The Weed Science Society of America defines an adjuvant as any substance in a herbicide formulation or added to the spray tank to modify herbicidal activity or application characteristics. This article will discuss the types of adjuvants commonly used with postemergence products to improve herbicide absorption, and describe the mechanisms by which they function. Other additives commonly used with postemergence herbicides, such as drift retardants, compatibility agents, etc., will not be covered in this paper. These products often are called utility adjuvants.
Surfactants are a class of adjuvant widely used with herbicides in corn and soybean production.
The lipophilic portion of the molecule typically is a long alkyl chain. The two portions of the surfactant molecule allow it to associate with liquids having wide ranging solubilities. In the figure the surfactant allows the oil to go into suspension in water by creating an emulsion. There are several types of surfactants available, but most products marketed for use with postemergence herbicides are classified as nonionic because they have a neutral charge. Cationic surfactants are formulated with several herbicides, including most glyphosate products.