Behaviour of the herbicide quinclorac in a rice plant-grown lysimeter

In order to elucidate the environmental behaviour of the herbicide quinclorac (3,7-dichloroquinoline-8-carboxylic acid) applied to rice paddies, [3- super(14)C]quinclorac (specific activity: 1.50 MBq/mg) was applied at the rate of 300g a.i./ha to a lysimeter (0.564 m ID x 1 m soil depth) simulating the rice paddy conditions. The super(14)C-labeled herbicide was applied 22 days after transplanting and the rice plants were grown by the conventional cultivating method for 88 days until harvest. The amount of super(14)CO sub(2) evolved from the surface of the lysimeter soil was 0.71% of the original radioactivity up to the 14th week after the application, while that of volatile super(14)C lost from the surface exhibited the background level, suggesting that quinclorac was stable microbiologically and chemically in this condition. No super(14)C-activity was detected in the leachates from the lysimeter during that period, indicating a very slow downward movement of the herbicide in the soil. The average super(14)C-activities detected in straw, ear without grains, and chaff after harvest amounted to 0.41, 0.10, and 0.19 mg/kg quinclorac equivalents, respectively, whereas that in brown rice grain was 0.15 mg/kg, indicating that it is far less than the maximum residue limits (MRL) of 0.5 mg/kg quinclorac set by Japan. That is, about 95% of the originally applied super(14)C remained in the 30-cm layer from the surface after harvest, whereas a very small fraction was distributed in the different parts of rice plants.