Resistant Weeds Research Articles

Survey of ACCase and ALS resistance in winter annual grasses identifies target-site and nontarget-site imazamox resistance in Secale cereale.

Early detection of herbicide resistance in weeds is crucial for successful implementation of integrated weed management. We conducted a herbicide resistance survey of the winter annual grasses feral rye (Secale cereale), downy brome (Bromus tectorum), and jointed goatgrass (Aegilops cylindrica) from Colorado winter wheat production areas for resistance to imazamox and quizalofop. All samples were susceptible to quizalofop. All downy brome and jointed goatgrass samples were susceptible to imazamox. Out of 314 field collected samples, we identified three feral rye populations (named A, B, and C) that were imazamox resistant. Populations B and C had a target-site mechanism with mutations in the Ser653 residue of the acetolactate synthase (ALS) gene to Asn in B and to Thr in C. Both populations B and C had greatly reduced ALS in vitro enzyme inhibition by imazamox. ALS feral rye protein modeling showed that steric interactions induced by the amino acid substitutions at Ser653 impaired imazamox binding. Individuals from population A had no mutations in the ALS gene. The ALS enzyme from population A was equally sensitive to imazamox as to known susceptible feral rye populations. Imazamox was degraded two times faster in population A compared with a susceptible control. An oxidized imazamox metabolite formed faster in population A and this detoxification reaction was inhibited by malathion. Population A has a nontarget-site mechanism of enhanced imazamox metabolism that may be conferred by cytochrome P450 enzymes. This is the first report of both target-site and metabolism-based imazamox resistance in feral rye. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The Effect of Sulfentrazone and Glyphosate Herbicides to Control Eleusine Indica L. a Resistant weed to Herbicide

Weed Eleusine indica L. was reported to have been difficult to control using glyphosate in Oil palm plantation during the time. Sulfentrazone was used as an alternative herbicide for the management of resistant weeds. The aim of this study was to determine the effects of two active ingredients Glyphosate and Sulfentrazone on weed Eleusine indica L. resistant to herbicide. The study was conducted using Completely Randomly Designed (CRD) with 14 treatments and 3 replications. 14 treatments consisted of single herbicide Glyphosate, which was applied at field rate 2 l/ha, Sulfentrazone was sprayed at the numerous dose of 500, 750, 1000, and 1250 ml/ha, and the mixture of Glyphosate and sulfentrazone at the dose of 500 ml/ha + 2 l/ha and 750 ml/ha + 2 l/ha, respectively. The research was conducted at the Greenhouse of the Faculty of Agriculture, Andalas University, Padang, from August to December, 2021. The results showed that there were significantly effect when used in combination of herbicides Glyphosate and Sulfentrazone, showed 100% of mortality weed after 3 week application. For a single active ingredient, Glyphosate controlled eficiently weed at the 4-leaf stage, Sulfentrazone had same impact on managing weed on the 4 and 6-leaf stage, both caused 100% weed dead. However, weed Eleusine indica L. was treated with Glyphosate still survived on 6 and 8 leaves weeds. Similar to Glyphosate, Sulfentrazone could not completely control resistant weeds at the 8-leaf stage. Classification of resistant weed Eleusine indica L. are herbicide-developing resistance 14%, herbicide-resistant 22%, and herbicide-susceptible 64%.

Micro Electric Shocks Control Broadleaved and Grass Weeds

A search for energy efficient, non-herbicide weed control methods led to development of a novel electrical weeding technology. This study focuses on weed control efficiency and energy as elements of a system that would include machine vision and robotics to control escape weeds in field crops. Two pulse generation systems, one single and one multiple, were developed and evaluated at different delivered voltages and energies. Greenhouse trials using specially designed and built application and recording technology showed the application of precisely applied micro-shocks with precisely controlled direct current (DC) voltage, pulse number, pulse length and period (hereafter PMS) can kill small Lolium multiflorum Lam., Chenopodium album L., Amaranthus powellii S. Wats. and Solanum nigrum L. plants with minimal energy. Plants took as much as two weeks to die. Increasing applied energy increased effectiveness as determined by plant biomass reduction and death rate. Grasses appear difficult to control once tillering has commenced, and high voltages may destroy leaf blades but not growing points. Broadleaved plants took several days to show evidence of chlorosis which preceded senescence and death. Our results showed that 5 J is sufficient energy to bring about death or severe growth limitation in many seedlings up to 15 cm height. This is as little as 1% of the energy of, and more effective than, ultra-low energy treatments reported in other recent research. To control five herbicide resistant weeds m−2, the required energy would be about 0.25 MJ ha−1 plus transport and actuation energy for weed destruction, as compared to an optimum target of about 20–40 MJ ha−1 including transport suggested in the literature. PMS can effectively control broadleaved weed seedlings and small non-tillering grasses at a fraction of the energy required by commercially available systems. This indicates PMS has potential as a viable technology for hand-held electric weeders or as part of a site-specific robotic weeding system.

Design of a novel hapten and development of a sensitive monoclonal immunoassay for dicamba analysis in environmental water samples

Weed resistance to glyphosate has been a driving force behind the increased use of alternative herbicides in agriculture. Recently, dicamba-tolerant recombinant plants were introduced to the market, which may result in residues of this agrochemical contaminating environmental waters. Given that restrictions on the use of dicamba have consequently been established by regulatory agencies, it is therefore also desirable to conduct extensive controls on dicamba residues. Immunoassays are currently the most powerful bioanalytical technology for the rapid monitoring of chemical residues and contaminants. In the present study, a novel hapten was designed maintaining unaltered all the antigenic moieties of the target molecule, and this was used to generate high-affinity monoclonal antibodies against dicamba for the first time. Additionally, a collection of haptens with different linker composition or linker tethering site was synthesized and conjugated to proteins. Using these novel immunoreagents, a direct competitive enzyme-linked immunosorbent assay with a limit of detection for dicamba of 0.24 ng/mL was developed and validated. Analysis of water samples from different origins afforded recovery values between 90 % and 120 %, and coefficients of variation below 20 % were obtained. These results indicate that the developed immunochemical assay is suitable for the rapid determination of dicamba residues in environmental water samples.

Target-site mutation and cytochrome P450s confer resistance to multiple herbicides in Italian ryegrass (Lolium multiflorum Lam.) from China

Italian ryegrass (Lolium multiflorum Lam.) is a highly competitive grass weed capable of severely infesting wheat fields worldwide. In recent years, Chinese farmers are finding it increasingly difficult to control L. multiflorum when using acetyl-CoA carboxylase (ACCase)-inhibiting herbicides. Accordingly, clarifying the resistance pattern of putative resistant populations of L. multiflorum is important for selecting alternative herbicides for effective weed control. In this study, a putative resistant L. multiflorum population, HN-03, which apparently has survived clodinafop-propargyl treatment under the recommended field application rate, was collected from a wheat field in Zhumadian, Henan, China. Whole-plant bioassay revealed that the HN-03 population had evolved high-level resistance to clodinafop-propargyl. Sequencing of the ACCase gene revealed a point mutation, of TGG to TGT, at codon position 1999, causing a Trp-to-Cys substitution in the resistant plants. The HN-03 population also exhibited cross-resistance to fenoxaprop-P-ethyl, clethodim, and pinoxaden and multiple-resistance to mesosulfuron-methyl and bixlozone, but it remained susceptible to cypyrafluone. Pre-treatment with piperonyl butoxide and malathion considerably increased the susceptibility of the HN-03 population to fenoxaprop-P-ethyl and mesosulfuron-methyl, by 25.3% and 36.6%, respectively. This study shows the L. multiflorum population HN-03 has evolved multiple herbicide resistance due to both a target-site mutation and P450s-mediated enhanced metabolism. Cypyrafluone can be used as an alternative herbicide to control resistant L. multiflorum, and it might be potentially helpful for improving the management of herbicide resistance in other aggressive weeds.

Pesticide dynamics in agroecosystems: Assessing climatic and hydro-physical effects in a soybean cycle under no-tillage

The high rate of No-Tillage (NT) adoption in Argentina has been followed by conspicuous application of herbicides due to RR crops selection and recently due to weed resistance. In addition, no-tillage in the Rolling Pampa (Argentina) with absence of crop rotation and scarce crop residues, led to physical degradation (low infiltration rates, platy structure development) having strong implications on water dynamics and the risk of soil and water contamination. Several factors can influence the mobility of pesticides in the landscape. However, measurement of soil hydro-physical and climate traits as determinants of pesticide dynamics are still scarce. In this study, pesticides from soil and runoff (Glyphosate, AMPA, S-Metolachlor, Sulfentrazone), and hydro-physical and climatic variables were measured at the field in an entire soybean cycle under NT. Temporal dynamics of runoff and saturated hydraulic conductivity was measured by rainfall simulations and disc permeameters together with soil bulk density, soil moisture, crop residues, rainfall and rainfall erosivity. Besides, soil structure was evaluated through cultural profile method. In the Vertic Argiudoll (silty clay loam) pesticides mobility were related to the number of days after agrochemical application modulated also by heavy natural rainfalls occurred during the experiment that lowered agrochemical concentration mostly in runoff. Temporal hydro-physical properties and spatial soil structural variability, together with strong climatic events governed runoff process affecting pesticides mobility while no strong pesticide accumulation in the plots was observed in this study due to low saturated hydraulic conductivity, high runoff coefficients and poor soil structural traits. Thus, the selection of appropriate agricultural management is a key factor to modify hydro-physical top soil dynamics to change hydrological partition from horizontal to vertical fluxes, in particular in soils with poor structural condition.

Up-regulation of bZIP88 transcription factor is involved in resistance to three different herbicides in both Echinochloa crus-galli and E. glabrescens.

The resistance of weeds to herbicides poses a major threat to agricultural production, and non-target-site resistance (NTSR) is often a serious problem as its mechanisms can in some cases confer resistance to herbicides with different modes of action. In this study, we hypothesized that bZIP transcription factors (TFs), which regulate abiotic stress responses in many plants, play a regulatory role in NTSR. Whole-plant assays indicated that the wild grasses Echinochloa crus-galli and E. glabrescens are resistant to the herbicides penoxsulam, cyhalofop-butyl, and quintrione. Transcriptome sequencing then identified 101 and 49 bZIP TFs with differential expression following penoxsulam treatment in E. crus-galli and E. glabrescens, respectively. Twelve of these genes had >60% homology with rice genes. The expression of bZIP88 was considerably up-regulated 6 h after treatment with the three different herbicides, and it was similar between resistant and susceptible populations; however, the relative expression levels before herbicide treatment and 24 h after were the same. We used rice (Oryza sativa ssp. japonica cv Nipponbare) as a model system for functional validation and found that CRISPR-Cas9-knockout of the rice bZIP88 ortholog increased the sensitivity to herbicide, whereas overexpression reduced it. The OsbZIP88 protein was localized to the nucleus. Using ChIP coupled with high-throughput sequencing, OsbZIP88 was found to form a network regulatory center with other TFs such as bZIP20/52/59 to regulate OsKS1, OsCOE1, and OsIM1, which are related to auxin, abscisic acid, brassinosteroids, and gibberellic acid. Based on these results, we have established a database of bZIP TFs corresponding to herbicide stress, and resolved the mechanisms of the positive regulation of herbicide resistance by bZIP88, thereby providing new insights for NTSR.

Weed Management in Millets- A Holistic Approach

Millets are the traditional staple food of dry land regions and have the potential to contribute substantially for food, fodder and the nutritional security. The millets are relatively poor competitors against weeds especially during the early growth stages due to their slow initial growth and wider spacing. Yield loss due to weeds in millets varies from 5 to 94 per cent depending on climatic, edaphic and biotic factors. Weeds compete with crops for nutrients, soil moisture, sunlight and space when they are limiting, resulting in reduction in yield, quality and increased cost of production. The objective of this paper is to review the research that have been conducted pertaining to various aspects of weed management in millets. The literature suggests that instead of relying on any single method of weed control, all the feasible methods are to be integrated for the effective and sustainable management of weeds in millets. Integrated weed management can effectively overcome the problems of weed shift and development of resistance in weeds and reduce the weed seed bank and manage the weeds below the economic threshold level to avoid any economic loss.

Digital transformation for a sustainable agriculture in the United States: Opportunities and challenges

AbstractAgriculture faces key challenges of increasing productivity while reducing adverse impacts on the environment. Conventional practices that rely on tillage, inefficient and over‐application of chemicals, and monoculture row cropping are leading to growing resistance of weeds and pests to chemicals, nutrient and sediment run‐off, and declining soil carbon stocks in the United States. Digital technologies and artificial intelligence (AI) technologies are enabling the collection of vast amounts of geo‐referenced information about growing conditions within the field, automated implementation of spatially varying input applications, and reduced reliance on chemical inputs. We discuss the pathways by which digital agricultural technologies have the potential to address the challenge of herbicide‐resistant weeds, over‐application of nitrogen and irrigation water, and cover crop planting for restoring soil health and contribute to the environmental sustainability of agriculture. Then, we discuss the economic factors, behavioral preferences of farmers, peer pressure, and social networks that can be expected to play a role in adoption decisions. We conclude with a discussion of approaches for ex ante assessments of the determinants of farmer willingness to adopt digital technologies and their diffusion in a region.

Searching of Novel Herbicides for Paddy Field Weed Management—A Case Study with Acetyl-CoA Carboxylase

Weed management is the major biological constraint in paddy (Oryza sativa L.) producing areas. Predominantly, barnyard grass (Echinochloa crus-galli) is a rice-mimicking weed that causes 57% of yield loss in rice production. Conventionally, herbicides are the site-specific weed inhibitors often used to suppress E. crus-galli growth. Acetyl-CoA carboxylase (ACCase) is an important target for developing novel herbicides with remarkable selectivity against gramineous weeds. Notably, fenoxaprop-P-ethyl (FPPE) is a selective ACCase herbicide extensively used in paddy fields to inhibit barnyard grass. However, prolonged use of FPPE herbicide elicits phytotoxicity in cultivated rice and herbicide resistance in weeds. Recently, phytotoxins are emerging as an alternative to commercial herbicides with safer environmental profiles. Nevertheless, discovering natural herbicides through in vivo and in vitro techniques is time-consuming and expensive. Therefore, high-end computational screening strategies including Tanimoto similarity, docking, binding free energy, and herbicide-likeness were used to pinpoint the lead molecule. Finally, molecular dynamics and MM/PBSA calculations were employed to validate the binding kinetics of the hit compound. Indeed, sinigrin was identified as a promising phytotoxic inhibitor against the ACCase enzyme. The findings of our study were well correlated with the existing experimental results. Overall, the current work will aid in the development of commercializing phytotoxin herbicides in foreseeable future.

Identification and Comprehensive Evaluation of Resistant Weeds Using Unmanned Aerial Vehicle-Based Multispectral Imagery.

Atrazine is one of the most widely used herbicides in weed management. However, the widespread use of atrazine has concurrently accelerated the evolution of weed resistance mechanisms. Resistant weeds were identified early to contribute to crop protection in precision agriculture before visible symptoms of atrazine application to weeds in actual field environments. New developments in unmanned aerial vehicle (UAV) platforms and sensor technologies promote cost-effective data collection by collecting multi-modal data at very high spatial and spectral resolution. In this study, we obtained multispectral and RGB images using UAVs, increased available information with the help of image fusion technology, and developed a weed spectral resistance index, WSRI = (RE-R)/(RE-B), based on the difference between susceptible and resistant weed biotypes. A deep convolutional neural network (DCNN) was applied to evaluate the potential for identifying resistant weeds in the field. Comparing the WSRI introduced in this study with previously published vegetation indices (VIs) shows that the WSRI is better at classifying susceptible and resistant weed biotypes. Fusing multispectral and RGB images improved the resistance identification accuracy, and the DCNN achieved high field accuracies of 81.1% for barnyardgrass and 92.4% for velvetleaf. Time series and weed density influenced the study of weed resistance, with 4 days after application (4DAA) identified as a watershed timeframe in the study of weed resistance, while different weed densities resulted in changes in classification accuracy. Multispectral and deep learning proved to be effective phenotypic techniques that can thoroughly analyze weed resistance dynamic response and provide valuable methods for high-throughput phenotyping and accurate field management of resistant weeds.

Comparison of herbicide specificity of CYP81A cytochrome P450s from rice and a multiple-herbicide resistant weed, Echinochloa phyllopogon.

CYP81A cytochrome P450s (CYP81As) play a key role in herbicide detoxification in Poaceae plants. Crop CYP81As confer natural tolerance to multiple herbicides, whereas CYP81As in weeds disrupt herbicide action. Identifying differences in CYP81A herbicide specificity between crops and weeds could provide valuable information for controlling weeds. In this study, we quantitatively compared herbicide specificity between CYP81A6 from rice (Oryza sativa) and CYP81A12 and CYP81A21 from a weed, Echinochloa phyllopogon, using transgenic Escherichia coli and Arabidopsis. All three CYP81As metabolized the five tested herbicides and formed similar metabolites, with the highest relative activities of 400 to 580% toward bentazone compared to their activity on bensulfuron-methyl (defined as 100%). However, they showed differing activity toward propyrisulfuron. The relative activities of Echinochloa phyllopogon CYP81A12 (12.2%) and CYP81A21 (34.4%) toward propyrisulfuron were lower than that of rice CYP81A6 (98.5%). Additionally, rice CYP81A6 produced O-demethylated propyrisulfuron and hydroxylated products, whereas Echinochloa phyllopogon CYP81As produced only hydroxylated products. Arabidopsis expressing CYP81A12 and CYP81A21 exhibited lower levels of resistance against propyrisulfuron compared to that in Arabidopsis expressing CYP81A6. Homology modeling and in silico docking revealed that bensulfuron-methyl docked well into the active centers of all three CYP81As, whereas propyrisulfuron docked into rice CYP81A6 but not into Echinochloa phyllopogon CYP81As. The differing herbicide specificity displayed by rice CYP81A6 and Echinochloa phyllopogon CYP81A12 and CYP81A21 will help design inhibitors (synergists) of weed CYP81As, as well as develop novel herbicide ingredients that are selectively metabolized by crop CYP81As, to overcome the problem of herbicide resistance. © 2022 Society of Chemical Industry.

Plant life span and persistence of soil seedbanks predict the emergence of herbicide resistance in noxious weeds

AbstractPlant control methods have been developed to reduce weed species that are often problematic in agricultural systems. However, these methods can create new challenges, such as herbicide resistance. Determining which plant traits are associated with herbicide resistance can assist managers in identifying species with the potential to develop herbicide resistance and to better understand factors contributing to the evolution of herbicide resistance. We used random forest models to model herbicide resistance of noxious weeds as a function of 10 biological and ecological plant characteristics. Three noxious weed characteristics—plant life span, seedbank persistence, and occurrence in riparian or wetland microsites—predicted herbicide resistance with 87% accuracy. Species with persistent seedbanks and with short life spans (i.e., annuals) that occurred outside riparian or wetland areas were most likely to develop herbicide resistance. Short life spans indicate short generation times enabling faster evolution for herbicide resistance. Persistent seedbanks may increase the survival of resistant genotypes within a population or may be co-selected as an alternate form of escape from control methods. Species occurring in riparian or wetland microsites may be a case of “avoidance” rather than resistance, as managers typically avoid applying herbicide in these areas. Currently, 47 of the noxious weed species analyzed in this study are herbicide resistant, and our models identified an additional 63 species with traits that are highly associated with herbicide resistance, potentially indicating species that are at risk of developing resistance under conducive conditions. Further data-driven analyses with more plant traits and species from around the world could help refine current risk assessment of herbicide-resistance development.

Productivity and grain size of coffee grown in different weed management systems

Intensive weed management is one of the most common practices in coffee cultivation areas. Consequently, some problems, such as soil degradation and the selection of herbicide resistant weed, have increased over time, but, if properly managed, weeds at coffee planting inter-rows can offer benefits of erosion control, nutrient recycling and crop sustainability. The aim of this study is to evaluate the effect of different weed management strategies on the productivity and coffee grain size, i.e., quality. The experiment is installed onto a resprouting Coffea arabica L. site, four years after it was established. Treatments are implanted at planting inter-row Urochloa ruziziensis, Pueraria phaseoloides, and spontaneous vegetation maintained by mowing, herbicides, and weeding. To measure dry matter accumulation, samples are taken with a 0.25 m2 square template at plots maintained by mowing and herbicide application. To evaluate the yield and granulometry, coffee fruits are harvested, processed and classified in a set of 14 sieves (grouped in flat or “moca” shapes). The methods of controlling herbicide and weeding show significance in relation to grain production, with the production of grains having a higher market value standing out, when compared with the other treatments. The accumulation of dry matter above soil, in treatments with herbicides and spontaneous vegetation positively influenced the early coffee productivity (2018), and with U. ruziziensis and spontaneous vegetation, positively influenced the productivity of late harvest (2019). The accumulation of dry matter on the soil tends to be positively linked to coffee productivity, especially in periods when there is a shortage of rain in the region under study; however, it cannot be stated that this influence relationship (causality) has a direct positive effect between dry matter mass production and productivity of future coffee plantations.

Bio-efficacy of Herbicides with Different Water Volumes and Spray Timing under Zero Tillage Rice Residue Retention Scenario against Phalaris minor in Wheat

Background: Continuous use of the similar modes of action-based herbicides leads to the development of herbicide resistance in wheat-associated weeds in north-west India. Accelerated development of multiple resistance against most of the available post-emergence herbicides emphasize the use of pre-emergence herbicides. But, the efficacy of pre-emergence herbicide is a matter of concern as surface retained stubbles and/or straw alter herbicide efficacy associated with a direct interception of herbicide.
 Methods: A field study was conducted for two rabi seasons (2018-19 and 2019-20) to optimize spray volume and time of application for improving the bio-efficacy of pre-emergence herbicides under zero tillage full rice residue scenario. The herbicidal treatments (g/ha) included pendimethalin + pyroxasulfone (1000.0+127.5) as pre-emergence (PE), pyroxasulfone (127.5) as PE, pyroxasulfone (127.5) as early post emergence (EPoE), aclonifen+diflufenican (1002+200) EPoE, halauxifen methyl+ fluroxypyr (7.3+233.4) as EPoE, flumioxazin (100) as PE with two different water volumes 500 and 1000 liter/ha along with pendimethalin fb pinoxaden (1500+50) in 500 l/ha water volume, pendimethalin (1500) as PE in 375 l/ha water volume, weed free and weedy check.
 Results: The present study showed that pendimethalin + pyroxasulfone as PE with a higher spray volume of 1000 l/ha and alone application of pyroxasulfone as EPoE produced similar Phalaris minor control as with pendimethalin fb pinoxaden (PE fb Post) under full rice residue scenario in zero-till wheat.

Inhibition of acyl-ACP thioesterase as site of action of the commercial herbicides cumyluron, oxaziclomefone, bromobutide, methyldymron and tebutam.

Understanding the mode and site of action of a herbicide is key for its efficient development, the evaluation of its toxicological risk, efficient weed control and resistance management. Recently, the mode of action (MoA) of the herbicide cinmethylin was identified in lipid biosynthesis with acyl-ACP thioesterase (FAT) as the site of action (SoA). Cinmethylin was registered for selective use in cereal crops for the control of grass weeds in 2020. Here, we present a high-resolution co-crystal structure of FAT in complex with cumyluron identified by a high throughput crystallization screen. We show binding to and inhibition of FAT by cumyluron. Furthermore, in an array of experiments consisting of FAT binding assays, FAT inhibition assays, physiological and metabolic profiling, we tested compounds that are structurally related to cumyluron and identified the commercial herbicides oxaziclomefone, methyldymron, tebutam and bromobutide, with so far unknown sites of action, as FAT inhibitors. Additionally, we show that the previously described FAT inhibitors cinmethylin and methiozolin bind to FAT in a nanomolar range, inhibit FAT enzymatic activity and lead to similar metabolic changes. Based on presented data, we corroborate cinmethylin and methiozolin as potent FAT inhibitors and identify FAT as the SoA of the herbicides cumyluron, oxaziclomefone, bromobutide, methyldymron and tebutam. © 2022 Society of Chemical Industry.

A non‐chemical weed control strategy, introducing duckweed into the paddy field

Herbicide resistance in weeds and environmental pollution resulting from excessive application of chemical herbicides keeps increasing. Development of environment-friendly and effective weed management strategies are required for sustainable agricultural production. In this study we investigated the effects of duckweeds (Landoltia punctata (G. Meyer) Les & D. J. Crawford and Spirodela polyrhiza (Linnaeus) Schle iden) introduction on the weed community and rice growth in paddy fields. The study was conducted in the two rice-growing seasons (2018 and 2019) with three treatments: rice grown without duckweed introduction (CK), with L. punctata introduction (LP), and with S. polyrhiza introduction (SP). On average, LP and SP significantly reduced total weed density by more than 90% and 97%, respectively. Early in the rice-growing season, both duckweed species completely prevented weed growth. Further, both species significantly promoted rice plant growth in the advanced stages. SP significantly improved grain yield of rice by 23%. Light transmittance, temperature of the floodwater and soil, floodwater pH, and dissolved oxygen content significantly decreased following introduction of the duckweeds, indicating that the duckweeds introduction might inhibit weeds growth by altering environmental factors. This study provides a possible environment-friendly way to inhibit weed biomass in the paddy field by introducing duckweeds and interpreted the possible reasons of the impacts of duckweed on environmental variables. Weed control is beneficial for rice growth. Duckweed coverage might be limited in open fields and the associated practice requires additional investigation. This article is protected by copyright. All rights reserved.

Identification of acetolactate synthase resistant Amaranthus retroflexus in Ukraine

The problem of weed resistance to herbicides has become very important in the last decade and threatens to dramatically reduce the productivity and profitability of modern crop production. Herbicides – ALS inhibitors dominate among current herbicides and are used annually on large areas of sunflower, wheat, corn, soybean, and rapeseed. Also, in recent years, Clearfield seeds of sunflower, corn, canola, soybean and wheat have been sown in large areas. In recent years, there has been a sharp decrease in Amaranthus retroflexus L. control levels by imidazolinone class herbicides. Thus, the effects of herbicides with different modes of action on the development of A. retroflexus on sunflower after imidazolinone application were investigated in field research. In the conditions of the Cherkasy region of Ukraine, the biotype A. retroflexus was identified, which is resistant to the post-emergence application of herbicides - acetolactate synthase (ALS) inhibitors of the imidazolinone class – imazapyr and imazamox. Weed plants treated with imidazolinone derivatives in the maximum doses registered in Ukraine did not differ from untreated control plants. Also, in the conditions of field experiments, cross resistance of the weed biotype to herbicides – ALS inhibitors of the sulfonylurea class – foramsulfuron and iodosulfuron-methyl-sodium, thifensulfuron-methyl, tribenuron-methyl, nicosulfuron was established; and also, to the triazolinone derivative – thiencarbazone-methyl; to triazolpyrimidine derivatives – florasulam and flumetsulam. Multiple resistance of the A. retroflexus biotype to herbicides of the classes of glycine derivatives – glyphosate, phenoxycarboxylates – 2,4-D, benzoic acid – dicamba has not been established; compositions of dicamba with triketone – topramesone; diphenyl ethers – aclonifen; pyridine carboxylates – clopyralid, picloram and aminopyralid. It was shown for the first time that herbicide compositions with selected nutrients (ammonium pool) can increase the level of effectiveness of controlling resistant weed biotypes. Thus, the addition of ammonium sulfate increases the effectiveness of controlling ALS-resistant A. retroflexus with herbicides – a derivative of benzoic acid (dianate) and a derivative of benzoic acid with a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor (stellar – dicamba + topramesone). Thus, the A. retroflexus biotype resistant to ALS-herbicides of the imidazolinone class was identified for the first time in Ukraine, which is cross-resistant to other ALS-inhibitors of the sulfonylureas, triazolinones, and triazolpyrimidine classes. Multiple resistance of A. retroflexus to herbicides of the classes of glycine derivatives – glyphosate; phenoxycarboxylates – 2,4-D; benzoic acid – dicamba, triketones – topramesone; diphenyl ethers – aclonifen; pyridine carboxylates – clopyralid, picloram and aminopyralid has not been established. The identification of a highly harmful weed species resistant to widely used herbicides – ALS inhibitors in the central part of the "grain belt" of Ukraine requires a significant revision of the principles of crop rotation formation and ways of controlling weeds in the country in order to maintain high levels of profitability and productivity of agrophytocenoses.

Crop Rotations in California Rice Systems: Assessment of Barriers and Opportunities

Flooded rice soils are unique in terms of maintaining soil fertility and long-term productivity, allowing continuous rice systems to contribute greatly to global food supply. Yet increasing herbicide resistant weed pressure, water scarcity, and other sustainability challenges suggest a need to explore options for cropping system diversification. However, little research has evaluated the current obstacles limiting diversification of rice systems in different contexts. During summer and fall of 2020 we interviewed 42 rice growers to (i) assess the perceived benefits and challenges of crop rotation in the context of California rice systems and (ii) identify the factors influencing decision-making and barriers to adoption. Rice-based cropping systems ranged from high to low diversity across three different categories of growers (conventional rotations > organic > continuous rice). Key factors influencing the feasibility of rotations were soil limitations, production costs and productivity level of alternative crops, water and equipment requirements, market access, and regional differences. Generally, growers agreed that weed control and reduced reliance on herbicides were benefits of rotation. Similarly, growers who rotated described soil health as a primary benefit that decreases the need for fertilizer and pesticide inputs. However, there were many challenges to implementing rotations including heavy clay soils with poor drainage, lack of available contracts and markets for other crops, financial barriers such as land ownership and farm infrastructure (size of operation and available labor and equipment), and limited experience and knowledge of other viable crops. In terms of economic feasibility, those who only grow rice believed that other crops are less profitable, while those who rotate said that rotations increased profitability. Our research indicates that soil conditions are an important limitation, but other economic, social, and cultural barriers also strongly influence the potential for the diversification of rice systems.

Alkaline Soil Degradation and Crop Safety of 5-Substituted Chlorsulfuron Derivatives.

Sulfonylurea herbicides can lead to serious weed resistance due to their long degradation times and large-scale applications. This is especially true for chlorsulfuron, a widely used acetolactate synthase inhibitor used around the world. Its persistence in soil often affects the growth of crop seedlings in the following crop rotation, and leads to serious environmental pollution all over the world. Our research goal is to obtain chlorsulfuron-derived herbicides with high herbicidal activities, fast degradation times, as well as good crop safety. On account of the slow natural degradation of chlorsulfuron in alkaline soil, based on the previously reported results in acidic soil, the degradation behaviours of 5-substituted chlorsulfuron analogues (L101–L107) were investigated in a soil with pH 8.39. The experimental data indicated that 5-substituted chlorsulfuron compounds could accelerate degradation rates in alkaline soil, and thus, highlighted the potential for rational controllable degradation in soil. The degradation rates of these chlorsulfuron derivatives were accelerated by 1.84–77.22-fold, compared to chlorsulfuron, and exhibited excellent crop safety in wheat and corn (through pre-emergence treatment). In combination with bioassay activities, acidic and alkaline soil degradation, and crop safety, it was concluded that compounds L104 and L107, with ethyl or methyl groups, are potential green sulfonylurea herbicides for pre-emergence treatment on wheat and corn. This paper provides a reference for the further design of new sulfonylurea herbicides with high herbicidal activity, fast, controllable degradation rates, and high crop safety.