Behaviour and fate of 14C-2,4-dichlorophenoxyacetic acid herbicide in rice field water

A rice field model agroecosystem was set-up in a radioactive glass house at the Malaysian Institute for Nuclear Technology Research MINT, Dengkil Complex, Bangi, Selangor. Three containers were filled-up with rice field soil obtained from the Muda rice granary area and stream water. Rice plant Oryza sativa var indices Telok Kecai of approximately 30 days old was planted in the lysimeter. The herbicide 2,4-Dichlorophenoxyacetic acid (in short 2,4-D) was applied at the recommended rate of 1 kg/ha for post-emergence control of annual and perennial broad-leaved weeds in rice, as practiced by the Malaysian farmers. Being very soluble in water, Carbon-14 labeled 2,4-D herbicide was applied to surface water in each lysimeter. Surface water was top-up to a similar level throughout the experiment. The level of radioactivity was monitored in surface water until 74 days after application (DAA). Radioactivity in surface water declined very rapidly. At 1 DAA, 77.7 X 1 0-1% of the applied radioactivity was measured in surface water. At 4 DAA, only 8.9 x 10-3% of radioactivity left, followed by 1.1 x 10-3% at 6 DAA, and 0.7 X 10-3 % at 8 DAA. Radioactivity in surface water was fluctuating between 0.1 x 10-3 % and 0.6 x 10-3% of the total applied radioactivity, from 11 DAA until 74 DAA. At 57 DAA, 0.6 x 10-3% of applied radioactivity was measured in water samples possibly due to its uptake in a very small slimy aquatic plant found growing in surface water. Under the tropical climate of Malaysia, the half-life of 2,4-D herbicide in rice field water was less than 4 days. (Author)