Improved efficiency of a divalent herbicide in the presence of clay, by addition of monovalent organocations

A severe decrease in efficacy of contact herbicides such as diquat (DQ) is caused by dust accumulated on the weeds. Such inactivation was explained by the adsorption of herbicides on dust particles. As a result of this inactivation, the rates of pesticides needed for efficient pest control are usually larger than the recommended rates, which increases the expenses, and pose a real danger to the environment and to the ecological balance of watersheds. We proposed the hypothesis that the addition of a monovalent organic cation to the herbicide formulation might improve the efficacy by lowering the amount of herbicide that is inactivated by the sorption to clay minerals. This hypothesis was tested on lettuce and pepper plants dusted with clay, using the commercial formulation of diquat (“Reglone”) and adding the monovalent organic cations acriflavin (AF) and mepiquat (MQ). We observed that a clay mineral with a high cation exchange capacity (CEC), such as montmorillonite, reduced the herbicidal activity of DQ more than a lower CEC clay as sepiolite. However, addition of 5–20 mM concentrations of AF or MQ may overcome the influences of a clay dust on the activity of a cationic herbicide. Such addition allowed the obtainment of good herbicidal results with less than 1/3 of the normal used amount of herbicide. The results of our research point out to a mechanism that may lower the needed doses of contact herbicides, due to a competitive adsorption on the clay–dust particles between the pesticide and monovalent organic cations that leaves more herbicide available to act on the weed.