Why does the label say to include an adjuvant?

Defining the terms

Firstly, let’s work through the definitions, because we need to all be on the same page before we can get into the fun details. You have probably heard these terms many times before – adjuvants, non-ionic surfactants, spreaders, stickers, penetrants, emulsifiers, wetting agents etc. But what exactly are they? And, for instance, which are suited for use on turfgrass when including with a pesticide spray mix?

Adjuvants is the general term for materials added to spray solutions to improve the performance of plant protection compounds, like herbicides, insecticides, miticides and some fungicides. They form a broad range of products including surfactants, spreaders, penetrants, stickers, crop oils, anti-foaming agents and compatibility agents.

Surfactants are a type of adjuvant, and the most commonly used by turfgrass managers. The word surfactant comes from the contraction of "SURFace-ACTive-AgeNT", but they are also known as spreaders, penetrants, stickers, emulsifiers, or wetting agents. A surfactant is a chemical substance that increases the spreading, retention and/or penetrating properties of a liquid by lowering its surface tension. There are four main types of surfactants: Anionic surfactants, Cationic surfactants, Non-ionic surfactants and Amphoteric surfactants.

Non-Ionic Surfactants (NIS) are water soluble chemical and lipid compounds, comprised of linear or nonyl-phenol alcohols and/or fatty acids, that are not molecularly charged (positive or negative) and are non-reactive. Non-ionic surfactants are most commonly used as soil wetting agents because of their universal fit, and a high degree of turf safety.

Anionic Surfactants are those in which the active portion of the molecule attached to the lipophilic (‘fat loving’) segment has a negative ion (anion) charge. These are also used in blends of soil wetting agents with high penetrating abilities due to the negative charge.

Cationic Surfactants are those in which the active portion of the molecule attached to the lipophilic (‘fat loving’) segment has a positive ion (cation) charge. These were the first type of soil wetting agents but carried a risk of phytotoxicity.

Amphoteric Surfactants are neutrally charged as they carry both a positive and negative charge. They can form positive or negative charges when in solution, depending on the pH of the water.

In professional turf management, the most common types of adjuvants used are spreader and penetrants:

Spreader type adjuvants are used where good coverage for short periods is the primary objective. Most often used in conjunction with foliar applied products to increase the spread over the treated surface and increase sticking and herbicide uptake. For example, the addition of an adjuvant is recommended on the MONUMENT® and CASPER® Herbicide labels to ensure full coverage across difficult to control weeds such as sedge due to their waxy leaf coating. Another example is BARRICADE® Herbicide, which when mixed with MONUMENT® Liquid Herbicide is supported by a surfactant to move evenly across emerged plants of Winter Grass.

Penetrant type adjuvants are used with products to improve the transfer of active ingredients from the leaf surface to its interior tissues. They increase uptake and result in greater herbicide activity on difficult-to-penetrate weeds. Some soil wetting agents are also classed as penetrants allowing for greater evenness of penetration. For example, when treating Fairy Ring with VELISTA® Fungicide, a penetrant helps to move the active into the zone of activity evenly.

Always refer to the product label in order to choose the correct additive for a specific product.

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Understanding Adjuvants & Surfactants in Turf Management

How does a surfactant work?

Pure water will stand as a droplet, with only a small area of contact with a leaf’s waxy surface due to its cohesive force. However, water droplets containing a surfactant will spread in a thin layer over a waxy leaf surface, which improves its adhesive force. The reason for this is that surfactants reduce the surface tension, or the tendency of the water molecules to adhere to each other. This enables the water droplet to cover a greater leaf surface area, while also allowing movement inwards to tighter areas like leaf sheaths and plant crowns. This is critical when we are looking to control weeds in grasses or transitioning from overseeding, such as ryegrass removal when using MONUMENT® Liquid Herbicide.

Surfactants Water Droplet Diagram

Figure 1: Water beads and how they are affected by surfactants

How do they do that?

There are many different types of surfactants, but in general, they are made up of a long chain hydrocarbon group on one end that is “fat loving” (lipophilic) and on the other end a more “water loving” (hydrophilic) group of atoms form a rounded head, as represented in the diagram below (Figure 2).

Surfactants Lipophilic Adjuvant Diagram

Figure 2: Components of a surfactant molecule

Surfactant molecules move between water molecules forcing them further apart. They do this by moving to the surface of the water droplet where they form a ‘micelle’ layer allowing the product to be more evenly dispersed on a surface (see Figure 3).

Surfactants Droplet Surface Tension Diagram

Figure 3: Interaction of hydrophilic and lipophilic parts of a surfactant to reduce surface tension and spread the droplet.

The lower the surface tension in a spray solution, the better the coverage, allowing more product to reach its target and vastly improving efficacy. Surface tension decreases until the concentration reaches a certain point, and once this point is reached then adding more surfactant will add no further benefit.

What benefits do you get from using an adjuvant on turfgrass?

Using an adjuvant when recommended with a herbicide, insecticide, miticide or fungicide is intended to either enhance the effectiveness of the product, or improve the ease of application. Adjuvants do this by improving retention, spreading, and leaf uptake, while minimising undesirable product loss.

The benefits from using an adjuvant on turfgrass

Let’s get into the detail of each of these benefits:

Retention

Droplets with a high surface tension will be more likely to bounce off target surfaces while those with a low surface tension will tend to spread on contact and be retained. A spray solution with spherical shaped droplets on contact with a surface are said to have a high surface tension. Droplets that readily deform and spread on contact have a lower surface tension, and are more likely to stick or be retained on the leaf surface.

Surfactants Low Surface Tension Diagram

Figure 4: Low surface tensions tends to be retained on contact.

Spreading

As outlined previously, a surfactant enables the water droplet to cover a greater leaf surface area, while also allowing movement inwards to tighter areas like leaf sheaths, plant crowns along with soil positioning. This ensures that more of the product reaches the areas that it needs to.

Leaf Uptake

When an adjuvant is added to a herbicide solution, the result is more complex than simply lowering the surface tension of the solution. They can also alter the permeability of the leaf cuticle – meaning that the herbicide’s active ingredient is more easily able to pass through the outermost protection layer of the plant. Adjuvants form a bridge between unlike chemicals such as oil and water, or water and the wax on a leaf surface. The result can be an increase not only in surface contact and reduced runoff, but also increased leaf penetration.

Surfactants Increasing Leaf Penetration Diagram

Undesired loss

Reducing surface tension to the lowest level possible may not always have beneficial effects. Droplets may run off the target surface as they coalesce due to the overly low surface tension or where a high-volume application causes droplets to run together. To prevent this, follow the label guidance on water volume and surfactant rate to get the best result.

Tank mixing with Syngenta products

Some products require no additional adjuvants to perform well, but some do perform better with adjuvants – this is detailed on the product label. To be able to recommend an adjuvant on the label, study results must be provided to the regulator to demonstrate the added benefit that this will have on the end result.

The labels of CASPER® Herbicide and MONUMENT® Herbicide, as well as AGADOR® and HIGRAN® Miticides, recommend the inclusion of a non-ionic surfactant by a volume/volume percentage.

The general Syngenta recommendation, depending on the label, is to use AGRAL (600 g active per litre) non-ionic surfactant at 0.42% v/v or 0.5% v/v of a 1000 g of active ingredient loaded product at 0.25% v/v.

This can easily be calculated as per the examples below:

0.25% v/v = 250 mL of a 1000 g/a.i/L product per 100 L of spray mix
0.42% v/v = 420 mL AGRAL® per 100 L of spray mix
0.50% v/v = 500 mL AGRAL® per 100 L of spray mix

Using the appropriate adjuvant can aid in delivering maximum product performance and plant safety. However, if the wrong adjuvant is used, it can increase the risk of poor product performance and injury to the plant. Always refer to the product label in order to choose the correct additive for a specific product.