Performance of Slow‐Release Formulations of Alachlor

Abstract

Alachlor [2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide] is a widely used herbicide that has been identified as a groundwater contaminant. Several slow‐release formulations (SRFs) of this herbicide have been examined for their capacity to reduce its leaching and enhance weed control. A field experiment was performed to determine alachlor leaching and bioefficacy in (i) a commercial formulation, Alanex, and (ii) SRFs based on ethylcellulose (EC) microencapsulation or phosphatidylcholine (PC)–montmorillonite complexes. Alachlor distribution within the top 30 cm of the soil layer was modeled with the pesticide‐leaching model PEARL, using release constants for the SRFs obtained from in vitro experiments. Compared with the commercial formulation, leaching to the 20‐ to 30‐cm depth was reduced by 33% for EC and 25% for PC–clay formulations, but only in the latter was herbicide bioefficacy retained. The poor bioefficacy of the EC formulations can be explained by their very slow release properties, which may nevertheless confer an advantage under prolonged heavy rainfall. The model yielded good predictions for the residual amounts of herbicide at 59 and 99 d after treatment (DAT). At 191 DAT, the amounts of alachlor at 0‐ to 20‐cm depth were overestimated by 13 to 23%, indicating limitations of the model at longer times, attributed to (i) the adaptation time of the microbial population for herbicide degradation, and (ii) the effect of herbicide sequestration in the soil matrix, a phenomenon denoted as “aging.