Slow release formulations of alachlor 1 Performance of slow-release formulations of alachlor

3 Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] is a 4 widely used herbicide that has been identified as a groundwater contaminant. Several 5 slow-release formulations (SRFs) of this herbicide have been examined for their 6 capacity to reduce its leaching and enhance weed control. A field experiment was 7 performed to determine alachlor leaching and bioefficacy in: (i) a commercial 8 formulation, Alanex, (ii) SRFs based on ethylcellulose (EC) microencapsulation or 9 phosphatidylcholine (PC)-montmorillonite complexes. Alachlor distribution within the 10 top 30 cm of the soil layer was modeled with the pesticide-leaching model PEARL, 11 using release constants for the SRFs obtained from in-vitro experiments. Compared to 12 the commercial formulation, leaching to the 20to 30 -cm depth was reduced by 33% 13 for EC and 25% for PC-clay formulations, but only in the latter was herbicide 14 bioefficacy retained. The poor bioefficacy of the EC formulations can be explained by 15 their very slow release properties, which may nevertheless confer an advantage under 16 prolonged heavy rainfall. The model yielded good predictions for the residual amounts 17 of herbicide at 59 and 99 d after treatment (DAT). At 191 DAT, the amounts of alachlor 18 at 0to 20 -cm depth were overestimated by 13 to 23%, indicating limitations of the 19 model at longer times, attributed to: (i) the adaptation time of the microbial population 20 for herbicide degradation and (ii) the effect of herbicide sequestration in the soil matrix, 21 a phenomenon denoted as “aging”. 22 23 Slow release formulations of alachlor 3 Abbreviations: SRF: slow-release formulation; EC: ethylcellulose; PC: 1 phosphatidylcholine; a.i.: active ingredient; RMSE: root mean square error; DAT: days 2 after treatment; AEW: acetone/ethyl acetate/water 3