Behaviour of the Herbicide Quinclorae in a Riceplant-grow Lysimeter

Abstract

In order to elucidate the environmental behaviour of the herbicide quinclorac (3,7-dichloroquinoline-8-carboxylic acid) applied to rice paddies, [3-¹⁴C]quinclorac (specific activity: 1.50 MBq/mg) was applied at the rate of 300g a.i./ha to a lysimeter (0.564 m ID × 1 m soil depth) simulating the rice paddy conditions. The ¹⁴C-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 ¹⁴CO₂ 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 ¹⁴C lost from the surface exhibited the background level, suggesting that quinclorac was stable microbiologically and chemically in this condition. No ¹⁴C-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 ¹⁴C-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 ¹⁴C 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.

Key Words:

• [¹⁴C]quinclorac
• lysimeter
• mineralization
• leaching
• bioavailability
• soil-bound residue