BACKGROUND Intensive application of acetolactate synthase (ALS)‐inhibiting herbicides has resulted in herbicide‐resistance in many weeds, including Bromus sterilis. The present study was conducted to identify the mechanisms conferring resistance to ALS‐inhibiting herbicides in a Bromus sterilis biotype. RESULTS Dose–response studies revealed the resistant biotype to be 288 times less sensitive to pyroxsulam than the susceptible biotype. Furthermore, experiment with a single‐dose, proved this biotype was also cross‐resistant to propoxycarbazone, iodosulfuron plus mesosulfuron and sulfosulfuron. Prior treatment with malathion, a known inhibitor of cytochrome P450s, reduced the level of resistance to pyroxsulam. No mutations were detected from the partial ALS gene sequencing. Flow cytometry and chromosome counting rejected ploidy level variation between the susceptible and resistant biotypes. Relative copy number variation ruled out gene amplification. Quantitative real‐time polymerase chain reaction (PCR) detected a significant difference in ALS gene expression between the susceptible and resistant biotypes. CONCLUSIONS Target gene overexpression and enhanced metabolism by cytochrome P450s are likely mechanisms of resistance to pyroxsulam in Bromus sterilis. The current findings highlight the need to monitor additional brome populations for herbicide resistance in Europe and endorse the need for alternate herbicides in integrated weed management to delay the possible evolution of herbicide resistance in these species. © 2020 Society of Chemical Industry

Loose silky bent grass (Apera spica-venti (L.) P. Beauv) is a winter outcrossing annual grass, widespread in the Central and East European countries, mainly, in Germany, Poland and the Czech Republic (Soukup et al., 2006). Although, it is also found in North European countries, such as Denmark, Sweden and Lithuania (e.g. Auškalniené et al., 2020). It can be controlled with numerous herbicides, including ureas, dinitroanilines and thiocarbamates. The first case of resistance to ureas was found in Germany in 1996 (Niemann, 2000). Lately,

Scentless mayweed (Tripleurospermum inodorum (L.) Sch. Bip.) is a competitive weed in the disturbed and agricultural areas of Europe, including the Czech Republic (Štrobach and Mikulka 2019). This weed is difficult to manage, particularly in cereals such as winter wheat and winter rape (Nadtochii 2009, Reiss et al. 2018. Since this weed germinates at almost the same time to that of the crop and produces high biomass, it significantly affects the yield of these crops (Adamczewski et al. 2014). The prevalence of this weed is mostly due to minimum or no-tillage systems. In general, scentless mayweed is well controlled by the most widely used herbicides in cereals, especially inhibitors of enzyme acetolactate synthase (ALS) such as active ingredients tribenuron, pyroxsulam or florasulam (Majcen et al. 2013, Jursík et al. 2018. Some herbicides, such as carfentrazone, pendimethalin, and growth regulator-inhibiting herbicides, were found to be less effective against this weed. Pre-emergence herbicides containing metazachlor, dimetachlor and pethoxamide usually give the most effective scentless mayweed control in winter rape. In case of control failure or the absence of preemergent treatment, herbicides containing clopyralid, picloram and halauxifen (synthetic auxins) can be applied after emergence during autumn or spring (Jursík et al. 2018).Acetolactate synthase is a vital enzyme involved in the synthesis of the branched-chain amino acids valine, leucine and isoleucine (Duggleby and Pang 2000). Since the first cases of resistance against ALSinhibiting herbicides, there has been a continuous rise