Originally written by Gared Shaffer, former SDSU Extension Weeds Field Specialist.
With the rise of herbicide-resistant weeds, the use of an adjuvants is also on the rise and may be necessary to help control resistant weeds. Also, when weather conditions are not ideal at the time of application, adjuvants may help increase application efficacy for controlling weeds. Adjuvants are classified into two categories: activator adjuvants and modifier adjuvants. Activator adjuvants are products that help the herbicide move into the target plant to overcome the barriers that impede movement of the herbicide from the leaf surface to the cell. Activator adjuvants are oils, surfactants and fertilizers. Modifier adjuvants alter the application characteristics of the spray solution. Modifier adjuvants are anti-foaming agents, compatibility agents and drift control agents. The Weed Science Society of America defines a spray adjuvant as “any substance in an herbicide formulation or added to the spray tank to modify herbicidal activity or application characteristics.” There are some products that have adjuvants added to the formulation and need no additional adjuvant. Other products usually require to add adjuvant(s) for better application.
To select the best adjuvant there are several factors to consider.
- What does the pesticide label call for?
- What does the adjuvant claim to do?
- What is the cost of the adjuvant?
- What adjuvants are available at your location?
There are different requirements and best use practices for different herbicides on each federal label. If the label gives the option for either crop oils or non-ionic surfactant, use of the nonionic surfactant is a better choice under “normal” weather conditions and when weeds are within label guidelines. If the weeds are stressed and possibly outside of label recommendations for height or growth stage, then oil concentrates are preferred. If the label allows, oil concentrates would be the preferred choice for grass control. Nitrogen fertilizer is used only if the herbicide label recommends it. If there is a possibility of crop injury due to the current conditions at spraying, nonionic surfactants work better than oil concentrates. It is always a good idea not to use oil concentrates with plant growth regulator herbicides such as dicamba and 2,4D for improved crop safety.
Spray additives are not regulated by the EPA in the same way herbicides, insecticides and fungicides are. Pesticide adjuvant manufacturers are not required to prove or demonstrate that the spray additive product will fulfil the claims of functionality or that it will be consistent from one application or the next. The Chemical Producers and Distributors Association has a record of all voluntary certification of adjuvants.
A list of certified adjuvants can be found at the Council of Producers & Distributors of Agrotechnology website.
A list of certified and uncertified spray additives can be found in the Compendium of Herbicide Adjuvants, 13th Edition.