estimated to be Can$43 million annually.

Herbicide mixtures for post-emergence control of horseweed in coffee interrow

Glyphosate-resistant biotypes of horseweed (Conyza spp.) have been a constant problematic for coffee growers all over Brazil. Therefore, this work was developed with the objective of evaluating efficacy of different herbicides, applied isolated or in mixture, for horseweed control in coffee plantation interrows. A field trial was performed with the following treatments (g ha-1): glyphosate at 925, glufosinate-ammonium (GA) at 500, saflufenacil at 49, flumioxazin at 50, glyphosate + GA (925 + 500), glyphosate + saflufenacil (925 + 49), glyphosate + flumioxazin (925 + 50), GA + saflufenacil (500 + 49), GA + flumioxazin (500 + 50) and glyphosate + saflufenacil + clethodim (925 + 49 + 108), besides check plots without herbicide application. The treatment with clethodim was included considering a possible concomitant presence of sourgrass (Digitaria insularis (L) Fedde) in the areas. At the moment of herbicide application, horseweed was 40 cm tall in pre-flowering. Percent control was evaluated at 7, 14, 21 and 28 days after application (DAA), as well as mass of dry matter at 28 DAA. In general, adequate control of horseweed was obtained with glufosinate-ammonium applications, isolated or in mixture with glyphosate, saflufenacil or flumioxazin. Mixture of glyphosate and saflufenacil has also reached high control of horseweed infestation, without difference to GA treatments. As conclusion, horseweed infestations in coffee interrow may be efficiently controlled with tank mixtures of herbicide.

Confirmation and control of glyphosate-resistant biotype of goosegrass (Eleusine indica L.). In Sumatran oil palm

Glyphosate has been used as the main tool for weed control in an oil palm plantation for about four decades. In the last five years, in an oil palm plantation of PTP Nusantara III, a population of Eleusine indica was reported to have been difficult to control using glyphosate. The objectives of this study were first, to confirm whether the population of E. indica in the oil palm plantation has evolved resistance to glyphosate. Secondly, to evaluate post-emergent alternative herbicides, premixed 39.5% MSMA + 7.8% diuron, glufosinate ammonium, topramezone and glyphosate as a comparison, which were all of them applied at field rates of 1975 g + 390 g, 450 g, 41,25 g, and 1080 g a.i. ha−1, respectively. While pre-emergent herbicides, indaziflam, diuron, and flumoxiazin were applied 75, 1500, and 160 g a.i ha−1, respectively. The results showed that the suspected glyphosate-resistant E. indica population has been confirmed to have developed resistance to glyphosate. The population is highly resistant with level of resistance 28-fold of the susceptible population. No resistant plant was killed at the rate of 4320 g. a.e. ha−1 glyphosate (four-fold field rate for E. indica control) whereas the susceptible plant was killed 77,5%. Alternative herbicides tested effectively suppressed glyphosate-resistant E. indica population when applied at field rate. The alternative herbicides could be used to overcome glyphosate-resistance in the oil palm plantation.

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.

Hyperspectral plant sensing for differentiating glyphosate‐resistant and glyphosate‐susceptible johnsongrass through machine learning algorithms

Johnsongrass (Sorghum halepense) is one of the weeds that evolves resistance to glyphosate [N-(phosphonomethyl)-glycine], the most widely used herbicide, and the weed may cause agronomic troublesome in the southern USA. This paper reports a study on developing a hyperspectral plant sensing approach to explore the spectral features of glyphosate-resistant (GR) and glyphosate-sensitive (GS) plants to evaluate this approach using machine learning algorithms to differentiate between GR and GS plants. On average, GR plants have higher spectral reflectance compared with GS plants. The sensitive spectral bands were optimally selected using the successive projections algorithm respectively wrapped with the machine learning algorithms of k-nearest neighbors (KNN), random forest (RF), and support vector machine (SVM) with Fisher linear discriminant analysis (FLDA) to classify between GS and GS plants. At 3 weeks after transplanting (WAT) KNN and SVM could not acceptably classify the GR and GS plants but they improved significantly with the stages to have their overall accuracies reaching 73% and 77%, respectively, at 5 WAT. RF and FLDA had a better ability to classify the plants at 3 WAT but RF was low in accuracy at 2 WAT while FLDA dropped accuracy to 50% at 4 WAT from 57% at 3 WAT and raised it to 73% at 5 WAT. Previous studies were conducted developing the hyperspectral imaging approach to differentiate GR Palmer amaranth from GS Palmer amaranth and GR Italian ryegrass from GS Italian ryegrass with classification accuracies of 90% and 80%, respectively. This study demonstrated that the hyperspectral plant sensing approach could be developed to differentiate GR johnsongrass from glyphosate-sensitive GS johnsongrass with the highest classification accuracy of 77%. The comparison with our previous studies indicated that the similar hyperspectral approach could be used and transferred from classification across different GR and GS weed biotypes, such as Palmer amaranth, Italian ryegrass and johnsongrass, so it is highly possible for classification of more other GR and GS weed biotypes as well. On the basis of classic pattern recognition approaches the process of plant classification can be enhanced by modeling using machine learning algorithms. © 2022 Society of Chemical Industry. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

The world's first glyphosate-resistant case of Avena fatua L. and Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne and alternative herbicide options for their control.

Avena fatua and A. ludoviciana (commonly known as wild oats) are the most problematic winter grass species in fallows and winter crops in the northeast region of Australia. A series of experiments were conducted to evaluate the performance of glyphosate and alternative post-emergence herbicides on A. fatua and A. ludoviciana. This study reports the world’s first glyphosate-resistant (GR) biotypes of A. fatua and A. ludoviciana. The glyphosate dose required to kill 50% of the plants (LD50) and to reduce 50% of the biomass (GR50) for the GR biotype of A. fatua was 556 g a.e./ha and 351 g a.e./ha, respectively. These values for A. ludoviciana were 848 g a.e./ha and 289 g a.e./ha. Regardless of the growth stage (3–4 or 6–7 leaf stages), clethodim (120 g a.i./ha), haloxyfop (78 g a.i./ha), pinoxaden (20 g a.i./ha), and propaquizafop (30 g a.i./ha) were the best alternative herbicide options for the control of A. fatua and A. ludoviciana. The efficacy of butroxydim (45 g a.i./ha), clodinafop (120 g a.i./ha), imazamox + imazapyr (36 g a.i./ha), and paraquat (600 g a.i./ha) reduced at the advanced growth stage. Glufosinate (750 g a.i./ha), flamprop (225 g a.i./ha), and pyroxsulam + halauxifen (20 g a.i./ha) did not provide effective control of Avena species. This study identified alternative herbicide options to manage GR biotypes of A. fatua and A. ludoviciana.

Effect of low glyphosate doses on flowering and seed germination of glyphosate-resistant and -susceptible Digitaria insularis.

Herbicide hormesis is characterized by stimulation of various growth and developmental parameters, such as biomass and height, at low herbicide doses. Other possible hormetic effects are earlier flowering, higher seed weight, more seeds, and a shorter plant life cycle, which could favor the propagation of the species. This study tested the early flowering in glyphosate-resistant and -susceptible Digitaria insularis biotypes under treatment with low glyphosate doses. Hormesis caused by low glyphosate doses occurred in all experiments. The hormetic effects were a decrease in time necessary for the formation of inflorescences and increased seed weight and germination speed. Higher glyphosate doses were required for the hormetic effect in the glyphosate-resistant than the -susceptible D. insularis biotype. Hormesis caused by low glyphosate doses in D. insularis may provide an advantage for the dissemination of this species, helping to alter the weed flora. As the doses that cause stimulation in glyphosate-resistant biotypes are higher than in glyphosate-susceptible biotypes, the selection of resistant biotypes may be favored in glyphosate-sprayed fields, increasing the rate of infestation of glyphosate-resistant biotypes.

Spatio-Temporal Distribution of Digitaria insularis: Risk Analysis of Areas with Potential for Selection of Glyphosate-Resistant Biotypes in Eucalyptus Crops in Brazil

The objective of this study was to model the spatio-temporal distribution of Digitaria insularis (D. insularis) and analyze the risk of selection of glyphosate-resistant biotypes in eucalyptus cultivation in Brazil. Global data on the distribution of the specie were collected and associated with their ideal growth characteristics. The models were generated using Climex software, providing a predictive modeling technique. Biological data, species distribution, and climatic parameters were used to predict and map potential areas for the species of interest through the combination of growth and stress indices, giving rise to the Ecoclimatic Index (EI). The spatial distribution of D. insularis is predominantly in South and Central America and southern North America. The model had a good fit with the collected data and predicted higher EI values for tropical and subtropical regions, as was the case in Brazil. Species growth can occur throughout the year, with lower rates in winter, mainly in the country’s southern regions. Brazil has high climatic suitability for the occurrence of Digitaria insularis. Due to the climate suitability evidenced by the models and the expressive use of the same active ingredient, there is a risk of selecting glyphosate-resistant Digitaria insularis biotypes in eucalyptus cultivation areas.

Cross-resistance to diquat in glyphosate/paraquat-resistant hairy fleabane (Conyza bonariensis) and horseweed (Conyza canadensis) and confirmation of 2,4-D resistance in Conyza bonariensis

AbstractHairy fleabane and horseweed are pervasive weed species in agriculture. Glyphosate-resistant (GR) and glyphosate/paraquat–resistant (GPR) biotypes challenge current management strategies. These GR and GPR biotypes have non–target site resistance, which can confer resistance to herbicides with different sites of action (SOAs). This study’s objective was to characterize the response of GR, GPR, and glyphosate/paraquat–susceptible (GPS) biotypes of both weed species to herbicides with a different SOA. Whole-plant dose–response bioassays indicated a similar response among tested biotypes of both weed species to rimsulfuron, dicamba, hexazinone, glufosinate, flumioxazin, saflufenacil, or mesotrione. The hairy fleabane GR and GPR biotypes were 2.7- and 2.9-fold resistant to 2,4-D relative to the GPS biotype (GR50 766.7 g ai ha–1), confirming 2,4-D resistance in hairy fleabane for the first time in California. The GR and GPR biotypes were not cross-resistant to dicamba. No differences in response to 2,4-D were observed among horseweed biotypes with a GR50 ranging from 150.2 to 277.4 g ai ha–1. The GPR biotypes of both species were cross-resistant to diquat, with a 44.0-fold resistance in hairy fleabane (GR50 863.7 g ai ha–1) and 15.6-fold resistance in horseweed (GR50 563.1 g ai ha–1). The confirmation of multiple resistances to glyphosate, paraquat, and 2,4-D in hairy fleabane curtails herbicide SOA alternatives and jeopardizes resistance management strategies based on herbicide rotation and tank mixtures, underscoring the critical need for nonchemical weed control alternatives.

Control of glyphosate-resistant horseweed and giant ragweed in soybean with halauxifen-methyl applied preplant

AbstractHorseweed and giant ragweed are competitive, annual weeds that can negatively impact crop yield. Biotypes of glyphosate-resistant (GR) giant ragweed and horseweed were first reported in 2008 and 2010 in Ontario, respectively. GR horseweed has spread throughout the southern portion of the province. The presence of GR biotypes poses new challenges for soybean producers in Canada and the United States. Halauxifen-methyl is a recently registered selective herbicide against broadleaf weeds for preplant use in corn and soybean. There is limited literature on the efficacy of halauxifen-methyl on GR horseweed and giant ragweed when combined with currently registered products in Canada. The purpose of this study was to determine the effectiveness of halauxifen-methyl applied alone and tank-mixed to control GR giant ragweed and GR horseweed in glyphosate and dicamba-resistant (GDR) soybean in southwestern Ontario. Six field experiments were conducted separately for each weed species over 2018 and 2019. Halauxifen-methyl applied alone offered 72% control of GR horseweed at 8 wk after application (WAA). Control was improved to >91% when halauxifen-methyl applied in combination with metribuzin, saflufenacil, chlorimuron-ethyl + metribuzin, and saflufenacil + metribuzin. At 8 WAA, halauxifen-methyl provided 11% control of GR giant ragweed, and 76% to 88% control when glyphosate/2,4-D choline, glyphosate/dicamba, glyphosate/2,4-D choline + halauxifen-methyl, and glyphosate/dicamba + halauxifen-methyl were used. We conclude that halauxifen-methyl applied preplant in a tank-mixture can provide effective control of GR giant ragweed and horseweed in GDR soybean.

First Report of Glyphosate-Resistant Biotype of Eleusine Indica (L.) Gaertn. in Europe

Glyphosate-resistant biotypes of Eleusine indica (L.) Gaertn. have been detected in Asia, the Americas but not in Europe. The aim of this study was to evaluate resistance levels and possible target site resistance mechanisms of an E. indica biotype (19-1) collected from a plant nursery in Southern Italy where poor glyphosate efficacy was reported. Two dose-response experiments were conducted to evaluate the sensitivity of biotype 19-1 to glyphosate in comparison with two susceptible checks. 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) sequencing was performed to identify possible mutations conferring the resistance. The susceptible biotypes were completely controlled at the glyphosate recommended field dose of 360 g ae ha−1, while 50% of the plants of biotype 19-1 survived at 1440 g ae ha−1. The resistance index of biotype 19-1 ranged between 5.8 and 7.3 for the response variables of fresh weight reduction and plant survival, respectively. All the plants surviving glyphosate application and sampled for DNA analyses had the point mutation P106A. The biotype 19-1 can be confirmed as glyphosate-resistant, representing the first glyphosate-resistant population of E. indica in Europe.

Glyphosate Resistance in Sonchus oleraceus and Alternative Herbicide Options for Its Control in Southeast Australia

Sonchus oleraceus is becoming a hard-to-control weed in Australian cropping systems, especially in glyphosate-tolerant cotton and during summer fallows. Several biotypes of this weed have developed resistance to glyphosate as a result of common management practices under conservation agriculture systems in the country. A series of pot experiments were conducted to evaluate the effect of temperature on glyphosate efficacy and performance of several post-emergence and pre-emergence herbicides on a glyphosate-resistant (GR) and a glyphosate-susceptible (GS) biotype of S. oleraceus. At low temperatures (19–24 °C), no plants of the GS biotype survived glyphosate application at 570 g/ha; however, in the high-temperature regime (28–30 °C), 83% of the plants survived this rate of glyphosate. Similarly, for the GR biotype, up to 58% of the plants survived at 2280 g/ha of glyphosate when applied during the high-temperature regime and no plants survived this rate during the low-temperature regime. A number of post-emergence herbicides were found to be effective for S. oleraceus control. However, herbicide application delayed to the six-leaf stage compared with the four-leaf stage reduced control, especially for bromoxynil and saflufenacil herbicides. Glufosinate and paraquat were the most effective herbicides for S. oleraceus control, resulting in no seedling survival for both biotypes. Isoxaflutole, pendimethalin or s-metolachlor efficacy was not reduced by the presence of crop residue, suggesting that these herbicides could be used to control S. oleraceus in conservation agriculture systems. The results of this study suggest that growers will need to reduce over-reliance on glyphosate for weed control in summer fallows and use alternative post-emergence herbicides.

Homogeneity among glyphosate-resistant Amaranthus palmeri in geographically distant locations.

Since the initial report of glyphosate-resistant (GR) Amaranthus palmeri S. Watson in 2006, resistant populations have been reported in 28 states. The mechanism of resistance is amplification of a 399-kb extrachromosomal circular DNA, called the EPSPS replicon, and is unique to glyphosate-resistant plants. The replicon contains a single copy of the 10-kb 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene which causes the concomitant increased expression of EPSP synthase, the target enzyme of glyphosate. It is not known whether the resistance by this amplification mechanism evolved once and then spread across the country or evolved independently in several locations. To compare genomic representation and variation across the EPSPS replicon, whole genome shotgun sequencing (WGS) and mapping of sequences from both GR and susceptible (GS) biotypes to the replicon consensus sequence was performed. Sampling of GR biotypes from AZ, KS, GA, MD and DE and GS biotypes from AZ, KS and GA revealed complete contiguity and deep representation with sequences from GR plants, but lack of homogeneity and contiguity with breaks in coverage were observed with sequences from GS biotypes. The high sequence conservation among GR biotypes with very few polymorphisms which were widely distributed across the USA further supports the hypothesis that glyphosate resistance most likely originated from a single population. We show that the replicon from different populations was unique to GR plants and had similar levels of amplification.

Target-Site Resistance to Glyphosate in Chloris Virgata Biotypes and Alternative Herbicide Options for its Control

Due to the overdependence on glyphosate to manage weeds in fallow conditions, glyphosate resistance has developed in various biotypes of several grass weeds, including Chloris virgata Sw. The first case of glyphosate resistance in C. virgata was found in 2015 in Australia, and since then several cases have been confirmed in several biotypes across Australia. Pot studies were conducted with 10 biotypes of C. virgata to determine glyphosate resistance levels. The biotypes were identified as either susceptible, moderately resistant or highly resistant based on the glyphosate dose required to kill 50% of plants. Two glyphosate-susceptible (GS) and two glyphosate-resistant (GR) biotypes were identified by the dose-response study and analyzed for the presence of target-site mutation in the 5–enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene. Performance of alternative herbicides to glyphosate as well as the double-knock herbicide approach was evaluated on the two GS (Ch and SGM2) and two GR (SGW2 and CP2) biotypes. Three herbicides, clethodim, haloxyfop and paraquat, were found to be effective (100% control) against all four biotypes when applied at the 4–5 leaf stage. All the sequential herbicide treatments, such as glyphosate followed by paraquat and glufosinate-ammonium followed by paraquat, provided 100% control of all four biotypes of C. virgata. This study identified effective herbicide options for the control of GR C. virgata and showed that target-site mutations were involved in the resistance of two biotypes to glyphosate (SGW2 and CP2). Results could aid farmers in selecting herbicides to manage C. virgata in their fields.

Response of glyphosate-resistant and glyphosate-susceptible biotypes of annual sowthistle (Sonchus oleraceus) to increased carbon dioxide and variable soil moisture

AbstractThe growth response of annual sowthistle (Sonchus oleraceusL.) to anticipated future climate conditions is currently unknown, and thus two parallel studies were conducted dealing with glyphosate-resistant (GR) and glyphosate-susceptible (GS) biotypes ofS. oleraceus. The glyphosate efficacy study was conducted using different doses of glyphosate (0 [control], 180, 360, 720 [recommended dose], and 1,440 g ae ha−1) at two different moisture levels (well-watered and water-stressed conditions). In the second study, the growth and seed production of these biotypes were studied under different atmospheric carbon dioxide (CO2) concentrations (450 and 750 ppm) and under well-watered (100% field capacity) and water-stressed (50% field capacity) conditions. Results showed that the GR biotype survived (>68%) at 1,440 g ha−1, but for the GS biotype, no plant survived, and both biotypes were slightly (<10%) affected by moisture regimes. In the elevated CO2condition, the GS biotype plants were >38% taller and produced >44%, >18%, and >21% more leaves, buds, and seeds, respectively, compared with the ambient CO2concentration under both moisture regimes. The biomass also increased by 27% in comparison with the ambient CO2concentration. For the GR biotype, plants at the elevated CO2level, while they also grew 38% taller in comparison with the ambient CO2concentration, the numbers of leaves, buds, and seeds and biomass were not affected by this increase in CO2. Results showed that there were minimal changes in response to glyphosate for GR and GS biotypes ofS. oleraceuswith or without moisture stress. Our study suggests that future climate change with elevated CO2levels can affect the response ofS. oleraceusto glyphosate, and such knowledge will be helpful for weed management in the future.