Spread Of Herbicide Resistance Research Articles

Early adopter insights on physical impact mill technology for harvest weed seed control in Canada

The evolution and spread of herbicide resistance among the weed community has increased interest in alternative weed management strategies such as harvest weed seed control. Western Canadian producers have begun adopting physical impact mills as an additional weed management strategy. A survey of early adopters of physical impact mill technology in Canada was conducted to better understand the motivations behind producers adopting, initial experiences, and research needs. Ten producers responded to the survey, accounting for 18 out of an estimated 30 impact mills in use in Canada, believed to be located primarily in the Canadian Prairies. These producers were mainly from larger farms (> 4,000 ha), equipped the majority of their combines (75% average) and used the mills in essentially all crops grown. The majority of respondents were located in Saskatchewan, with 2 mills being used in Alberta. Wild oat (60%) and kochia (50%) were the weeds most frequently mentioned as specific motivators of impact mill adoption. Average increased fuel cost from the mill was estimated at CAD $3.46 ha-1, with average annual maintenance costs of about $1,500 per impact mill. Producers relied on information from mill companies and other early-adopting farmers primarily, followed by extension talks and social media. Research needs were also identified by producers that could inform the future direction of harvest weed seed control research in Canada. Future research should focus on confirming efficacy, optimizing combine settings, and looking at integrated systems with precision agriculture technologies.

Meanings, materials and competences of area-wide weed management in cropping systems

CONTEXTArea-wide collaboration across private and public property boundaries can enhance the management of weeds and minimise the spread of herbicide resistance. Yet we know little about the practices individual land managers engage in to achieve area-wide weed management (AWWM). OBJECTIVEThis paper uses Social Practice Theory (SPT) as a framework to understand how cropping land managers engage with the practices of AWWM, and what the drivers and barriers are to their participation. METHODS30 qualitative interviews were undertaken with land managers in Australian cropping regions of the Darling Downs (Queensland), Gwydir (New South Wales), Riverina (New South Wales) and Sunraysia (Victoria). Thematic analysis of the interviews explored the three dimensions of SPT—meanings, materials, and competences—of AWWM and the interactions between these elements. RESULTS AND CONCLUSIONSThere is a value-action gap between growers' desire to participate in AWWM practices and their capacity to do so. The analysis reveals that narrowing this gap requires interventions at the points where the SPT elements intersect. It recommends beginning by leveraging existing commonalities between growers who already have a desire to participate in AWWM then scaling out by encouraging trusted agronomist networks to link diverse growers to facilitate collaborative weed management practices. There is also a role for government in supporting the development of leadership capabilities among growers, providing an enabling environment for agronomists to act as systemic facilitators and leading by example on public land. SIGNIFICANCEMost of the research on collective management of weeds has focused on formal groups in grazing systems. This study provides new insights into how and why weeds are largely managed independently in cropping systems and proposes ways growers may be supported to adopt more collaborative practices to address this landscape-scale problem.

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.

Occurrence and management of herbicide resistance in annual vegetable production systems in North America

AbstractHerbicide resistance has been studied extensively in agronomic crops across North America but is rarely examined in vegetables. It is widely assumed that the limited number of registered herbicides combined with the adoption of diverse weed management strategies in most vegetable crops effectively inhibits the development of resistance. It is difficult to determine whether resistance is truly less common in vegetable crops or whether the lack of reported cases is due to the lack of resources focused on detection. This review highlights incidences of resistance that are thought to have arisen within vegetable crops. It also includes situations in which herbicide-resistant weeds were likely selected for within agronomic crops but became a problem when vegetables were grown in sequence or in adjacent fields. Occurrence of herbicide resistance can have severe consequences for vegetable growers, and resistance management plans should be adopted to limit selection pressure. This review also highlights resistance management techniques that should slow the development and spread of herbicide resistance in vegetable crops.

Weak population differentiation and high diversity in Salsola tragus in the inland Pacific Northwest, USA

Salsola tragus is a widespread and problematic weed of semi-arid wheat production globally, and in the inland Pacific Northwest region of the USA. The species exhibits high levels of phenotypic diversity across its range and, at least in California USA, previous work has described cryptic diversity comprising a multi-species complex. Such cryptic diversity could suggest the potential for a differential response to management inputs between groups, and have important implications for the spread of herbicide resistance or other adaptive traits within populations. We used a genotyping-by-sequencing approach to characterize the population structure of S. tragus in the inland Pacific Northwest. Our results indicated that the population in this region is comprised of a single, tetraploid species (S. tragus sensu latu) with weak population structure on a regional scale. Isolation-by-distance appears to be the primary pattern of structure, but an independent set of weakly differentiated clusters of unknown origin were also apparent, along with a mixed mating system and high levels of largely unstructured genetic diversity. Despite considerable phenotypic variability within S. tragus in the region, agronomic weed managers can likely consider it as a single entity across the region, rather than a collection of cryptic subgroups with possible differential responses to management inputs or agroecosystem conditions. A lack of strong barriers to migration and gene flow mean that adaptive traits, such as herbicide resistance, can be expected to spread rapidly through populations across the region. © 2022 Society of Chemical Industry.

Pollen-mediated flow of herbicide resistance genes in Beckmannia syzigachne.

Beckmannia syzigachne (Steud.) Fernald has evolved herbicide resistance due to the long-term sole use of herbicides and has become a dominant weed in wheat fields in the middle and lower reaches of the Yangtze River in China. In addition to the selection pressure imposed by herbicides, pollen-mediated gene flow (PMGF) has been reported to cause the spread of herbicide resistance between populations within a certain range in some farmland weeds. It is not clear whether the same is true for the self-pollinated grass weed B. syzigachne. In this study, we confirmed and quantified the level of PMGF in B. syzigachne through concentric circle planting and herbicide resistance tests. Results show that when the B. syzigachne pollen donor was close to the recipient (0.5m), the average gene flow was 0.66%. Gene flow was detected as far as 10m (the farthest distance studied) and decreased exponentially with increasing distance, which could be described by a double exponential decay model. Temperature also affected gene flow, whilst the average level of gene flow in all directions of wind was similar and wind speed caused insignificant difference in gene flow. The results of this study confirmed that PMGF can occur between B. syzigachne populations in adjacent fields. Although the level of resistance spreading by pollen was low, especially across long distance, the results were relevant for smallholding farms, which is the dominant form of agricultural operation in China. It is therefore important to take proactive measures and integrate chemical and ecological weed control methods to prevent the spread of resistant B. syzigachne via both seeds and pollens. © 2022 Society of Chemical Industry.

Opportunities to Manage Herbicide Resistance through Area-Wide Management: Lessons from Australian Cropping Regions

Herbicide resistance management is often understood as a decision for individual land managers, but their decisions have far-reaching impacts for social-ecological systems. Area-wide management can reduce these impacts by supporting many land managers to cooperatively work towards a shared goal of reducing the spread of resistance. The aim of this research is to identify what support is needed for area-wide herbicide resistance management in cropping systems. Data was collected from 84 interviews with growers, public land managers and weed management advisors. Sixty-five interviews were conducted across three cropping regions of eastern Australia—Darling Downs (Queensland), Riverina (New South Wales) and Sunraysia (Victoria)—and 19 interviews were conducted with stakeholders beyond these regions. The majority (51%) of interviewees expressed concern about the spread of herbicide resistance, but only 14% described involvement in area-wide resistance management programs. Area-wide management was mostly reported to involve sharing information among stakeholders, rather than coordination or joint activities. Key barriers to participation were perceived to be the diverse agricultural industries in each region and the costs of participation. Future area-wide management program designs need to build working relationships among diverse stakeholders, clearly define the boundaries of the program and demonstrate the benefits that accrue from participation.

Pesticide-free but not organic: Adoption of a large-scale wheat production standard in Switzerland

The sustainable intensification of agriculture requires solutions for a large-scale reduction of pesticide use while sustaining agricultural yields. Pesticide-free production standards, which bring together the strengths of all the food value chain actors, could be a cornerstone of this transformation. In Switzerland, a non-organic, private–public standard for pesticide-free wheat production is currently being introduced by the producer organization IP-SUISSE. It is the first of its kind in Europe and may reach a market share of 50% of Swiss wheat production. We here assess the determinants of farmers' participation and willingness to participate in the future. For our analysis, we combine a survey of the entire population of IP-SUISSE wheat producers (4749 farmers, 23.3% response rate) with data on historical farm-level wheat yields, soil properties, weather, climate, weed pressure, and spread of herbicide resistance. Our results indicate that a large-scale establishment of pesticide-free wheat production in Switzerland is possible. We find that farmers' perceptions of positive environmental effects of the production program are key for adoption. Moreover, farmers' expectations of the program’s production effects play a central role. Farmers perceiving large yield losses and increases in production risks are less likely to enter the program. Based on our results, we discuss implications, leverage points, and challenges for designing and implementing large-scale pesticide-free production programs.

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.

Genetic diversity and structure of Lolium perenne ssp. multiflorum in California vineyards and orchards indicate potential for spread of herbicide resistance via gene flow.

Management of agroecosystems with herbicides imposes strong selection pressures on weedy plants leading to the evolution of resistance against those herbicides. Resistance to glyphosate in populations of Lolium perenne L. ssp. multiflorum is increasingly common in California, USA, causing economic losses and the loss of effective management tools. To gain insights into the recent evolution of glyphosate resistance in L. perenne in perennial cropping systems of northwest California and to inform management, we investigated the frequency of glyphosate resistance and the genetic diversity and structure of 14 populations. The sampled populations contained frequencies of resistant plants ranging from 10% to 89%. Analyses of neutral genetic variation using microsatellite markers indicated very high genetic diversity within all populations regardless of resistance frequency. Genetic variation was distributed predominantly among individuals within populations rather than among populations or sampled counties, as would be expected for a wide‐ranging outcrossing weed species. Bayesian clustering analysis provided evidence of population structuring with extensive admixture between two genetic clusters or gene pools. High genetic diversity and admixture, and low differentiation between populations, strongly suggest the potential for spread of resistance through gene flow and the need for management that limits seed and pollen dispersal in L. perenne.

Atmospheric pollen dispersion from herbicide-resistant horseweed (Conyza canadensis L.)

Horseweed (Conyza canadensis (L.) Cronq.) with evolved herbicide resistance has become an especially problematic weed in crop production across the USA and on four continents (North America, South America, Asia, and Europe). Spread of herbicide resistance can occur through pollen-mediated gene flow between resistant and susceptible horseweed populations. However, there are little knowledge, preventive guidelines, and mechanism modeling for pollen transport in this system. We need to better understand pollen dispersion and deposition in the context of atmospheric conditions, herbicide-resistant horseweed patch size, and buffer crop type, height, and field size. A mechanistic model is needed to account for these. A pollen dispersion and deposition model was calibrated and validated using 2013 experimental field data. The validated model was run for various combinations of atmospheric conditions, horseweed characteristics (source strength), and buffer species and size (pollen can be intercepted by crop plants). Large fields with crops with a high leaf area density and tall plants can effectively prevent pollen dispersion. The information will help provide guidelines for preventing herbicide resistance spread from herbicide-resistant weeds and genetically modified plants in general.

Multiple-Herbicide Resistance Is Widespread in Roadside Palmer Amaranth Populations.

Herbicide-resistant Palmer amaranth is a widespread issue in row-crop production in the Midsouthern US. Palmer amaranth is commonly found on roadside habitats in this region, but little is known on the degree of herbicide resistance in these populations. Herbicide resistance in roadside Palmer amaranth populations can represent the spread of an adaptive trait across a selective landscape. A large-scale survey was carried out in the Mississippi Delta region of eastern Arkansas to document the level of resistance in roadside Palmer amaranth populations to pyrithiobac and glyphosate, two important herbicides with broad history of use in the region. A total of 215 Palmer amaranth populations collected across 500 random survey sites were used in the evaluations. About 89 and 73% of the surveyed populations showed >90% survival to pyrithiobac and glyphosate, respectively. Further, only 3% of the populations were completely susceptible to glyphosate, while none of the populations was completely controlled by pyrithiobac. Among the 215 populations evaluated, 209 populations showed multiple resistance to both pyrithiobac and glyphosate at varying degrees. Dose-response assays confirmed the presence of high levels of herbicide resistance in the five selected populations (≥ 25-fold compared to a susceptible standard). Results demonstrate the prevalence of multiple-herbicide resistance in roadside Palmer amaranth populations in this region. Growers should be vigilant of Palmer amaranth infestation in roadsides adjacent to their fields and implement appropriate control measures to prevent likely spread of herbicide resistance into their fields.

Seeding rate and cultivar effects on canola (Brassica napus) competition with volunteer wheat (Triticum aestivum)

Canola (Brassica napus L.) is an important rotational crop in the temperate cropping zone of southern Australia. Herbicide-resistant weeds are rapidly spreading and reducing canola grain yield and quality. Crop competition is a useful tool for reducing weed costs and dependence on herbicides, and retarding the spread of herbicide resistance. The potential interaction of canola seeding rate and cultivar for weed management has not been quantified in Australia. A field experiment was conducted in three environments to examine the impact of two contrasting canola cultivars (a low vigour type and a high vigour hybrid) at four seeding rates (10–100 plants/m2) on volunteer wheat (~50 plants/m2). Significant but variable effects of crop seeding rate, cultivar and weed were detected on canola density and grain yield, and on the suppression of volunteer wheat. The canola hybrids suppressed volunteer wheat more than the less vigorous cultivars in all the experiments. There was no benefit of increasing canola seeding rate above the normally recommended rate of 40 plants/m2 for weed suppression. The seed production of volunteer wheat on average doubled when canola density dropped from 40 to 10 plants/m2. Treatment effects on canola grain yield losses from weeds were less than those on weed suppression. The grain yield of both cultivars was reduced between 30% and 40% with weeds at a canola density of 40 plants/m2 and plateaued above this density in weedy conditions. Maintaining canola plant establishment and using competitive cultivars is critical to avoiding weed seedbank replenishment, and reducing canola yield losses from weed competition.

Characterization of PPO-Inhibitor–Resistant Waterhemp (Amaranthus tuberculatus) Response to Soil-Applied PPO-Inhibiting Herbicides

Waterhemp resistance to foliar applications of protoporphyrinogen oxidase (PPO)–inhibiting herbicides has become increasingly disconcerting given the widespread distribution of glyphosate resistance. Fortunately, soil-residual PPO-inhibiting herbicides remain efficacious in waterhemp populations resistant to PPO-inhibiting herbicides; however, these herbicides should theoretically select for the resistant biotype as herbicide concentrations diminish in the soil. Accordingly, the objectives of this research were twofold: (1) evaluate the efficacy of three PPO-inhibiting herbicides, foliar- and soil-applied, on PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) waterhemp, and (2) investigate the differential effects of PPO-inhibiting herbicides on an R biotype and an S biotype during several discrete developmental events relevant to soil–residual herbicide activity (i.e., radicle protrusion, radicle elongation, and waterhemp emergence). Greenhouse and growth chamber experiments indicated that the R biotype was least sensitive to the diphenylether herbicide fomesafen, followed by sulfentrazone and flumioxazin; however, fomesafen pluss-metolachlor improved soil-residual efficacy over fomesafen alone. Growth stage considerably influenced the R : S ratio, decreasing from 38× to 3.4×, when comparing ratios generated from foliar applications and soil-residual applications measuring radicle protrusion, respectively. Overall, this research supports the use of full soil-residual herbicide rates, reinforcing the importance of best management practices to manage the spread of herbicide resistance.

Segregation of non‐target‐site resistance to herbicides in multiple‐resistant Alopecurus myosuroides plants

SummaryNon‐target‐site resistance (NTSR) comprises a set of mechanisms conferring resistance to multiple modes of action. Investigation of the number of loci involved in NTSR will aid in the understanding of these resistance mechanisms. Therefore, six different multiple herbicide‐resistant Alopecurus myosuroides plants with different herbicide history were crossed in two generations with a susceptible wild type. Seeds from the backcrossing generation were studied for their segregation rate for resistance to five herbicides with four different modes of action (HRAC groups C2, A, B and K3). Taking into account that NTSR is a set of quantitative traits, the numbers of loci controlling NTSR were estimated using a normal mixture model fitted by the NLMIXED procedure of SAS. Each herbicide was controlled by a different number of loci comparing the six plants. In most of the cases, chlorotoluron resistance was controlled by one locus, whereas resistance to fenoxaprop‐P‐ethyl needed one or two loci. Resistance to pinoxaden was in all plants conferred by two loci. Cross‐resistance of fenoxaprop‐P‐ethyl and pinoxaden was found in all backcrossings, indicating that at least one of the two loci is responsible for both resistances. Resistance to mesosulfuron + iodosulfuron was conferred by a minimum of two loci. Results indicated that a minimum of five different loci can be involved in a multiple NTSR plant. Furthermore, the plant‐specific accumulation of NTSR loci was demonstrated. Such behaviour should be taken into account when evaluating the development and further spread of herbicide resistance.

Distribution of Weedy Red Rice (Oryza sativa) Resistant to Imidazolinone Herbicides and its Relationship to Rice Cultivars and WildOryzaSpecies

Several weedy red rice populations have evolved resistance to imidazolinone herbicides worldwide. The understanding of the factors related to the herbicide resistance in weedy red rice is important to prevent its occurrence in new areas where imidazolinone-resistant rice cultivars are being used, and to manage the new rice cultivars resistant to herbicides with modes of action other than the acetolactate synthase (ALS)-inhibitors that are being developed. The objectives of this study were to analyze the relationship of weedy red rice populations from southern Brazil with rice cultivars and wildOryzaspecies and to evaluate the occurrence of introgression from rice cultivars and seed migration as the origin of resistance to imidazolinone herbicides in weedy rice. The study was based on 27 weedy red rice populations, seven rice cultivars, and four wildOryzaspecies that were genotyped with 24 simple sequence repeats and threeALS-specific single-nucleotide polymorphism markers. A large proportion of the genetic variation of the weedy red rice populations was found within (74%) rather than among populations (26%). The weedy red rice populations were more closely related to the newer rice cultivars that are imidazolinone-resistant than to the older cultivars. The South American nativeOryza glumaepatulaand the other wildOryzaspecies—Oryza rufipogon, Oryza longistaminata, andOryza glaberrima—clustered separately from weedy red rice populations, indicating a low likelihood of introgression among weedy red rice and these wild species. Seed migration was an important factor in the genetic structure of the evaluated weedy red rice populations, although gene flow by pollen from resistant cultivars was the principal reason for the spread of herbicide resistance.

Pollen‐mediated transfer of herbicide resistance in Echinochloa crus‐galli

Pollen-mediated gene flow (PMGF) can facilitate the dispersal and spread of herbicide resistance from one weed population to another within an agricultural landscape. The aim of this study was to quantify the extent of PMGF in Echinochloa crus-galli (barnyardgrass), an important herbicide-resistant weed species in the United States and across the world. Gene flow declined exponentially with distance, and the double exponential decay model predicted an average gene flow of 5.6% when the pollen donor and recipient plants were at a close distance of 0.25 m from each other (12.5% at 0 m). Gene flow declined by 90% at 0.9 m from the pollen source, yet gene flow was detected as far as 50 m (the farthest distance studied). The farthest gene flow occurred in directions of the fastest wind events, but mean gene flow levels were similar among the directions. Results indicate that long-distance, landscape-scale PMGF is unlikely in barnyardgrass, but gene flow is likely to occur between adjacent fields at levels greater than initial frequencies of resistance alleles in natural, unselected populations. Thus, any resistance management strategy should consider the likelihood that PMGF can contribute to the spread of herbicide resistance between production fields.

Development of a Geo-Referenced Database for Weed Mapping and Analysis of Agronomic Factors Affecting Herbicide Resistance in Apera spica-venti L. Beauv. (Silky Windgrass)

In this work, we evaluate the role of agronomic factors in the selection for herbicide resistance in Apera spica-venti L. Beauv. (silky windgrass). During a period of three years, populations were collected in more than 250 conventional fields across Europe and tested for resistance in the greenhouse. After recording the field history of locations, a geo-referenced database has been developed to map the distribution of herbicide-resistant A. spica-venti populations in Europe. A Logistic Regression Model was used to assess whether and to what extent agricultural and biological factors (crop rotation, soil tillage, sowing date, soil texture and weed density) affect the probability of resistance selection apart from the selection pressure due to herbicide application. Our results revealed that rotation management and soil tillage are the factors that have the greatest influence on the model. In addition, first order interactions between these two variables were highly significant. Under conventional tillage, a percentage of winter crops in the rotation exceeding 75% resulted in a 1280-times higher risk of resistance selection compared to rotations with less than 50% of winter crops. Under conservation tillage, the adoption of >75% of winter crops increased the risk of resistance 13-times compared to rotations with less than 50% of winter crops. Finally, early sowing and high weed density significantly increased the risk of resistance compared to the reference categories (later sowing and low weed density, respectively). Soil texture had no significant influence. The developed model can find application in management programs aimed at preventing the evolution and spread of herbicide resistance in weed populations.

Seed production of barnyardgrass (Echinochloa crus-galli) in response to time of emergence in cotton and rice

SUMMARYThe spread of herbicide resistance in barnyardgrass (Echinochloa crus-galli(L.) Beauv.) poses a serious threat to crop production in the southern United States. A thorough knowledge of the biology of barnyardgrass is fundamental for designing effective resistance-management programmes. In the present study, seed production of barnyardgrass in response to time of emergence was investigated in cotton and rice, respectively, in Fayetteville and Rohwer, Arkansas, over a 2-year period (2008–09). Barnyardgrass seed production was greater when seedlings emerged with the crop, but some seed production was observed even if seedlings emerged several weeks after crop emergence. Moreover, barnyardgrass seed production was highly variable across environments. When emerging with the crop (0 weeks after crop emergence (WAE)), barnyardgrass producedc. 35 500 and 16 500 seeds/plant in cotton, andc. 39 000 and 2900 seeds/plant in rice, in 2008 and 2009, respectively. Seed production was observed when seedlings emerged up to 5 WAE (2008) or 7 WAE (2009) in cotton and up to 5 WAE (2008, 2009) in rice; corresponding seed production wasc. 2500 and 1500 seeds/plant in cotton, andc. 14 700 and 110 seeds/plant in rice, in 2008 and 2009, respectively. The results suggest that cultural approaches that delay the emergence of barnyardgrass or approaches that make the associated crop more competitive will be useful in integrated management programmes. In the context of herbicide resistance management, it may be valuable to prevent seed return to the seedbank, irrespective of cohorts. The findings are vital for parameterizing herbicide resistance simulation models for barnyardgrass.

The spread of resistance to acetolactate synthase inhibiting herbicides in a wind borne, self-pollinated weed species, Lactuca serriola L.

Resistance to ALS-inhibiting herbicides in Lactuca serriola first appeared in the northern Yorke Peninsula in South Australia in 1994, with resistance soon observed at a number of additional sites. The rapid appearance of resistance at many sites could be attributed to a number of independent selection events or to movement of resistant seed from the original field. ISSRs were used to genotype plants collected in 1999 and 2004 from roadsides or fields in an attempt to determine the importance of these two factors in the spread of herbicide resistance in L. serriola. In 1999 and 2004, chlorsulfuron-resistant L. serriola plants were found in both fields and roadsides with resistant plants being more frequent in fields than roadsides and more frequent in 2004 than in 1999. Genetic relationships generated using UPGMA analysis indicated the presence of more than one genotype within the herbicide resistant populations sampled for both years and suggested independent selection as well as movement of resistant seed had occurred. DNA extracted from samples collected in 1999 was used to sequence a highly conserved region of the ALS gene that coded for a single amino acid modification within the gene. Four different mutations were identified within the resistant samples and these mutations tended to cluster on a geographical basis. Together these data provide evidence for both multiple independent evolutionary events and for the potential movement of individual genotypes as far as 43 km in the region.