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

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.