Adjuvants generally referred to as "activators" most often affect the ability of a herbicide to penetrate a plant's leaf surface.
There are several types: Non-ionic surfactants (NIS) are the workhorses of activator adjuvants. NIS adjuvants usually aid in wetting, spreading, dispersing and emulsifying the spray to enhance herbicide activity.
NIS products do their task by combining water-loving and oil-loving properties in the same molecule. Foaming is a common trait of many NIS products; high-quality non-ionic surfactants have antifoaming agents added.
Crop oil concentrates (COCs) are petroleum-based products that work primarily as material spreaders and to help spray penetrate the waxy coating on leaf cuticles. Since oil doesn't readily mix with water, COC adjuvants are formulated with 15-20% surfactant or emulsifier.
COCs are generally more effective than other adjuvants under conditions of low humidity and high temperature, or when weed size is at the upper end of the herbicide label.
Methylated seed oils (MSOs) are highly refined oil concentrates from oilseed crops (soybeans, sunflowers, etc.). They are the most active of the activator adjuvants and perform better than other products on large or on drought-stressed weeds.
Poor-quality MSOs often contain free (unreacted) methanol, which can lower the cost but also increase the potential for poor crop response.
Fertilizers are recommended on many postemergent herbicide labels. Ammonium sulfate (21-0-0-24S), ammonium polyphosphate (10-34-0) and urea ammonium nitrate (28-0-0) are often used to enhance the activity of herbicides that are absorbed into a plant by the process called "ion-trapping." The ammonium ions create a pH difference between the outside and inside of the plant's cells.
Adding ammonium to a spray mixture may also lower the pH. Ammonium sulfate serves as a conditioner for hard water, since the sulfur component binds with free salts.
Sources: Compendium of Herbicide Adjuvants, published by Southern Illinois University; Terra Industries' Riverside Adjuvant Technical Handbook; and Crop Advantage Adjuvants, from MFA, Inc.