In response to growers reports of poor weed control, resistance to ACCase inhibitors pinoxaden, propaquizafop and cycloxydim was investigated in populations of six wild oats, Avena fatua, collected from cereal-dominated crop rotations in Ireland. Glasshouse assays confirmed reduced sensitivity to all three ACCase inhibitors in four of the six populations, R2 to R5. R1 was cross-resistant to pinoxaden and propaquizafop and R6 was resistant to propaquizafop only. Dose-response studies confirmed significant differences in the severity of resistance amongst these populations (p < 0.05). For pinoxaden, the ED50 or GR50 resistance factor (RF) of R1, R3 and R5 were between 11.6 and 13.1 times or 25.1 and 30.2 times more resistant, respectively, compared with the susceptible populations. For propaquizafop, the ED50 and GR50 RF of R1, R2, R3, R5 and R6 were between >7.8 and >32 or 16.6 and 59 times more resistant, respectively. For cycloxydim, only R5 had both high ED50 and GR50 RF values of >43.2 and 98.4 respectively. In R2, although the ED50 values to both pinoxaden and cycloxydim and additionally, R3 to cycloxydim, were above recommended field rates, their GR50 values remained below, suggesting a shift towards cross-resistance. While R4 was the only population, where both ED50 and GR50 for all ACCase inhibitors remained below recommended field rates, they would not give effective control at these rates, strongly indicating evolving resistance. This is the first study reporting variable cross-resistance types and levels to ACCase inhibitors in A. fatua from Ireland.

The challenges of grass-weed control in a mild Atlantic climate, added to by the use of non-inversion tillage and limited herbicide options necessitated on-farm knowledge acquisition on grass weeds in Ireland. We surveyed 103 farms from 2020 to 2021 of which, 62 were plough-based and 41 non-inversion tillage. The survey comprised a questionnaire to determine grower demographics, grass-weed challenges, and the adoption of integrated grass-weed control methods (IWM); a grid-square assessment of key grass weeds encountered in one or more cereal fields in both years using weed scores from 0 (absent) to 10 (total weed cover); with samples collected for herbicide-resistance testing. Across the 103 farms, Bromus spp., (62%) and Avena fatua (56%) were the most prevalent but with moderate resistance (8% acetolactate synthase (ALS)-resistant Bromus, 10% acetyl-CoA carboxylase (ACCase)-resistant A. fatua); Lolium multiflorum (13%) and Alopecurus myosuroides (16%) were found on fewer farms but with higher resistance levels (56% ACCase and/or ALS-resistant L. multiflorum, 43% ACCase/ALS-resistant A. myosuroides). Of those who adopted non-inversion tillage, a higher proportion of growers practiced crop rotation (88% cereal/non-cereal break or 83% alternate spring/winter-sown) and used cover crops (71%) than those ploughing (52-66% crop rotation or 19% cover crops), but herbicide use was similar in both systems. Despite higher levels of IWM practices, non-inversion tillage farms had higher weed scores (2.2 ± 0.2 or 3.8 ± 0.7) of B. sterilis and L. multiflorum than ploughing (1.2 ± 0.2 or 2.0 ± 0.7). Considering the complex resistance profile of high resistance-risk species, there is a need for urgent determination of and adoption of effective IWM across systems, specifically, non-inversion tillage. Most of the growers were aware of herbicide-resistant grass weeds (&gt;80%) and most (&gt;90%) used IWM practices (4 or more) to some extent. In our survey, non-inversion tillage farms tended to have younger growers with more formal education and larger farms. This is the first multi-component survey in Ireland establishing long-term monitoring of grass-weed challenges, grower characteristics and IWM practices.

Understanding the resistance spectrum and underlying genetic mechanisms is critical for managing herbicide-resistant populations. In this study, resistance to acetyl CoA carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors was investigated in four suspected resistant populations of Alopecurus myosuroides (ALOMY-001 to ALOMY-004) and Lolium multiflorum (LOLMU-001 to LOLMU-004), collected from cereal production fields in Ireland. Glasshouse assays with three ALOMY-active herbicides [propaquizafop, cycloxydim (ACCase) and mesosulfuron + iodosulfuron (ALS)] or five LOLMU-active herbicides [pinoxaden, propaquizafop, cycloxydim (ACCase) and mesosulfuron + iodosulfuron, pyroxsulam (ALS)], and target-site resistance mechanism studies, based on pyrosequencing, were carried out in each of those populations. For A. myosuroides, Ile-1781-Leu ACCase mutation contributed to propaquizafop and cycloxydim resistance (shoot dry weight GR50 resistance factor (RF) = 7.5–35.5) in all ALOMY populations, and the independent Pro-197-Thr or Pro-197-Ser ALS mutation contributed to mesosulfuron + iodosulfuron resistance (RF = 3.6–6.6), in ALOMY-002 to ALOMY-004. Most of the analyzed plants for these mutations were homo/heterozygous combinations or only heterozygous. For L. multiflorum, phenotypic resistance to mesosulfuron + iodosulfuron (RF = 11.9–14.6) and pyroxsulam (RF = 2.3–3.1) was noted in all LOLMU populations, but the Pro-197-Gln or Pro-197-Leu ALS mutation (mostly in homozygous status) was identified in LOLMU-001, LOLMU-002 and LOLMU-004 only. Additionally, despite no known ACCase mutations in any LOLMU populations, LOLMU-002 survived pinoxaden and propaquizafop application (RF = 3.4 or 1.3), and LOLMU-003 survived pinoxaden (RF = 2.3), suggesting the possibility of non-target-site resistance mechanisms for ACCase and/or ALS resistance in these populations. Different resistance levels, as evidenced by a reduction in growth as dose increased above field rates in ALOMY and LOLMU, were due to variations in mutation rate and the level of heterozygosity, resulting in an overall resistance rating of low to moderate. This is the first study confirming cross- and multiple resistance to ACCase- and ALS-inhibiting herbicides, highlighting that resistance monitoring in A. myosuroides and L. multiflorum in Ireland is critical, and the adoption of integrated weed management strategies (chemical and non-chemical/cultural strategies) is essential.