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BACKGROUND: Brome grass (Bromus diandrus Roth) is prevalent in the southern and western cropping regions of Australia, where it causes significant economic damage. A targeted herbicide resistance survey was conducted in 2020 by collecting brome grass populations from 40 farms in Western Australia and subjecting these samples to comprehensive herbicide screening. One sample (population 172‐20), from a field that had received 12 applications of clethodim over 20 years of continuous cropping, was found to be highly resistant to the acetyl‐CoA carboxylase (ACCase)‐inhibiting herbicides clethodim and quizalofop, and so the molecular basis of resistance was investigated. RESULTS: All 31 individuals examined from population 172‐20 carried the same resistance‐endowing point mutation causing an aspartate‐to‐glycine substitution at position 2078 in the translated ACCase protein sequence. A wild‐type susceptible population and the resistant population had similar expression levels of plastidic ACCase genes. The level of resistance to quizalofop, either standalone or in mixture with clethodim, in population 172‐20 was lower under cooler growing conditions. CONCLUSION: Target‐site resistance to ACCase‐inhibiting herbicides, conferred by one ACCase mutation, was selected in all tested brome plants infesting a field with a history of repeated clethodim use. This mutation appears to have been fixed in the infesting population. Notably, clethodim resistance in this population was not detected by the farmer, and a high future incidence of quizalofop resistance is anticipated. Herbicide resistance testing is essential for the detection of evolving weed resistance issues and to inform effective management strategies. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

BACKGROUND Lolium rigidum is the weed of greatest economic impact in Australia due to its formidable capacity to evolve herbicide resistance. In this study, 579 field‐sampled L. rigidum populations were tested for resistance to 21 herbicides applied at the recommended rate. Nine herbicide treatments were binary mixtures. RESULTS: A total of 15 876 individual resistance tests were conducted by screening two million seeds at the recommended label rate. The overall frequency of resistant populations was 31%, 14%, 71%, 6% and 0% in response to the post‐emergence herbicide treatments clethodim, clethodim + butroxydim, imazamox + imazapyr, glyphosate and paraquat, respectively. The resistance frequency to stand‐alone pre‐emergence wheat‐selective herbicides ranged from 10% to 34%. Conversely, the levels of resistance to pre‐emergence mixtures or stand‐alone propyzamide were significantly lower, ranging from 6% to 0%. In winter, the responses to glyphosate, paraquat, cinmethylin, prosulfocarb, pyroxasulfone and trifluralin were reassessed, with 7%, 0%, 0%, 21%, 21% and 28% as the respective resistance frequencies. South Australia and Victoria are identified as epicenters for L. rigidum population resistance to pyroxasulfone, whereas populations in New South Wales have the greatest resistance to glyphosate and in Western Australia to clethodim. CONCLUSIONS: For the first time, resistance levels to stand‐alone herbicides and binary mixtures are geographically ranked across the Australian continent by benchmark statistical analysis of resistance frequencies and distribution. The extension of these results will raise awareness of rapidly emerging patterns of herbicide resistance, encouraging the adoption of cost‐effective modes of action and integration of diverse strategies for weed resistance management. [ABSTRACT FROM AUTHOR]