Stability of herbicide solution systems under temperature variations across different rice-growing seasons

To explore the temporary storage of paddy herbicide solutions due to weather changes, operational scheduling conflicts, and other factors, the stability of six herbicides for paddy fields, including pretilachlor 300 g/L EC, butachlor 60% EC, cyhalofop-butyl 100 g/L EC, florpyrauxifen-benzyl 3% EC, metamifop + cyhalofop-butyl 10% EC, and MCPA + bentazone 38% AS was clarified under temperature variations among different rice-growing seasons. Herbicide solutions were prepared in ultrapure water corresponding to a field spray volume of 450 L/hm² and stored in dark for 24 h. The storage conditions simulated diurnal temperature fluctuations during early- (22/15 °C, 25/18 °C), middle- (29/22 °C, 31/24 °C), and late-season (33/26 °C, 36/27 °C) rice application periods. Then, their physical, chemical, and bioactivity stability were evaluated. Physical stability results showed phase separation in solutions of pretilachlor 300 g/L EC and cyhalofop-butyl 100 g/L EC. Under early-season rice temperature regimes, phase separation initiated later but progressed to its maximum volume more rapidly compared to middle- and late-season conditions. No phase separation was observed in the other four herbicide formulations. Regarding to chemical stability, there were no significant differences (P > 0.05) in the mass concentrations of active ingredients in all solutions within 24 h after preparation. Bioactivity of solutions stored for 24 h was comparable to that of immediate application after preparation. For instance, metamifop + cyhalofop-butyl 10% EC, applied at the dosage of 2250 mL/hm², provided over 95% control of barnyardgrass at 15 and 30 days after application. MCPA + bentazone 38% AS applied at 1500 mL/hm² achieved 100% control of broadleaf weeds and sedges at 15 and 30 days after application, with no observed phytotoxicity to rice. These results indicate that the tested paddy herbicide solutions exhibit satisfactory chemical stability within 24 h after preparation under conventional spray volumes, without compromising the stability of their bioactivity.