May 20, 2010

This spring as you prepare to apply your postemergence weed control, many herbicides will recommend using an additive to improve herbicide efficacy and/or reduce drift. These spray additives, also called tank-mix adjuvants, are materials added to a tank mix to aid or modify the action of an agricultural chemical or the physical characteristics of the mixture.

Adjuvants are classified into two categories: activator adjuvants and modifier adjuvants. Activator additives increase the postemergence activity of herbicides, usually by increasing herbicide movement into the leaf tissue. They include oils, surfactants, and fertilizers. Modifier additives alter the application characteristics of the spray solution and include anti-foaming agents, compatibility agents, and drift control agents.

Role of Additives

Additives are commonly used with postemergence herbicides because they increase herbicide penetration of the treated plant surface and improve herbicide performance. Additive response varies with the herbicide, weed species, and environmental conditions. It is important that additives not be indiscriminately added to the spray mixture.

The degree of weed control and the potential for crop injury are both influenced by additives. Additives that increase weed control also may increase crop injury. There can be a fine line between increased weed control and increased crop injury.

The 2010 Guide for Weed Management in Nebraska includes reference tables that summarize adjuvant recommendations for corn, soybean and sorghum burndown and postemergence herbicides on pages 34-36. (This resource is updated annually and available online and in print.)

Product Certification Voluntary

Spray additives are not regulated by the EPA in the same way as herbicides, insecticides, and fungicides. As a result, the manufacturer is not required to demonstrate that the product matches the claims of functionality, nor that the formulation is consistent between batches.

The Chemical Producers and Distributors Association began a certification program in 2001 to increase the consistency of adjuvant product performance. The program is voluntary. Manufacturers may submit products for certification, and must meet 17 separate standards that are based on scientific standards developed by ASTM International.

Some herbicide manufacturers recommend the use of certified adjuvants. The language on the pesticide label will read, “When an adjuvant is to be used with this product, [name of registrant of the pesticide] recommends the use of a Chemical Producers and Distributors Association certified adjuvant.” The list of certified adjuvants is available at http://www.cpda.com/Adjuvant-Certification-Program.

For a comprehensive list of spray additives (certified and non-certified) see a Compendium of Herbicide Adjuvants, available from: Bryan Young, Southern Illinois University, 1205 Lincoln Drive, Carbondale, IL 62901. Further information is available on the Web at www.herbicide-adjuvants.com.

Mark Bernards
Extension Weeds Specialist