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Do adjuvants boost herbicide efficacy?

with Peter Jones, senior chemist, VICCHEM

The old saying ‘oils ain’t oils’ certainly holds when it comes to adjuvant products available to enhance the performance of herbicide actives and other agricultural chemicals.  

Peter Jones, VICCHEM’s development and technical services manager, says the array of adjuvant products available to growers is extensive, and it pays to have a good working knowledge of when and how to use them in a tank mix that includes herbicides.

“An adjuvant is any material added to a tank mix that aids or modifies the action of the herbicide or the physical integrity of the mix,” he says. “Adjuvants contribute to the WeedSmart Big 6 tactic to ‘Optimise spray efficacy’ to maximise the level of weed control from every application.”

There are three categories of adjuvants – oils, surfactants (aka wetting agents) and buffers/water conditioners. Oils help oil-soluble herbicides penetrate through the weed’s waxy cuticle and reduce the evaporation rate of spray droplets from leaves. Surfactants reduce the surface tension of the spray droplets to enhance leaf coverage and adhesion to the leaf surface. They also assist in the uptake of water-soluble actives into the leaf. Buffers and water conditioners maintain chemical equilibrium in the tank to prevent the loss or decay of the actives.

“The use of appropriate adjuvant products in a tank mix can make a noticeable difference to the percentage of weeds that survive and set seed following a herbicide application,” says Peter. “This is how adjuvants can preserve yield and potentially reduce the risk of herbicide resistance evolution.”

Not every tank mix needs adjuvants, and adjuvants are not required to optimise the performance of every herbicide.

The WeedSmart Big 6 strategy embraces herbicide and non-herbicide weed control tactics to keep weed numbers low in cropping systems. The latest weed control tactics and technologies will feature at WeedSmart Week in Port Lincoln on 29–30 July 2024.

How do I know if an adjuvant should be included in a particular mix?

‘Tank mix science’ can be complex. It pays to consult a few sources of information and advice for your specific situation. Start with the herbicide label. In the past, it was common for a herbicide label to have a very general statement regarding adjuvant use, such as ‘add 1% oil’. An increasing number of herbicide labels now include detailed instructions on the use of adjuvants and the appropriate rate.

Where there is a claim that an adjuvant enhances the performance of another agrichemical, this claim is backed by data submitted to the APVMA as part of the product’s registration.

If the tank mix includes multiple herbicide actives, and possibly including a fungicide and or insecticide, be aware that conflicts can easily arise. Consulting with the chemical supplier and local field agronomist can help make important decisions about what can be safely included in a mix.

Remember that the recommended adjuvants are included in the tank mix to enhance the performance of the actives or to help protect the active from degradation.

Do I always need to include an adjuvant?

No. Many herbicides do not benefit from the addition of any adjuvants. Using the wrong adjuvant can have adverse outcomes such as phytotoxicity causing crop damage or physical incompatibility causing spray blockages.

Where an adjuvant is recommended on the herbicide label, this indicates that the efficacy of the herbicide on the target weeds will most likely be enhanced when the adjuvant is included in the spray mix. The outcome may vary depending on many factors, including the weed’s size and health, and environmental conditions.

The increased level of weed control may be moderate (e.g. wetting agents added to glyphosate) or very significant (e.g. HASTEN Spray Adjuvant added to clethodim to treat ryegrass). While a small increase in efficacy may seem hard to justify, a shift from 90% to 95% control might not improve crop yield, but could decrease the potential for herbicide resistance to evolve.

Buffer and water conditioner adjuvants should always be considered if the water source is not rainwater. These adjuvants can counter the negative impacts of water pH and hardness.

Do adjuvants influence spray droplet delivery?

As a general rule, oil adjuvants tend to increase the spray droplet size, and surfactants (or wetting agents) tend to decrease the droplet size. This should be considered when planning a spray operation to minimise the production of fine (driftable) droplets.

Adjuvants are formulated to ensure spray-ability and droplet formation, minimise droplet shatter and evaporation, and maximise leaf-to-active contact and absorption.

The choice of nozzles, sprayer set-up and operation do the heavy lifting when it comes to minimising spray drift, and adjuvants can provide an additional element of spray safety.

Other resources

Adjuvants’ role in combatting herbicide resistance

Adjuvants – Oils, surfactants and other additives for farm chemicals

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