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Clearing up confusion on adjuvants and additives
by Brent Pringnitz
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April 27, 1998 - The label on many postemergence herbicides call for a variety of additives to increase the product's effectiveness.  These may be in the form of fertilizers, surfactants, and adjuvants.   However, there remains confusion over what constitutes a surfactant, a crop oil concentrate or and adjuvant.  Since these materials are not regulated by consistent labeling laws, it becomes difficult to determine what category a product falls into.

As manufacturers combine many of these materials into a single packaged product, it can be difficult to determine if the product matches what the label requires.  Reference materials may available from the dealer or manufacturer that will list the characteristics of materials in their products.  Another source of information on herbicide adjuvants is A Compendium of Herbicide Adjuvants prepared by George Kapusta at Southern Illinois University.  It contains 411 entries from 37 companies and is available for $3.

The term "crop oil concentrate" means the material is designed for application to a crop.  It may or may not actually contain oils from a crop.  They may be either vegetable oils or phytobland petroleum oils.   Because of this confusion, the United Soybean Board (USB) has designed a guide to many common COC's that lists whether they contain vegetable or petroleum oils.  Many times adjuvants that contain vegetable oils, such as soybean, sunflower, canola, or cotton, are designated "vegetable oil concentrate" or "methylated seed oil."

For further information, there is also an online slide show describing types of adjuvants and their uses.

An adjuvant is something which is added to a spray solution to increase the effectiveness of the active ingredient.  They may be packaged and formulated with the herbicide product or they may be added to the spray solution as a tank mix.

Activator adjuvants
Surfactants are used to increase the dispersing, spreading, wetting, or other properties of the liquids. This term is derived from the words surface active agent.  Surfactant molecules are comprised of two parts:  a strong polar group that is attracted to water, and a non-polar group attracted to non-aqueous materials, such as oil.  Of the types of surfactants, the nonionic surfactants are the most common in agricultural sprays.  It should also be noted that surfactants make up a large class of adjuvants, but not all adjuvants are surfactants.

A crop oil is an emulsifiable petroleum oil-based product that may contain up to 5 percent surfactants with the remainder being a phytobland oil.

A crop oil concentrate (COC) is comprised of a non-phytotoxic (not causing injury) mineral and/or vegetable oil.  Many times a COC also contains up to 20 percent surfactant.  The principal function of these materials is to aid in moving the herbicide across the leaf cuticle and reduce the surface tension of the spray droplets.  Crop oils are also effective at increasing spray retension on leaf surfaces and reducing drying times.  This allows more time for the herbicide to penetrate the leaf.

Spray modifiers
These materials modify the spray solution in the tank or on the target.  Spreaders increase the surface area of the spray droplets.  Spreaders are usually nonionic surfactants.  Stickers cause the spray droplets to adhere or "stick" to the target to decrease run-off or wash-off during a rainfall.  A thickening agent increases the spray solution viscosity.  Most spray drift reduction products use a thickening agent.

Utility modifier
Anti-foam agents assist with reducing foaming in the tank during agitation and spraying.  To assist with mixing multiple products that may separate after mixing, a compatability agent is added.  This is most common when applying products in combination with liquid fertilizers.  Buffering agents are used to increase the solubility and dispersion of a product when used with extremely acid or alkaline water.

Selecting adjuvants
How do you determine which type of additive to use? Also, since a large number of additives are available for use with herbicides, how do you know whether a specific product will perform adequately? Herbicides differ widely in chemical properties, therefore they differ in requirements for spray additives. Improper selection of an additive for a specific use may reduce weed control or increase crop injury. The best source of information to determine the type of additive to use is the herbicide label. Herbicide manufacturers conduct extensive research to determine how to achieve the most consistent performance with their products, thus label recommendations should be followed. However, there are many situations where the user will need to make a decision among different additive options provided on the label.

What criteria should be used in choosing between additives listed on the label? The two primary factors to consider are how the additives will influence weed control and crop safety. A COC will generally enhance herbicide activity more than a surfactant; however, COC’s often will increase the potential for crop injury. In situations where weeds are under stress or reaching the maximum size specified on the herbicide label, the higher activity of the COC may be necessary to achieve acceptable control.

The relative margin of crop safety for the herbicide and the vigor of the crop should also be considered when selecting a spray additive. The potential for crop injury may be increased when using COC instead of a surfactant. Crops that are under stress due to the environment or other factors are more susceptible to herbicide injury, thus an additive that reduces herbicide stress would be beneficial under these conditions. In some situations, either weed control or crop safety may be compromised when selecting an additive.

Tank-mix partners also will influence additive selection. Read labels of all products in the tank mix to determine any restrictions regarding additive use. For example, the Accent label recommends the use of either a COC or surfactant when used alone. However, if Accent is tank-mixed with Clarity the label states not to substitute a COC for the surfactant.

Since the manufacture of additives is not regulated, a grower may be overwhelmed by the number of different products sold for this purpose. Most products sold at agricultural outlets are reputable and should perform the task they are designed. Compare the quantity of active ingredients in similar types of additives to help determine value. Be wary of any product that makes exaggerated claims such as ‘This additive will allow you to reduce herbicide rates by 25 to 50%’.

Adjuvants in Postemergence Herbicide Applications, a publication of the American Soybean Association.
A Guide to Soy-based Adjuvants, United Soybean Board.


Prepared by Brent A. Pringnitz, extension program specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.