Resistant Weeds Research Articles

Bioherbicide effects of Trichoderma koningiopsis associated with commercial formulations of glyphosate in weeds and soybean plants

Given the environmental impacts and the need to control resistant weeds, there is a growing demand for alternative products that are less harmful to human health and the environment. In this sense, this work evaluated the effect of the bioherbicide produced based on Trichoderma koningiopsis as alternative weed control, which was associated with different doses of three formulations of the commercial herbicide glyphosate (isopropylamine salt, ammonium salt, and potassium salt), on the morpho-physiological characteristics of weeds of Euphorbia heterophylla L. and Brachiaria plantaginea (Link) Hitch.; and in the soybean crop (Glycine max L.). The experiment was carried out in vases with a completely randomized design, with five replications. The treatments consisted of keeping the plants in the presence and absence of the bioproduct (Trichoderma koningiopsis) in isolation and associated with 10, 20, 50, 70, 90, and 100% (m/v) of the herbicide dose recommended by the manufacturer. Plant performance was evaluated by morphological characteristics of height and weight of plants, length, and diameter of roots and stem, and visual phytotoxic effects. The herbicides Zapp Qi®, Roundup Original®, and Roundup WG® associated with T. koningiopsis controlled the E. heterophylla plant from the lowest concentration (10% m/v) that caused 80%–90% of plant control. In the control of B. plantaginea, the treatments were efficient (100%) from half the recommended dose concentration of the commercial products. In the soybean crop, no significant phytotoxic visual damage or interference in the morphological characteristics of plants was observed. The association of synthetic herbicides with the bioherbicide proved efficient in controlling weeds and promising for reducing doses of synthetic herbicides.

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.

Phytotoxicity of Rich Oxygenated Terpenes Essential Oil of Prosopis farcta against the Weed Dactyloctenium aegyptium

Weeds are considered the main reason for crop yield loss in the world. Weed control and management include various treatments such as cultural, physical, chemical, and biological methods. Chemical control of weeds is the most common method; however, the application of commercial synthetic herbicides caused several dangerous hazards in the environment including the appearance of resistant weed biotypes. Prosopis farcta (Banks & Sol.) J.F.Macbr. (Family: Fabaceae), is a common weed plant in the Middle East, where it is hard to eliminate due to its deep and overlapped roots. On the other side, it has many traditional uses around the world. Herein, the essential oil (EO) of P. farcta above-ground parts was extracted via hydrodistillation techniques and then analyzed using gas chromatography-mass spectroscopy (GC-MS). From the GC-MS analysis, 47 compounds were identified with a relative concentration of 98.02%, including terpenes as the main components (95.08%). From overall identified compounds, cubenol (19.07%), trans-chrysanthenyl acetate (17.69%), torreyol (8.28%), davana ether (3.50%), camphor (3.35%), and farnesyl acetone (3.13%) represented the abundant constituents. Furthermore, the phytotoxic activity of the P. farcta EO was assessed against the weed Dactyloctenium aegyptium (L.) Willd. The EO of P. farcta, at a concentration of 100 µL L−1, significantly inhibited the germination, seedling shoot growth, and seedling root growth by 64.1, 64.0, and 73.4%, respectively. The results exhibited that the seedling root growth is the most affected followed by the seed germination and seedling shoot growth with respective IC50 at 64.5, 80.5, and 92.9 µL L−1. It can be concluded that weeds are not absolutely harmful, but they may have beneficial uses, such as, for example as a source of phytochemicals with application in weed control practices (bioherbicides). It is advised to conduct additional research to characterize the allelopathic action of the major chemicals in their pure form, either alone or in combination, against a variety of weeds.

Herbicide Resistance: Managing Weeds in a Changing World

Over the years, several agricultural interventions and technologies have contributed immensely towards intensifying food production globally. The introduction of herbicides provided a revolutionary tool for managing the difficult task of weed control contributing significantly towards global food security and human survival. However, in recent times, the successes achieved with chemical weed control have taken a turn, threatening the very existence we have tried to protect. The side effects of conventional farming, particularly the increasing cases of herbicide resistance agricultural weeds, is quite alarming. Global calls for sustainable weed management approaches to be used in food production is mounting. This paper provides detailed information on the molecular biological background of herbicide resistant weed biotypes and highlights the alternative, non-chemical weed management methods which can be used to prevent the development and spreading of herbicide-resistant weeds.

Comparative Efficacy of Different Herbicidal Combinations on Weed Growth and Yield Attributes of Wheat

Weeds are the main biotic obstacles on wheat production, which can impair wheat productivity by up to 60%, if not handled under critical stages of crop life cycle. Chemical weed control through herbicides has been the most popular and effective method among farmers. The weed management using similar herbicides however has led to herbicide resistance in weeds. This requires the evaluation of newer herbicidal combinations for the control of weeds in wheat. Therefore, an experiment was conducted at Research Farm, Department of Agronomy, JNKVV, Jabalpur, Madhya Pradesh during the Rabi season of 2016-17 to evaluate the effect of post emergence application of herbicides on the weeds and yield of wheat. The experiment was laid out in randomized block design with ten treatments comprising of eight herbicidal combinations along with a hand weeding and a weed check and replicated thrice. Observations on different weed parameters, growth parameters and yield of wheat were recorded. Among the different herbicidal combinations, the post emergence application of halauxifen-methyl ester + florasulam at 10.20 g/ha resulted in greatest suppression of weeds and had highest weed control efficiency (56.73%). It also led to highest growth in wheat which resulted in highest grain yield (5.81 t/ha) and harvest index (44.99%). From this study, it can be concluded that the post emergence application of halauxifen-methyl ester + florasulam at 10.20 g/ha can not only control weeds effectively but also increase the yield of wheat.

Synergistic test of atrazine 72% and mesotrion 8% herbicide against broadleaf weeds and grass weeds in corn (Zea mays L.) cultivation

Weed control employing single pesticide continually in the long-term causes weeds to become resistant. The usage of combined pesticides helps limit the occurrence of weed resistance. Herbicide formulations containing two or more active ingredients will exhibit interactions between constituents which could be synergistic, additive or antagonistic. The purpose of this study was to determine the effectiveness and synergism test of the mixture with the active ingredients atrazine 72%+mesotrion 8% against several types of weeds in sweet corn plantations. The experiment was carried out during 2022 at the Agricultural Training and Development Research Center, Padjadjaran University, Bandung, Indonesia. The treatment consisted of three types of herbicides with six dose levels, there was mixture of atrazine 72%+mesotrion 8%, herbicide atrazine 72% and herbicide mesotrion 8% with four replications. Target weeds included Ageratum conyzoides, Borreria alata, Synedrella nodiflora, Richardia brassiliensis, Eleusine indica, Digitaria ciliaris and Paspalum conjugatum. Data analysis used the percentage of weed damage to determine the effectiveness of each herbicide and linear regression analysis using the Multiplicative Survival Model (MSM) method to determine the LD50 of each herbicide. The results showed that the mixed herbicides atrazine 72%+mesotrion 8% suppressed the growth of the target weeds and had a treatment LD50 of 0.32 l/ha and an expected LD50 of 0.36 l/ha with a co-toxicity value of 1.125 ( greater than 1), so the mixed herbicides were synergistic.

Bioisosteric Tactics in the Discovery of Tetflupyrolimet: A New Mode-of-Action Herbicide.

The last new herbicidal modes-of-action with commercial significance were introduced to the marketplace multiple decades ago. Serious weed resistance to most herbicidal classes have since emerged with widespread use. Aryl pyrrolidinone anilides represent an entirely new mode-of-action class of herbicides that interfere with de novo pyrimidine biosynthesis in plants via inhibition of dihydroorotate dehydrogenase. The chemical lead for this new herbicide class discovery was identified from high-volume sourced greenhouse screening that required structural reassignment of the hit molecule followed by an extensive synthetic optimization effort. With excellent grass weed control and pronounced safety on rice, the selected commercial development candidate has a proposed common name of tetflupyrolimet and represents the first member of the new HRAC (Herbicide Resistance Action Committee) Group 28. This paper describes the discovery path to tetflupyrolimet with an added focus on the bioisosteric modifications pursued in optimization, including replacements of the lactam core itself.

Designing New Protoporphyrinogen Oxidase-Inhibitors Carrying Potential Side Chain Isosteres to Enhance Crop Safety and Spectrum of Activity.

There are several methods to control weeds, which impose particular challenges for farmers in all parts of the world, although applying small molecular compounds still remains the most efficient technology to date. However, plants can evolve to become resistant toward active ingredients which is also the case for protoporphyrinogen oxidase (PPO) inhibitors, a class of highly effective herbicides in use for more than 50 years. Hence, it is essential to continuously discover and develop new herbicidal PPO inhibitors with enhanced intrinsic activity, an improved resistance profile, enhanced crop safety, favorable physicochemical properties, and a clean toxicological profile. By modifying structural key features from known PPO inhibitors such as tiafenacil, inspired by isostere and mix&match concepts in combination with modeling investigations based on a wild-type Amaranthus crystal structure, we have found new promising lead structures showing strong activity in vitro and in vivo against several notorious dicotyledon and monocotyledon weeds with emerging resistance (e.g., Amaranthus palmeri, Amaranthus tuberculatus, Lolium rigidum, and Alopecurus myosuroides). While several phenyl uracils carrying an isoxazoline motif in their thio-linked side chain showed promising resistance-breaking potential against different Amaranthus species, introducing a thioacrylamide side chain afforded outstanding efficacy against resistant grass weeds.

The social foundations for re-solving herbicide resistance in Canterbury, New Zealand.

Synthetic herbicides have revolutionised agricultural weed control. Herbicide resistance (HR) is a natural process through which weeds evolve to be no longer susceptible to a herbicide. Repeated use of similar herbicides can lead to the proliferation of resistant weed populations, with detrimental on-farm effects. To date, 267 weed species worldwide are resistant to at least one herbicide. Yet, achieving universal uptake of best practice principles to manage HR remains difficult. Historically not a high priority for New Zealand cropping farmers, resistance may be more prevalent than commonly assumed. This article contributes to emerging national management strategies and the international scholarship on the human dimensions of HR. Regarding resistance as a socio-biological challenge, we draw on qualitative social research with agricultural stakeholders in New Zealand's main cropping region to outline important psychosocial preconditions for effective resistance management. Our findings show that these preconditions include: influencing awareness and attitudes, knowledge and skills; approaching HR as a shared responsibility; and supporting long-term and holistic thinking. We conclude that these preconditions form the social foundations for agricultural stakeholders' capacity to enact best practice principles to continuously re-solve HR. This novel framing allows analytical differentiation between the capacity and ability to act, with practical recommendations and future research needing to address both components of effective HR management.

Weed resistance assessment through airborne multimodal data fusion and deep learning: A novel approach towards sustainable agriculture

Herbicide-resistant weeds represent a significant challenge to modern agriculture. The need for innovative and sustainable weed management strategies has become increasingly pressing as the threat of herbicide-resistant weeds continues to escalate. Unmanned aerial vehicles (UAVs) and various sensors have become indispensable tools in plant phenotyping studies. In this study, a comprehensive resistance score (CRS) was proposed to effectively quantify weed resistance in the field. Multimodal data fusion and deep learning were utilized to perform regression of CRS, three different fusion methods for 3D-CNN and 2D-CNN to extract and fuse multimodal information collected by UAVs including spectral, structural, and texture information for weed resistance. Our findings demonstrate that (1) discernible differences in spectral response exist between susceptible and resistant weeds, with the optimal band for the Successive Projections Algorithm (SPA) selection coinciding with the optimal band for resistance expression band; (2) resistance assessment accuracy is enhanced through multimodal data fusion, with the late deep fusion network exhibiting the best accuracy, R2 of 0.777 and RMSE of 0.547; (3) the multimodal fusion network model displays robust adaptability in resistance assessment across varying densities and effectively generates weed resistance map. Overall, this research demonstrates the effectiveness of using multimodal data fusion and CRS, combined with deep learning for achieving accurate and reliable weed resistance assessment in agricultural fields.

Evaluation of different weed management strategies including non‐chemical tools and cinmethylin for control of multiple herbicide‐resistant Alopecurus myosuroides

AbstractThe development of resistance in Alopecurus myosuroides populations to different herbicide active ingredients in Europe exacerbates efforts toward its chemical control. In Europe, cinmethylin provides an additional mode of action for the control of A. myosuroides. The objective of this study was to evaluate three resistance weed management (RWM) strategies, including cinmethylin and other pre‐emergence herbicides, for the control of multiple herbicide–resistant A. myosuroides. The RWM strategies used in this study differed in the extent of preventive cultural practices (tillage, crop rotation, sowing date and stale seedbed). Two field trials were conducted at sites with ACCase‐ and ALS‐resistant A. myosuroides biotypes in Germany between 2017 and 2020. Cinmethylin was included in the herbicide regime for A. myosuroides control. The combination of initial inversion tillage, delayed sowing, crop rotation diversification, and stale seedbed reduced the density of A. myosuroides bup to 90% compared to a strategy with pre‐emergence herbicides used as main components in control. At both sites, the amount of viable A. myosuroides seeds in the soil seed bank was reduced by over 90% after a 3‐year trial period in plots with initial inversion tillage. Molecular analyses revealed that the omission of ACCase and ALS inhibitors did not reduce the proportion of resistant A. myosuroides biotypes. Long‐term control of resistant A. myosuroides must be based on a combination of cultural practices that reduce the soil seed bank and suppress the occurrence of A. myosuroides and the use of still‐effective active ingredients for preventing the renewed entry of seeds into the soil.

Transcriptionally linked simultaneous overexpression of P450 genes for broad-spectrum herbicide resistance.

Broad-spectrum herbicide resistance (BSHR), often linked to weeds with metabolism-based herbicide resistance, poses a threat to food production. Past studies have revealed that overexpression of catalytically-promiscuous enzymes explains BSHR in some weeds; however, the mechanism of BSHR expression remains poorly understood. Here, we investigated the molecular basis of high-level resistance to diclofop-methyl in BSHR late watergrass (Echinochloa phyllopogon) found in the US, which cannot be solely explained by the overexpression of promiscuous cytochrome P450 monooxygenases CYP81A12/21. The BSHR late watergrass line rapidly produced two distinct hydroxylated-diclofop-acids, only one of which was the major metabolite produced by CYP81A12/21. RNA-seq and subsequent RT-qPCR-based segregation screening identified the transcriptionally-linked overexpression of a gene, CYP709C69, with CYP81A12/21 in the BSHR line. The gene conferred diclofop-methyl resistance in plants and produced another hydroxylated-diclofop-acid in yeast (Saccharomyces cerevisiae). Unlike CYP81A12/21, CYP709C69 showed no other herbicide-metabolizing function except for a presumed clomazone-activating function. The overexpression of the three herbicide-metabolizing genes was also identified in another BSHR late watergrass in Japan, suggesting a convergence of BSHR evolution at the molecular level. Synteny analysis of the P450 genes implied that they are located at mutually independent loci, which supports the idea that a single trans-element regulates the three genes. We propose that transcriptionally-linked simultaneous overexpression of herbicide-metabolizing genes enhances and broadens the metabolic resistance in weeds. The convergence of the complex mechanism in BSHR late watergrass from two countries suggests that BSHR evolved through co-opting a conserved gene-regulatory system in late watergrass.

Vineyard floor management intensity impacts soil health indicators and plant diversity across South Australian viticultural landscapes

Throughout the last thirty years, major shifts in vineyard floor management have been observed. Challenges initially posed by intensive tillage included high rates of soil erosion and the degeneration of soil structure and soil organic matter, which lead viticulturists to depend more heavily on herbicide use as an effective weed control strategy. However, an increase in herbicide persistence and toxicity in water, soils, and grapevines, increasing resistance of common weeds; and pressure from consumers and regulators to reduce their use is directing a shift towards an overall reduction in herbicide usage. This has led to more frequent tillage to manage vegetation in vineyards, while in some instances, cultural practices including slashing and animal grazing are used solely or in conjunction. However, little is known about the holistic effects of these varying practices on vineyard soils and biodiversity across landscapes in Australia. Thus, to comparatively assess the environmental impacts of different floor management practices, soil health indicators and plant dynamics were seasonally measured in the mid- and under-vine rows at twenty-four vineyard sites and four native sites in the Barossa Valley, Eden Valley, and McLaren Vale, all located in South Australia, where different intensities of floor management were implemented. Vineyard sites were categorised based on the frequency of herbicide and/or tillage passes particularly in the under-vine area into Low (no annual management passes), Medium (one annual management pass), and High (two to four annual management passes) intensity groups. Findings revealed similarities in the vineyard mid-rows across the management intensities, yet the under-vine rows displayed many differences; in particular, there were more plant species, higher plant coverage, and greater plant biomass in the Low management intensity group. Furthermore, as management intensity decreased, the relative richness of ruderal plant species also decreased, giving way to a plant community mainly comprised of slow-growing, perennial Poaceae and Fabaceae species in the Low-intensity management group. These differences in plant dynamics drove a suite of soil responses including faster water infiltration, higher soil ammonium-N and total nitrogen, and a tendency of higher soil gravimetric water content at the time of sampling. These results suggest that after an initial period of establishing these more extensive vineyard floor management practices, low levels of soil disturbance in the under-vine rows may contribute positively to improving natural ecosystem synergy and functionality between soil and plants. Therefore, our findings lend insights into how the varying intensity of floor management practices, rather than differing management ideologies per se, across a viticultural landscape can be intrinsic supporters of agroecosystem resilience under South Australian conditions.

Extrachromosomal circular DNA-mediated spread of herbicide resistance in interspecific hybrids of pigweed.

Extrachromosomal circular DNAs (eccDNAs) are found in many eukaryotic organisms. EccDNA-powered copy number variation plays diverse roles, from oncogenesis in humans to herbicide resistance in crop weeds. Here, we report interspecific eccDNA flow and its dynamic behavior in soma cells of natural populations and F1 hybrids of Amaranthus sp. The glyphosate-resistance (GR) trait is controlled by eccDNA-based amplification harboring the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene (eccDNA replicon), the molecular target of glyphosate. We documented pollen-mediated transfer of eccDNA in experimental hybrids between glyphosate-susceptible Amaranthus tuberculatus and GR Amaranthus palmeri. Experimental hybridization and fluorescence in situ hybridization (FISH) analysis revealed that the eccDNA replicon in Amaranthus spinosus derived from GR A. palmeri by natural hybridization. FISH analysis also revealed random chromosome anchoring and massive eccDNA replicon copy number variation in soma cells of weedy hybrids. The results suggest that eccDNAs are inheritable across compatible species, contributing to genome plasticity and rapid adaptive evolution.

Genetically engineered varieties and applied pesticide toxicity in U.S. maize and soybeans: Heterogeneous and evolving impacts

The extensive adoption of genetically engineered (GE) varieties in U.S. agriculture has dramatically changed the patterns of pesticide use. How this process ultimately affects environmental risk remains an open question. Previous studies have typically relied on aggregate trends to infer the impact of GE crop adoption on pesticide use, which fails to address selection bias and unobserved heterogeneity. We overcome this limitation by analyzing applied pesticide toxicity using farm-level fixed effects models, estimated with rich plot-level data on >200,000 seed and pesticide choices by U.S. maize and soybean farmers during the 1998–2016 period. We find that applied toxicity was, on average, lowered by the adoption of GE varieties across four target organism groups: mammals, birds, fish, and honey bees. However, most of the toxicity benefits conferred by GE adoption dissipated over time. For herbicide tolerant varieties, this was due to the increased use of old-line herbicides by GE adopters, a likely consequence of the growing problem of glyphosate weed resistance. Applied honey bee toxicity saw the sharpest increase during the GE era, but most of this increase was driven by the adoption of neonicotinoid seed treatments, rather than GE insect resistant traits.

Discovery of tetrazolium nicotinamide derivatives as novel 4-Hydroxyphenylpyruvate dioxygenase inhibiting-based herbicides

4-Hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27, HPPD) is currently one of the popular targets of herbicide research, and herbicides targeting it have shown promising results in treating resistant weeds. However, the long-term application of HPPD inhibitors with in their classical scaffolds inevitably leads to the development of drug resistance. To further delay the underlying development of weed resistance to HPPD inhibitors, we successfully screened HPPD inhibitor pharmacophore with novel chelate structures using computer-aided drug molecular design and obtained 2-trifluoromethyltetrazolium nicotinamide derivatives through active fragment splicing. Among these derivatives, 6-(3,3-diethyl-1-methylureido)-N-(1-methyl-1H-tetrazol-5-yl)-2-(trifluoromethyl)nicotinamide (compound 31), exhibited the most promising in vitro enzyme inhibitory activity, with IC50 value of 0.072 μM, which was approximately five times better than that of the control agent mesotrione (IC50 = 0.363 μM). The crystal structure of the AtHPPD complexed with compound 31 (2.0 Å) and the binding energy calculations of the representative compounds revealed several important interactions of the ligand binding to the target protein, which explained the superior enzyme inhibitory activity of the compounds at the molecular level. In addition, compounds 7 and 31 possessed 100% inhibition against the five target weeds at the tested dosage and both were more effective against Setaria viridis than mesotrione. In general, it is promising to design novel HPPD inhibitors by developing novel chelating structures, and compounds 7 or 31 could be used as the leads for further development of valuable HPPD inhibitors.

Evolution of Herbicide Resistant Weeds in Agro-ecological Systems

Weed management through herbicides continues to be the most efficient and cost-effective component of integrated weed management (IWM) in crop production systems. However, the current production systems are characterised by intensive, inappropriate use of herbicides along with misapplication which creates selection pressure and leads to the rapid evolution of herbicide resistance in weeds. Currently, there are 511 unique cases of herbicide-resistant weed species globally, involving 266 species and these comprise 153 and 113 dicots and monocots respectively. To date, herbicide resistance has been reported from 96 crops in more than 71 countries. Furthermore, weeds have evolved resistance to 21 of the 31 known sites of herbicide action in different crops. Understanding the mechanisms of herbicide resistance is therefore essential which when coupled with the ecological and management factors that affect herbicide resistance would lead to the development of appropriate, profitable, and sustainable weed management strategies. The review is aimed at the development of herbicide resistance, mechanisms of herbicide resistance, factors that influence the rate of resistance development and management of herbicide resistance by growers.

The phytotoxicity of soil-applied herbicides is enhanced in the first-year post strategic deep tillage

The sandplain soils of WA are inherently fragile with surface layers that are very low in organic matter and clay content. The advent of minimum- and no-till farming has seen the increase in frequency and intensity of cropping on these soils. However, a combination of soil physio-chemical constraints and agronomic issues remain a challenge to the sustainability of cropping systems on them. These constraints include sub-soil compaction, soil water repellence, sub soil acidity and herbicide resistant weeds. Strategic deep tillage such, as soil inversion and deep soil mixing, have been shown to ameliorate these multiple constraints and dramatically increase crop production. For WA soils, an increase in herbicide usage is correlated with a decrease in regular tillage, and how the two interact is imperfectly understood. As a result, current herbicide strategies and rates are designed to perform optimally in a minimum tillage environment. Two field experiments were established to compare crop damage from a range of commonly used pre-emergent herbicides when grown in soil that remained under minimum tillage, was deep mixed or inverted. These trials demonstrated that both strategic tillage methods significantly changed the soil surface composition that would be expected to directly affect the bioavailability of some herbicides. Two commonly used herbicides, Metribuzin and Diuron, detrimentally impacted crop performance following tillage in both trials. These same treatments reduced yield by a greater extent on both the soil inversion and deep mixing treatments (p < 0.001). No other herbicides, when applied at either label or triple label rates, significantly impacted yield on any of the soil treatments. There was a substantial crop production benefit from strategic deep tillage at Esperance but not at Geraldton. These results reflect that the influence of deep tillage on the toxicity of herbicides is highly dependent upon soil properties and rainfall.

Target and nontarget mechanisms of AHAS inhibitor cross-resistance patterns in Cyperus difformis

Weed resistance to acetohydroxyacid synthase (AHAS) inhibiting herbicides has been a critical issue for rice growers worldwide since the early 1990's. In California, resistance to bensulfuron-methyl was first detected in Cyperus difformis in 1993. Since then, populations of most major weeds of rice in California have been reported to show resistance to at least one AHAS inhibitor. We sought to describe the magnitude and mechanisms of AHAS inhibitor cross-resistance in California populations of C. difformis. Sixty-two populations were collected and screened for cross-resistance to bensulfuron-methyl (BEN), halosulfuron-methyl (HAL), bispyribac‑sodium (BIS), and penoxsulam (PEN), revealing six major patterns of cross-resistance. Representative C. difformis populations from each cross-resistance pattern were then subjected to dose-response, cytochrome P450 inhibition, AHAS gene sequencing, and metabolic studies with the same herbicides as in the screening. Dose-response confirmed the detected resistances in the representative populations, and suggested that the majority of observed resistance was dose-dependent. Cytochrome P450 inhibition via malathion revealed evidence of increased metabolic activity in resistant populations to BEN, BIS, and PEN. AHAS gene sequencing revealed amino acid substitutions in five of six populations: R3 (Pro197-Ser), R4 (Pro97-His), R10 (Asp376), R41 (Ala122-Asn), and R18 (Trp574-Leu). Metabolic studies confirmed evidence of increased activity of cytochrome P450s in all populations. Metabolic BEN and HAL analysis did not yield similar results to malathion inhibition, suggesting different P450's or other pathways. Taken together, the results of the studies confirm the complexity of AHAS inhibitor cross-resistance in C. difformis, and the presence of both target-site and metabolic resistance in most of the representative populations underscores the importance of proper herbicide selection, rotation, and scouting in fields.

Molecular Basis of Resistance to Bensulfuron-Methyl in a Smallflower Umbrella Sedge (Cyperus difformis L.) Population from China

Smallflower umbrella sedge (Cyperus difformis L.) is an invasive weed, and infestations of C. difformis are increasing in rice (Oryza sativa L.) fields in China. Bensulfuron-methyl is a widely used sulfonylurea herbicide that inhibits the acetolactate synthase (ALS) enzyme and has been used in recent years for effectively controlling annual weeds in the Cyperaceae family. In this study, a suspected resistant population of C. difformis (BBHY1) was collected from a rice field in Huaiyuan County, Anhui Province, China, that survived treatment with bensulfuron-methyl at the field-recommended rate (FRR). Single-dose tests and whole-plant bioassays confirmed that the BBHY1 population was resistant to bensulfuron-methyl and had evolved a high level of resistance, with a resistance index (RI) of 12.87. Sequencing of the ALS gene revealed a CCT to CAT point mutation at codon 197, which caused a P-to-H substitution in the resistant plants. Analysis of the relative expression of ALS revealed no significant differences between the resistant and susceptible populations. Inhibiting the activity of cytochrome P450s (P450s) or glutathione S-transferases (GSTs) had no significant effect on bensulfuron-methyl resistance. The BBHY1 population exhibited cross-resistance to pyrazosulfuron-ethyl, penoxsulam, and bispyribac-sodium, with RIs ranging from 5.48 to 20.63, but remained susceptible to MCPA sodium, florpyrauxifen-benzyl, and bentazon, with RIs of &lt;1.00. These herbicides could be potentially used as alternatives for controlling resistant populations and managing herbicide resistance in other aggressive weeds in rice fields.