Effect of weather conditions on efficacy and residue amount of two acetyl‐ <scp>CoA</scp> carboxylase inhibiting herbicides in spinach

Abstract The risk of contamination by the acetyl‐CoA carboxylase inhibiting herbicides fluazifop‐P‐butyl and quizalofop‐P‐ethyl is significant, particularly in short‐season vegetables like spinach. This study examined meteorological factors affecting their efficacy and residue levels in spinach. Field trials were conducted in Prague, Czech Republic, in 2022 and 2023, using Echinochloa crus‐galli seeds at 150 seeds/m 2 . Herbicides were applied at the four‐true‐leaf growth stage, with fluazifop‐P‐butyl at 110 g/ha and quizalofop‐P‐ethyl at 40 g/ha, under three irrigation regimes. Statistical analysis showed significant effects of temperature and irrigation on herbicide efficacy against E. crus‐galli and residue levels in spinach leaves. Fluazifop‐P‐butyl residues were approximately 10 times greater (287–480 μg/kg in 2022 and 76–91 μg/kg in 2023) than quizalofop‐P‐ethyl (25–42 μg/kg in 2022 and 5–9 μg/kg in 2023), even though its maximum residue level (MRL) is 10 times lower (20 μg/kg for fluazifop‐P‐butyl vs. 200 μg/kg for quizalofop‐P‐ethyl). Residues were notably higher in 2022 likely due to lower temperatures (mean temperature in the growing period 11.2°C), which slowed degradation of herbicides. Irrigation shortly after herbicide application (20 mm) reduced residue levels but decreased efficacy. Warmer conditions in 2023 enhanced efficacy, with a mean daily temperature of 6.3°C higher during the application‐to‐harvest period. Herbicide degradation was more temperature‐dependent than efficacy. Fluazifop‐P‐butyl is unsuitable for spinach due to the high risk of exceeding MRL values, while quizalofop‐P‐ethyl can be used safely under varied weather conditions without exceeding MRL.