Assessing factors influencing the release of hexazinone from clay-based formulations

Abstract

New designs of pesticide formulations based on organoclays are receiving increasing attention in reducing offsite movement of pesticides in the environment and in prolonging the efficacy of soil-applied pesticides. In the present article, we report the results of laboratory and field experiments conducted to evaluate the influence of the saturating cation, the sorbent : herbicide ratio, and the type of preparation on the controlled release properties of organoclay-based formulations of the herbicide hexazinone. Two organoclays with different affinities for hexazinone (hexadecyltrimethylammonium-exchanged Arizona montmorillonite, HDTMA-SA, and phenyltrimethylammonium-exchanged Arizona montmorillonite, PTMA-SA), two herbicide loadings (10% and 20% AI), and three types of preparation were assayed. Laboratory experiments showed that hexazinone formulations based on HDTMA-SA displayed slow-release properties in water, retarded herbicide leaching through soil columns, and maintained a herbicidal efficacy similar to that of the currently available commercial hexazinone formulation (wettable powder). In contrast, PTMA-SA formulations released the herbicide instantaneously and did not display slow-release properties. High organoclay-herbicide ratios and the use of organic solvent made the interaction between HDTMA-SA and the herbicide more intimate and reduced the release rate of hexazinone, suggesting that a range of release and leaching behaviours can be achieved by selecting the type of preparation and the herbicide loading in the formulations. A field experiment confirmed the usefulness of HDTMA-SA formulations of hexazinone to reduce herbicide leaching while maintaining weed-control efficacy.