Phytotoxicity of Herbicide Vapor

Abstract

It has been known for more than 40 years, since the introduction of modern herbicides, that certain compounds can produce phytotoxic vapor during and after spray application (Baskin and Walker 1953; Zimmerman et al. 1953) and thus contaminate sensitive crops and natural vegetation. These volatile herbicides do not belong to one specific chemical type or category of use, but are often, although not exclusively, either an ester-based formulation or intended for soil incorporation. One advantage of herbicides formulated as esters is that they are generally more efficacious than the equivalent salt formulation due to the higher solubility of nonpolar compounds in the waxes of the leaf surface, which in turn leads to increased uptake and hence phytotoxicity. The result is that the same degree of weed control can be achieved with a smaller amount of ester-based herbicide than is possible with the salt. Another advantage of ester formulations is that they can be tank-mixed with other pesticides so that a crop can be sprayed with the minimum number of operations. Herbicide vapor itself may play a role in weed control because it is probably well-distributed in the crop canopy immediately after application and thus available for assimilation by weeds that were not adequately sprayed. In the case of soil-incorporated herbicides, vapor may facilitate the distribution through soil air spaces (Hance et al. 1973), as well as contribute to long-term weed control by its slow release from soil (Swann and Behrens 1972).