Using adjuvants with herbicides
An adjuvant is an additive that is intended to improve the effectiveness of a herbicide. Some herbicides require no additional adjuvants to perform well, but some perform better with adjuvants, and this is usually detailed on the product label. Many adjuvants have been developed to help herbicides to cover, stick to and penetrate the leaves of target plants.
The Australian Pesticide and Veterinary Medicine Authority recognises two categories of adjuvants:
- adjuvants that enhance product efficacy
- adjuvants that improve the ease of application.
Adjuvants that enhance product efficacy
Wetters/spreaders
Wetters/spreaders enhance spreading and adhesion of spray droplets over target surfaces by reducing the surface tension of the herbicide formulation, and include:
- non-ionic surfactants – these are non-reactive in that they are not negatively or positively charged. They remain on the leaf once dry and allow rewetting after rain, permitting additional pesticide uptake.
- anionic surfactants – have a negative charge.
- cationic surfactants – have a positive charge.
- amphoteric surfactants
- organo-silicate surfactants
- acidified surfactants
Stickers
Stickers increase adhesion of herbicides to leaf surfaces, and can be:
- latex-based
- terpene/pinolene
- pyrrolidone-based
Penetrants
Penetrants improve the transfer of active ingredients from the leaf surface to its interior tissues and include:
- mineral oils
- vegetable oils
- esterified vegetable oils
- organo-silicate surfactants
- acidified surfactants
Extenders
Extenders enhance the amount of time the active ingredient remains toxic by increasing its resistance to environmental degradation, and include:
- ammonium sulphate
- menthene-based extenders
Humectants
Humectants increase the density/drying time of a herbicide spray, including:
- glycerol
- propylene glycol
- diethyl glycol
Adjuvants that improve ease of application
Acidifying/buffering agents
Acidifying/buffering agents adjust the pH of alkaline or acidic water and minimise decomposition of the herbicide through alkaline hydrolysis.
Anti-foaming/de-foaming agents
Anti-foaming/de-foaming agents reduce or suppress the formation of foam in the spray tank preventing foam overflow, and include:
- dimethopolysiloxane
Compatibility agents
Compatibility agents permit the mixing of different herbicides by preventing antagonism between their ingredients in the spray solution, and include:
- ammonium sulphate
Drift control agents
Drift control agents alter the viscoelastic properties of the spray solution yielding a coarser spray with greater average droplet sizes, and include:
- polyacrylamides
- polysaccharides
Dyes
Dyes are commonly used to detect untreated areas or avoid treating an area twice.
Water conditioners
Water conditioners prevent reactions between hard water ions in spray solutions and suppress formation of precipitates or salts, and include:
- ammonium sulphate
Factors affecting adjuvant use
Crop safety
Addition of an adjuvant can reduce herbicide selectivity and thereby increase potential damage to crops and pastures. This is not an issue for fallow and pre-emergent herbicides.
Effectiveness or activity
Adjuvants are added to increase effectiveness of herbicides, however, the wrong type or rate of adjuvant can reduce herbicide effectiveness, by causing, for example, decreased herbicide retention on leaves.
Water hardness
Hard water can lead to poor mixing of chemicals with water. This particularly occurs with emulsifiable concentrates. High levels of calcium and magnesium ions bind with amine formulations, causing them to be less soluble and therefore less effective.
Water temperature
Low water temperature can lead to jelling in the tank. High-concentration herbicides may not mix and surfactants may perform poorly.
