Evolution Of Herbicide Resistance Research Articles

Herbicide-resistant weeds in turfgrass: current status and emerging threats

AbstractHerbicide-resistant weeds pose a severe threat to sustainable vegetation management in various production systems worldwide. The majority of the herbicide resistance cases reported thus far originate from agronomic production systems where herbicide use is intensive, especially in industrialized countries. Another notable sector with heavy reliance on herbicides for weed control is managed turfgrass systems, particularly golf courses and athletic fields. Intensive use of herbicides, coupled with a lack of tillage and other mechanical tools that are options in agronomic systems, increases the risk of herbicide-resistant weeds evolving in managed turfgrass systems. Among the notable weed species at high risk for evolving resistance under managed turf systems in the United States are annual bluegrass, goosegrass, and crabgrasses. The evolution and spread of multiple herbicide resistance, an emerging threat facing the turfgrass industry, should be addressed with the use of diversified management tools. Target-site resistance has been reported commonly as a mechanism of resistance for many herbicide groups, though non–target site resistance is an emerging concern. Despite the anecdotal evidence of the mounting weed resistance issues in managed turf systems, the lack of systematic and periodic surveys at regional and national scales means that confirmed reports are very limited and sparse. Furthermore, currently available information is widely scattered in the literature. This review provides a concise summary of the current status of herbicide-resistant weeds in managed turfgrass systems in the United States and highlights key emerging threats.

Response of glyphosate-resistant and susceptible biotypes of Echinochloa colona to low doses of glyphosate in different soil moisture conditions.

To evaluate the hormetic effect of glyphosate on Echinochloa colona, two pot studies were done in the screenhouse at the Gatton Campus, the University of Queensland, Australia. Glyphosate was sprayed at the 3–4 leaf stage using different doses [(0, 5, 10, 20, 40, 80 and 800 g a.e. ha-1) and (0, 2.5, 5, 10, 20 and 800 g a.e. ha-1)] in the first and second study, respectively. In the second study, two soil moistures (adequately-watered and water-stressed), and two E. colona biotypes, glyphosate-resistant and glyphosate-susceptible, were included. In both studies, plants that were treated with glyphosate at 2.5–40 g ha-1 grew taller and produced more leaves, tillers, inflorescences and seeds than the control treatment. In the first study, 5 g ha-1 glyphosate resulted in the maximum aboveground biomass (increase of 34% to 118%) compared with the control treatment. In the second study, the adequately-watered and glyphosate low dose treatments caused an increase in all the measured growth parameters for both biotypes. For example, total dry biomass was increased by 64% and 54% at 5 g ha-1 in the adequately-watered treatments for the resistant and susceptible biotypes, respectively, compared with the control treatment. All measured traits tended to decrease with increasing water stress and the stimulative growth of low doses of glyphosate could not compensate for the water stress effect. The results of both studies showed a hormetic effect of low doses of glyphosate on E. colona biotypes and such growth stimulation was significant in the range of 5 to 10 g ha-1 glyphosate. Water availability was found to be effective in modulating the stimulatory outcomes of glyphosate-induced hormesis. No significant difference was observed between the resistant and susceptible biotypes for hormesis phenomenon. The study showed the importance of precise herbicide application for suppressing weed growth and herbicide resistance evolution.

Adopting epidemiological approaches for herbicide resistance monitoring and management

AbstractThe widespread use and increasing reliance on herbicides for weed control has resulted in a global epidemic of evolved herbicide resistance in weed populations. In response, there has been a great deal of research effort to document resistance cases, understand the genetic and physiological mechanisms of resistance and use models and model organisms to explore resistance management strategies. Here, we argue that the field of epidemiology, which systematically studies the extent, distribution and determinants of a harmful organism or condition, can greatly contribute to our efforts to understand the emergence, selection and spread of herbicide resistance. By systematically collecting data on weed abundance and distribution, the frequency and mechanisms of resistance, and agronomic and environmental metadata, it is possible to develop statistical models that identify the underlying relationships between these elements. In doing so, these approaches can provide novel insight into the relative importance, origin and spread of different resistance mechanisms, and the agronomic, ecological and evolutionary drivers that dictate the dynamics of resistance evolution at local to global scales. Emerging technologies in weed surveillance, genomics and resistance diagnostics, statistics and data science will greatly facilitate the collection and analysis of large‐scale data sets, providing unprecedented potential for epidemiological analyses of the evolution of herbicide resistance at landscape scales.

StMADS11 Subfamily Gene PfMADS16 From Polypogon fugax Regulates Early Flowering and Seed Development.

The evolution of herbicide resistance in weedy plants leads to various adaptation traits including flowering time and seed germination. In our previous studies, we found an association of the early flowering phenotype with the ACCase inhibitor herbicide resistance genotype in a population of Polypogon fugax. MADS-box transcription factors are known to play pivotal roles in regulating plant flowering time. In this study, a SHORT VEGETATIVE PHASE (SVP)-like gene, belonging to the StMADS11 subfamily in the MADS-box family, was cloned from the early flowering P. fugax population (referred to as PfMADS16) and resistant to the herbicide clodinafop- propargyl. Overexpression of the SVP-like gene PfMADS16 in Arabidopsis thaliana resulted in early flowering and seed abortion. This is consistent with the phenotypic characters of resistant P. fugax plants, but contrary to the conventional role of SVP-like genes that usually suppress flowering. In addition, down regulation of the seed formation gene AtKTN1 in flowers of PfMADS16 transgenic Arabidopsis plants indicates that PfMADS16 may be indirectly associated with seed viability. Furthermore, one protein (PfMADS2) from the APETALA1 (AP1) subfamily interacting with PfMADS16 in P. fugax was identified with relevance to flowering time regulation. These results suggest that the PfMADS16 gene is an early flowering regulation gene associated with seed formation and viability in resistant P. fugax population. Our study provides potential application of PfMADS16 for integrated weed management (such as genetic-based weed control strategies) aiming to reduce the soil weed seedbank.

Incidence of multiple herbicide resistance in annual bluegrass (Poa annua) across southeastern Australia

AbstractAnnual bluegrass (Poa annua L.) is a problematic annual weed in established turf where the intensive use of herbicides has resulted in the evolution of herbicide resistance. In 2017, 31 populations of P. annua suspected to be resistant to herbicides commonly used to control this weed in turf were collected from golf courses across southeastern Australia to check the resistance status to different herbicide groups. All populations were found to be resistant to multiple turf herbicides. Dose–response experiments confirmed resistance to propyzamide, simazine, rimsulfuron, foramsulfuron, endothall, and pinoxaden. Levels of resistance to rimsulfuron (>56-fold), foramsulfuron (>19-fold), endothall (>7-fold), and pinoxaden (>4.3-fold) compared with the susceptible population were high, but levels of resistance to propyzamide (>2-fold) and simazine (>2-fold) were lower. Considerable variation in resistance to endothall and pinoxaden was observed among the populations of P. annua. Target-site resistance was confirmed for acetolactate synthase and acetyl-CoA carboxylase inhibitors, but not for photosystem II and microtubule assembly inhibitors. This study documented the extensive resistance to herbicides in P. annua from turf in Australia. Three of the populations investigated exhibited multiple resistance to herbicides from five mechanisms of action. The identification of multiple-resistant P. annua on several golf courses is a serious concern for turf managers.

Herbicide resistance in weeds and its management strategies

Weeds are much older than crop cultivation for food, fuel, fibre and shelter. Weed management is a big task for production of various crops. At present time, herbicides are most popular and effective means for weed management due to their higher efficacy and time saving nature. But their continuous and indiscriminate use has created several problems like weed flora shift, herbicide resistance and environmental hazards. Out of these, herbicide resistance is most serious problem of present time. Currently there are 480 cases of herbicide resistance in 252 plant species. There are various types of herbicide resistance such as cross resistance, multiple resistance, partial resistance, full resistance etc. Herbicide resistance has become a major issue at world level, and number of resistant biotypes is increasing continuously at very rapid rate. Therefore, some alternate management practices such as cultural, mechanical and biological methods should be integrated in weed management program to combat this problem. A proper understanding of development and mechanism of resistance (altered target sites, rapid metabolism and sequestration) would help us for its management. With the help of various physiological and biochemical studies, some resistant screening techniques should be developed for yielding quick results. This paper provides information on status of herbicide resistance, development of resistance, factors affecting development of herbicide resistance in weeds, resistance mechanisms and its management.

Low herbicide doses can change the responses of weeds to subsequent treatments in the next generation: metamitron exposed PSII-target-site resistant Chenopodium album as a case study.

It is well known that exposure to mild stress can precondition organisms to better tolerate subsequent stress exposure in the same or future generations. Since herbicide hormesis also represents a moderate stress to exposed plants, a transgenerational priming is likely but not proven. Especially in herbicide-resistant weeds showing enhanced reproductive fitness after regular herbicide treatments, the ability to induce resilient offspring phenotypes via hormesis may hasten the evolution of herbicide resistance in weeds. This hypothesis was studied for the triazinone metamitron in an F1 offspring generation of PSII target-site resistant (TSR) plants of Chenopodium album propagated after parental conditioning with various metamitron doses. In two independent dose-response greenhouse trials, there was a positive correlation between the strength of the stimulatory response during parental preconditioning and the magnitude of transgenerational changes in herbicide sensitivity and hormesis expression. Parental conditioning at subhormetic and toxic concentrations lead to less resilient offspring, while conditioning doses that induced a pronounced hormetic effect in F0 plants had a sensitivity-reducing and hormesis-promoting effect on the offspring. The observed reduction in sensitivity in F1 plants compared to unconditioned F1 plants was up to 2.2-fold. This study demonstrates that hormetic herbicide treatments have the ability to prime weeds for enhanced tolerance to subsequent treatments in the next generation. Effects proved dose sensitive and may act in concert with other stimulatory adaptations in plant populations. This is relevant for weed control and herbicide resistance evolution, but also for herbicide side-effects that go beyond the exposed area. © 2020 The Author. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Integrating cover crops for weed management in the semiarid U.S. Great Plains: opportunities and challenges

AbstractThe widespread evolution of herbicide resistance in weed populations has become an increasing concern for no-tillage (NT) growers in semiarid regions of the U.S. Great Plains. Lack of cost-effective and alternative new herbicide sites of action further exacerbates the problem of herbicide-resistant (HR) weeds and threatens the long-term sustainability of prevailing cropping systems in the region. A recent decline in commodity prices and increasing herbicide costs to manage HR weeds has spurred research efforts to build a strong rationale for developing ecologically based integrated weed management (IWM) strategies in the U.S. Great Plains. Integration of cover crops (CCs) in NT dryland production systems potentially offers several ecosystem services, including weed control, soil health improvement, decline in selective pest pressure, and overall reduction in pest management inputs. This review article aims to document the role of CCs for IWM, with emphasis on exploring emerging weed issues; ecological, economic, and agronomic benefits of growing CCs; and constraints preventing adoption of CCs in NT cropping systems in the semiarid Great Plains. We attempt to focus on changes in weed management practices, their long-term impacts on weed seedbanks, weed shifts, and herbicide-resistance evolution in the most common weed species in the region. We also highlight current knowledge gaps and propose new research priorities based on an improved understanding of CC management strategies that will ultimately aid in achieving sustainable weed management goals and preserving natural resources in water-limited environments.

Detection and characterization of resistance to acetolactate synthase inhibiting herbicides in Anisantha and Bromus species in the United Kingdom.

Anisantha and Bromus spp. are widespread and difficult to control, potentially due to the evolution of herbicide resistance. In this study, UK populations of four brome species have been tested for the early development of resistance to acetolactate synthase (ALS)-inhibiting herbicides commonly used in their control. Glasshouse assays confirmed reduced sensitivity to ALS-inhibiting herbicides in single populations of A. diandra, B. commutatus and B. secalinus, and in three populations of A. sterilis. By contrast, all 60 brome populations tested were sensitive to the ACCase-inhibiting herbicide propaquizafop and glyphosate. Dose-response with two ALS herbicides showed broad-ranging resistance in the A. diandra, A. sterilis and B. commutatus populations. In the B. commutatus population, this was associated with a point mutation in the ALS enzyme conferring target site resistance (TSR). Additionally, resistant populations of A. sterilis and B. commutatus populations contained enhanced amounts of an orthologue of the glutathione transferase phi (F) class 1 (GSTF1) protein, a functional biomarker of nontarget site resistance (NTSR) in Alopecurus myosuroides. There was further evidence of NTSR as these plants also demonstrated an enhanced capacity to detoxify herbicides. This study confirms the evolution of resistance to ALS inhibiting herbicides in brome species in the UK by mechanisms consistent with the evolution of both TSR and NTSR. These findings point to the need for increased vigilance in detecting and mitigating against the evolution of herbicide resistance in brome species in Northern Europe. © 2020 Society of Chemical Industry.

Response of Mungbean to the Residual Toxicity of Herbicides Used in Wheat under Strip Tillage System

The residual effect of eight herbicides (Pendimethalin, Pretilachlor, Triasulfuron Ethoxysulfuron, Pyrazosulfuron Ethyl, Carfentrazone – ethyl, Carfentrazone – ethyl+ Isoproteuron, 2, 4 –D) used in wheat of Agronomy Field Laboratory during March to June 2014 was evaluated for mungbean. The eighteen herbicide treatment combinations of the eight herbicides were used in wheat. The experiment was conducted in Random Complete Block Design (RCBD) with three replications. The effect of herbicide residues on the mungbean was evaluated in term of germination, seedling root and shoot length, leaf chlorophyll content and seeding dry matter. The result showed that seedling establishment of mungbean was not adversely affected by the herbicides applied the previous wheat crop. Also, the residue of herbicides applied in strip-tilled wheat did not show any adverse effect on germination, shoot length and seedling dry matter of mungbean and even shoot growth of mungbean was not inhibited by the residue of those herbicides. The reason might be related to the half-life and rate of degradation of herbicides in soil. In addition, farmers can easily grow mungbean in a cropping pattern as a subsequent crop of wheat because the study ensures that tested herbicides did not show any adverse residual effect on the establishment of this succeeding crop. However, continuous use of same herbicide or different herbicides with same mode of action in the same land year after year is strictly prohibited. Also, the prudent use of chemical/herbicide is essential to fulfill the goals of conservation agriculture by having reducing detrimental environmental impact as well as reducing herbicide resistance development in crops.

Genetic diversity and spread of glyphosate-resistant flaxleaf fleabane

BackgroundContinual application of herbicides for flaxleaf fleabane control readily results in the evolution of herbicide resistance. Flaxleaf fleabane has evolved resistance to different modes of action herbicides in many countries. Due to the comprehensive geographical distribution of flaxleaf fleabane in Australia, it was classified as a cosmopolitan weed and it therefore has no specific climatic requirement.ResultsThe high percentage of polymorphisms in the samples of the susceptible population (79.8%, 75%) suggests that susceptible populations of flaxleaf fleabane, even within one site, originated from a number of parents. However, the percentage of polymorphism in the resistant populations (51.5%, 66.8%) suggests that resistant populations of flaxleaf fleabane within one site could have originated from fewer parents. In addition, any site containing resistant and susceptible populations of flaxleaf fleabane may have been populated by a large number of parents, evidenced by the relatively high percentage of polymorphisms detected by amplified fragment length polymorphism (AFLP) analysis (86.5%). Despite the large geographic distances between collection locations, populations from across states clustered in several groups showing a close genetic relationship among these populations over these large distances. These high levels of genetic diversity within and between populations confirmed in the sequencing of enolpyruvylshikimate-3-phosphate above support the results of AFLP studies and gave the author more confidence to report the genetic diversity seen within and between population studies.ConclusionTo prevent further resistance spread, flaxleaf fleabane management in infested areas should focus on decreasing seed movement from resistant sites as well as reducing the selection pressure for resistance to glyphosate by adopting alternative management strategies.

Non-Chemical Weed Management in Vegetables by Using Cover Crops: A Review

Vegetables are a substantial part of our lives and possess great commercial and nutritional value. Weeds not only decrease vegetable yield but also reduce their quality. Non-chemical weed control is important both for the organic production of vegetables and achieving ecologically sustainable weed management. Estimates have shown that the yield of vegetables may be decreased by 45%–95% in the case of weed–vegetable competition. Non-chemical weed control in vegetables is desired for several reasons. For example, there are greater chances of contamination of vegetables by herbicide residue compared to cereals or pulse crops. Non-chemical weed control in vegetables is also needed due to environmental pollution, the evolution of herbicide resistance in weeds and a strong desire for organic vegetable cultivation. Although there are several ways to control weeds without the use of herbicides, cover crops are an attractive choice because these have a number of additional benefits (such as soil and water conservation) along with the provision of satisfactory and sustainable weed control. Several cover crops are available that may provide excellent weed control in vegetable production systems. Cover crops such as rye, vetch, or Brassicaceae plants can suppress weeds in rotations, including vegetables crops such as tomato, cabbage, or pumpkin. Growers should also consider the negative effects of using cover crops for weed control, such as the negative allelopathic effects of some cover crop residues on the main vegetable crop.

Herbicide drift exposure leads to reduced herbicide sensitivity in Amaranthus spp.

While the introduction of herbicide tolerant crops provided growers new options to manage weeds, the widespread adoption of these herbicides increased the risk for herbicide spray drift to surrounding vegetation. The impact of herbicide drift in sensitive crops is extensively investigated, whereas scarce information is available on the consequences of herbicide drift in non-target plants. Weeds are often abundant in field margins and ditches surrounding agricultural landscapes. Repeated herbicide drift exposure to weeds could be detrimental to long-term management as numerous weeds evolved herbicide resistance following recurrent-selection with low herbicide rates. The objective of this study was to evaluate if glyphosate, 2,4-D, and dicamba spray drift could select Amaranthus spp. biotypes with reduced herbicide sensitivity. Palmer amaranth and waterhemp populations were recurrently exposed to herbicide drift in a wind tunnel study over two generations. Seeds from survival plants were used for the subsequent rounds of herbicide drift exposure. Progenies were subjected to herbicide dose-response studies following drift selection. Herbicide drift exposure rapidly selected for Amaranthus spp. biotypes with reduced herbicide sensitivity over two generations. Weed management programs should consider strategies to mitigate near-field spray drift and suppress the establishment of resistance-prone weeds on field borders and ditches in agricultural landscapes.

Parallel and nonparallel genomic responses contribute to herbicide resistance in Ipomoea purpurea, a common agricultural weed.

The repeated evolution of herbicide resistance has been cited as an example of genetic parallelism, wherein separate species or genetic lineages utilize the same genetic solution in response to selection. However, most studies that investigate the genetic basis of herbicide resistance examine the potential for changes in the protein targeted by the herbicide rather than considering genome-wide changes. We used a population genomics screen and targeted exome re-sequencing to uncover the potential genetic basis of glyphosate resistance in the common morning glory, Ipomoea purpurea, and to determine if genetic parallelism underlies the repeated evolution of resistance across replicate resistant populations. We found no evidence for changes in 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS), glyphosate’s target protein, that were associated with resistance, and instead identified five genomic regions that showed evidence of selection. Within these regions, genes involved in herbicide detoxification—cytochrome P450s, ABC transporters, and glycosyltransferases—are enriched and exhibit signs of selective sweeps. One region under selection shows parallel changes across all assayed resistant populations whereas other regions exhibit signs of divergence. Thus, while it appears that the physiological mechanism of resistance in this species is likely the same among resistant populations, we find patterns of both similar and divergent selection across separate resistant populations at particular loci.

Weed-Competitive Ability of Forage Maize Cultivars against Barnyardgrass

Weed-competitive cultivars are desired in the wake of growing popularity of organic farming, environmental pollution and evolution of herbicide resistance in weeds. This research work evaluated the weed competitive ability of three forage maize cultivars (ADA-523, AGA and SASA-5) against the noxious weed barnyardgrass (Echinochloa crus-galli (L.) P.Beauv.). The study was conducted in spring 2018 and repeated in summer 2018. Results of this study showed that maize-barnyardgrass competition significantly decreased the growth of forage maize plants. For instance, barnyardgrass decreased the maize plant height by 11.9-16.9%, leaf length by 13.3-20.2%, leaf width by 20.2-27.4%, and number of leaves by 14.3-25.0%. Fresh and dry weights of maize plants were also significantly decreased as a result of weed-crop competition. Barnyardgrass decreased the shoot fresh weight (30.7-60.6%), shoot dry weight (33.3-52.2%), leaf fresh weight (33.4-56.5%) and leaf dry weight (31.9-50.0%) of the maize plants. An interactive effect of weed × maize cultivars was found non-significant. Forage maize cultivars also varied occasionally for their traits. Nevertheless, ADA-523 had a higher plant height, leaf length, leaf width, leaf fresh weight and leaf dry weight than the cultivars AGA and SASA-5. On the other hand, the cultivar SASA-5 had a higher shoot fresh weight, shoot dry weight and root fresh weight than the other cultivars in the study. This research work concluded that the forage maize cultivars in the study did not vary for the weed-competitive ability. Further, barnyardgrass-maize competition could decrease the growth and development of the maize cultivars.

Germinability and seed biochemical properties of susceptible and non–target site herbicide-resistant blackgrass (Alopecurus myosuroides) subpopulations exposed to abiotic stresses

AbstractQuantifying the level of ecophysiological, biochemical, and agronomical fitness of herbicide-resistant (R) and herbicide-susceptible (S) weeds is useful for understanding the evolutionary development of herbicide resistance, but also for implementing herbicide-resistance management strategies. Although germination is a key fitness component in the life cycle of weeds, germinability of S and R weeds has rarely been evaluated under stressful conditions. Germinability traits of S and non–target site resistant subpopulations of blackgrass (Alopecurus myosuroides Huds.) sharing closely related genetic background were tested under salinity, drought stress, and accelerated seed-aging (i.e., exposed to 100% relative humidity at 45 C from 0 to 134 h) conditions. In addition, the activity of three antioxidant enzymes and protein concentration of accelerated aged seeds of the subpopulations were studied. There were no differences in maximum seed germination (Gmax) and time to 50% germination between the two subpopulations under optimum conditions. However, under salinity, drought stress, and accelerated aging conditions, there were differences between the subpopulations. The salinity, drought, and accelerated aging treatments reducing Gmax of the S subpopulation by 50% were 18 dS m−1, 0.75 MPa, and 90 h, respectively, while for the R subpopulation the corresponding values were 15 dS m−1, 0.66 MPa, and 67 h. No differences were found in the activity of the antioxidant enzymes and the content of protein between non-aged seeds of the subpopulations. The aging treatments reducing the activity of catalase and superoxide dismutase enzymes by 50% were 118 and 82 h for the S subpopulation, respectively, while they were 54 and 58 h for the R subpopulation. In contrast, there were no differences in the effect of the aging treatments on the peroxidase activity and protein content between subpopulations. The results provided clear evidence that the non–target site resistant loci of blackgrass is associated with fitness costs under environmental stress.

Application of synthetic auxin herbicides to suppress seed viability of Italian ryegrass (Lolium perenne ssp. multiflorum) in tall fescue seed production

AbstractItalian ryegrass is one of the most troublesome weeds worldwide because of the rapid evolution of herbicide resistance in this species. Oregon tall fescue seed production requires high seed purity, demanding good control of Italian ryegrass. The necessity to control herbicide-resistant Italian ryegrass and maintain tall fescue seed purity created interest in new chemical management options. The objectives of this study were to assess the effects of synthetic auxin herbicides on seed viability of Italian ryegrass biotypes and the feasibility of this management strategy for use in tall fescue seed production. Eight treatments of synthetic auxin herbicides were applied to Italian ryegrass and tall fescue at two growth stages (boot and anthesis): dicamba (1.0 and 2.2 kg ae ha−1), 2,4-D (1.1 and 2.2 kg ae ha−1), aminopyralid (0.5 kg ae ha−1), dicamba + 2.4-D (0.8 + 1.1 kg ae ha−1), 2.4-D + clopyralid (1.1 + 0.3 kg ae ha−1), and halauxifen-methyl + florasulam (0.4 kg ae ha−1 + 0.4 kg ai ha−1). Aminopyralid applied at boot and anthesis stages of Italian ryegrass reduced seed viability. Aminopyralid treatments reduced seed viability and weight of Italian ryegrass more than 50% compared to the control. Four biotypes from different locations in western Oregon with different types of herbicide resistance were sprayed, and differences in aminopyralid effect among Italian ryegrass biotypes were documented. Aminopyralid reduced the speed of germination by 1 to 2 d. Aminopyralid treatments had a greater effect when applied at the anthesis stage and had a greater negative impact on tall fescue. Tall fescue plants were more susceptible to aminopyralid, so this management practice is not feasible for tall fescue seed production. Future studies are needed to understand the physiological mechanisms involved in the reduced seed viability and to define an optimum aminopyralid rate for different Italian ryegrass biotypes.

Herbicide programs for control of glyphosate-resistant canola (Brassica napus) in glyphosate-resistant soybean

AbstractGlyphosate-resistant (GR) canola is a widely grown crop across western Canada and has quickly become a prolific volunteer weed. Glyphosate-resistant soybean is rapidly gaining acreage in western Canada. Thus, there is a need to evaluate herbicide options to manage volunteer GR canola in GR soybean crops. We conducted an experiment to evaluate the efficacy of various PRE and POST herbicides applied sequentially to volunteer GR canola and to evaluate soybean injury caused by these herbicides. Trials were conducted across Saskatchewan and Manitoba in 2014 and 2015. All treatments provided a range of suppression (>70%) to control (>80%) of volunteer canola. All treatments with the exception of the glyphosate-treated control reduced aboveground canola biomass by an average of 96%. As well, canola seed contamination was reduced from 36% to less than 5% when a PRE and POST herbicide were both used. Moreover, all combinations of herbicides used had excellent crop safety (<10%). All PRE and POST herbicide combinations provided better control of volunteer canola compared with the glyphosate-only control, but tribenuron followed by bentazon and tribenuron followed by imazamox plus bentazon provided solutions that were low cost, currently available (registered in western Canada), and had the potential to minimize development of herbicide resistance in other weeds.

Interactions between crop sequences, weed populations and herbicide use in Western Australian broadacre farms: findings of a six-year survey

Six years of survey data taken from 184 paddocks spanning 14 million ha of land used for crop and pasture production in south-west Western Australia were used to assess weed populations, herbicide resistance, integrated weed management (IWM) actions and herbicide use patterns in a dryland agricultural system. Key findings were that weed density within crops was low, with 72% of cropping paddocks containing fewer than 10 grass weeds/m2 at anthesis. Weed density and herbicide resistance were not correlated, despite the most abundant grass weed species (annual ryegrass, Lolium rigidum Gaudin) testing positive for resistance to at least one herbicide chemistry in 92% of monitored paddocks. A wide range of herbicides were used (369 unique combinations) suggesting that the diversity of herbicide modes of action may be beneficial for reducing further development of herbicide resistance. However, there was a heavy reliance on glyphosate, the most commonly applied active ingredient. Of concern, in respect to the evolution of glyphosate resistant weeds, was that 45% of glyphosate applications to canola were applied as a single active ingredient and area sown to canola in Western Australia expanded from 0.4 to 1.4 million hectares from 2005 to 2015. In order to minimise the weed seed bank within crops, pastures were used infrequently in some regions and in 50% of cases pastures were actively managed to reduce weed seed set, by applying a non-selective herbicide in spring. The use of non-selective herbicides in this manner also kills pasture plants, consequently self-regenerating pastures were sparse and contained few legumes where cropping intensity was high. Overall, the study indicated that land use selection and utilisation of associated weed management actions were being used successfully to control weeds within the survey area. However, to successfully manage herbicide resistant weeds land use has become less diverse, with pastures utilised less and crops with efficacious weed control options utilised more. Further consideration needs to be given to the impacts of these changes in land use on other production factors, such as soil nutrient status and plant pathogens to assess sustainability of these weed management practices in a wider context.

Evaluation of thiencarbazone‐methyl for use in acetolactate synthase‐resistant soybean

AbstractTo combat herbicide‐resistant weeds, new options are needed to effectively rotate herbicide modes of action and slow the development of additional herbicide resistance. Thiencarbazone‐methyl (TCM), an acetolactate synthase (ALS)‐inhibiting herbicide, is currently being evaluated for preemergence (PRE) and postemergence (POST) activity on many troublesome summer annual and perennial weeds. Multiple experiments were conducted to assess the utility of this product in soybean (Glycine max) production systems. Field experiments were conducted across Arkansas in 2016 and 2017 to determine the tolerance of ALS‐resistant soybean varieties to PRE and POST applications of TCM at either 0.03 or 0.06 lb acre–1. For all locations, no significant interactions or main effects were observed for any parameters evaluated. Overall, soybean injury remained consistently low, with <3% injury at 21 days after treatment (DAT) on all ALS‐resistant varieties. However, the ALS‐susceptible variety showed high levels of injury for both PRE (75%) and POST (98%) applications at 21 DAT. Two additional experiments were conducted in 2016, 2017, and 2018 to evaluate the efficacy of PRE and POST applications of TCM in ALS/glufosinate‐resistant soybean. For PRE applications, residual activity of TCM, imazethapyr, chlorimuron, nicosulfuron, halosulfuron, and flumetsulam at labeled rates for soybean or corn were evaluated. TCM at 0.06 lb acre–1was effective in controlling ivyleaf morningglory (91 %) and broadleaf signalgrass (97%) at 21 DAT, but was not effective in controlling ALS‐resistant Palmer amaranth (58%). In the POST experiment, TCM at 0.03 or 0.06 lb acre–1was compared to imazethapyr, chlorimuron, nicosulfuron, and halosulfuron with and without glufosinate (0.5 lb acre–1). Alone, TCM produced higher control of ivyleaf morningglory (99 to 100%) and broadleaf signalgrass (94 to 99%) at 21 DAT compared to all other ALS‐inhibiting herbicides. However, TCM alone was not effective in controlling ALS‐resistant Palmer amaranth (55 to 61%) at 21 DAT, but when combined with glufosinate control increased to 95 to 99%. The results from this research suggest that TCM could have utility in soybean weed control systems due to the high level of resistance exhibited by ALS‐resistant soybean varieties and the efficacy of TCM on common soybean weed species at both the PRE and POST application timings.