no. 0.559

Herbicide Surfactants and Adjuvants

Quick Facts...

• Adjuvants and surfactants are materials added to spray solutions to improve the performance of crop protection compounds (herbicides).
• Based on chemical composition, surfactants can be divided into the following five groupings:
  1. non-ionic surfactants (NIS)
  2. crop oil concentrates (COC)
  3. nitrogen-surfactant blends
  4. esterified seed oils
  5. organo-silicones
• Not all oils are created equal with respect to surfactant performance and they should be evaluated carefully before using.

Adjuvants and surfactants are spray solution additives, and are considered to be any product added to an herbicide solution to improve the performance of the spray mixture.

Examples of adjuvants include compatibility agents (used to aid mixing two or more herbicides in a common spray solution), drift retardants (used to decrease the potential for herbicide drift), suspension aids (used to aid mixing and suspending herbicide formulations in solution), spray buffers (used to change the spray solution acidity), and surfactants.

Surfactants (surface active agents) are a type of adjuvant designed to improve the dispersing/emulsifying, absorbing, spreading, sticking and/or pest-penetrating properties of the spray mixture. Pure water will stand as a droplet, with a small area of contact with the waxy leaf surface. Water droplets containing a surfactant will spread in a thin layer over a waxy leaf surface.

Because postemergence herbicide effectiveness is greatly influenced by plant factors such as age, size and the growing conditions encountered before application, herbicide performance can vary. A way to minimize the variations in postemergence herbicide performance is to use an adjuvant or surfactant in the spray solution. Adjuvants, specifically surfactants, generally improve the effectiveness of postemergence herbicides. Typically, surfactants are not added to herbicides that are soil applied (pre-emergence).

The surfactants listed below are categorized into groups or classes based on their chemical composition and how they work. Generally, five surfactant classes are recognized. Table 1 lists their general usage.

Surfactant Class Definitions

Non-ionic surfactants are comprised of linear or nonyl-phenol alcohols and/or fatty acids. This class of surfactant reduces surface tension and improves spreading, sticking and herbicide uptake.

Crop oil concentrates are composed of a blend of paraffinic-based petroleum oil and surfactants. This surfactant class reduces surface tension and improves herbicide uptake and leaf surface spreading.

Nitrogen-surfactant blends consist of premix combinations of various forms of nitrogen and surfactants. They generally are used with herbicides recommending the addition of ammonium sulfate or 28 percent nitrogen. These surfactants reduce surface tension and improve leaf surface spreading.

Esterified seed oils are produced by reacting fatty acids from seed oils (corn, soybean, sunflower, canola) with an alcohol to form esters. The methyl or ethyl esters produced by this reaction are combined with surfactants/emulsifiers to form an esterified seed oil. These surfactants reduce surface tension and improve herbicide uptake by improving herbicide distribution on the leaf surface.

Organo-silicones are usually silicone/surfactant blends of silicone to non-ionic or other surfactants; a few within this classification are composed entirely of silicone. These surfactants provide a tremendous reduction in surface tension and spread more than conventional surfactants. In addition, this class of surfactant provides improved effectiveness through maximum rainfastness.

The addition of oils as surfactants in spray solutions is a relatively common practice, depending on the type of herbicide and the weeds involved. As such, it is important to note that not all oils provide the same surfactant effectiveness. In general, three types of oils are commonly referred to as surfactants: vegetable seed oils, crop oil concentrates, and esterified seed oils.

Vegetable seed oils are a blend of vegetable oil (cottonseed, soybean) and surfactants. These surfactants exhibit good crop tolerance but do not have good spreading, sticking or pest-penetrating properties.

Crop oil concentrates are a blend of paraffinic oil (petroleum based) and surfactants. These surfactants exhibit good spreading and penetrating properties but crop tolerance may be a problem.

Esterified seed oils are comprised of a methyl or ethyl ester of a vegetable seed oil (sunflower, soybean, corn, canola) combined with a surfactant/emulsifier. These spray solution additives have good spreading and pest-penetrating properties and convey good crop tolerance. However, these additives generally are more expensive than vegetable seed oils or crop oil concentrates.

Why Should You Select a Specific Surfactant Type?

The active ingredient is the agent in an herbicide formulation that has a specific effect on a weed. Select a surfactant that will best complement the action of the active ingredient and formulation type chosen for the specific application conditions. Consider the following points:

• What are the environmental conditions before herbicide application?
  • Conditions not ideal for plant growth (hot and dry) do not favor optimum herbicidal activity.
  • If an herbicide must be applied under less than ideal conditions, a surfactant (crop oil concentrate, esterified seed oil or organo-silicate) may greatly improve performance.
• What environmental conditions are likely to follow herbicide application?
  • Not all surfactants are equal with respect to degree of rainfastness.
  • Surfactant classes providing rapid rainfastness include esterified seed oils, organo-silicates and most nitrogen-surfactant blends.
• What are the leaf-surface characteristics of the plant(s) to be protected or controlled?
  • Thick and waxy leaf surfaces, hairy or narrow-leafed (grasses) plants are generally less susceptible to herbicide phytotoxicity than broad-leafed plants with little or no hair.
  • The addition of a surfactant may greatly improve herbicide performance where the leaf surface is thick and waxy, hairy or narrow-leafed.
  • In addition to improving the herbicidal activity on undesired plants, surfactants also can enhance the potential for increased phytotoxicity on desirable plants. In order to achieve maximum herbicide performance and lessen the potential for unwanted crop injury (phytotoxicity), always read and follow the herbicide label instructions on surfactant selection and use.
• What cost concerns are there?
  • Surfactant costs vary widely. In general, non-ionic surfactants and crop oil concentrates are the least expensive of the surfactant classes, followed by nitrogen-surfactant blends, esterified seed oils and organo-silicates.

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