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Pattern and Rate of Dissipation of Pretilachlor and Mefenacet in Plow Layer and Paddy Water under Lowland Field Conditions: A Three-year Study
Ferdinand F. FAJARDO, Kazuhiro TAKAGI, Masumi ISHIZAKA and Kenji USUI
J. Pestic. Sci.
25, 94-100 (2000)
Field trials were conducted at the National Institute of Agro-Environmental Sciences experiment station from 1995 to 1997. Each trial was carried out from mid-May to early July every year. Determining the pattern and rate of dissipation of pretilachlor and mefenacet in paddy soil at three different depths (0-1, 1-5 and 5-10cm) and in paddy water was the primary objective of the experiments. Half-lives (DT50) of the herbicides were calculated using both the simple first order kinetics (SFOK) and the biphasic first order kinetics (BFOK). The pattern and rate of dissipation of pretilachlor in both soil and water were quite different from that of mefenacet. At the 0-1cm soil layer (oxidative), dissipation of pretilachlor was quite rapid during the first 3 weeks but slowed down afterwards. In case of mefenacet, dissipation was a steady decline until the last sampling day. The DT50 in this layer was between 9 to11 days and 7 to10 days for mefenacet and pretilachlor, respectively. At the lower soil layers (1-5 and 5-10cm; reductive) pretilachlor appeared to leach from the upper layer within the first two weeks and then quickly disappeared afterwards. Faster degradation of pretilachlor was noted in these reductive soil layers. On the other hand, mefenacet dissipation declined steadily at these layers. Also, leaching of mefenacet was not as evident as in pretilachlor. Mefenacet concentration in paddy water peaked over the second to third day and its dissipation was rapid until the fourth week. Dissipation pattern of pretilachlor was a steady curvilinear decline. In paddy water, DT50 was about 3.3 to 4.1 days for mefenacet and 3.0 to 3.6 days for pretilachlor. Calculating the half-life in soil (0-1cm layer) using SFOK appears sufficient for mefenacet while a more complex model such as the BFOK describes that of pretilachlor better. The half-life and dissipation pattern of both herbicides did not vary considerably across years. There was no evidence of residue build-up due to continuous application of the herbicides in the same plots.
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