Imazethapyr and imazapic runoff under continuous and intermittent irrigation of paddy rice

• The intermittent flooding of rice reduced water runoff.
• The intermittent flooding of rice reduced the mass of herbicide transported.
• The runoff decrease was due to storage of rainwater inside the experimental plot.
• Herbicide dissipation in water was not affected by irrigation management.
• The highest concentrations of herbicides in runoff water was observed when runoff occurred closer to herbicide application.

In the Brazilian state of Rio Grande do Sul, rice (Oryza sativa) is predominantly grown using a continuous flood, which can cause herbicide runoff when fields are over-irrigated or heavy rainfall occurs. Proper irrigation and drainage management can reduce herbicide transport to the environment. The objective of this study was to evaluate the effect of different irrigation practices on water runoff and the transport and dissipation of the herbicides imazethapyr and imazapic. The experiments were conducted in the 2007/08 and 2008/09 growing seasons at the experimental area of the Universidade Federal de Santa Maria. The treatments comprised two irrigation management systems: continuous flooding of an approximate 10-cm flood depth and intermittent flooding where the flood was allowed to fully subside before being reestablished to a 10-cm height. The amount of water runoff and the rates of herbicide dissipation and transport were evaluated. Owing to increased storage of rainwater, intermittent flooding reduced water runoff by 47% and runoff of imazethapyr and imazapic by 89 and 77%, respectively, relative to the continuously flooded control. Imazapic was significantly more persistent than imazethapyr, but there was no effect of irrigation management on herbicide persistence in water.