By Scott McElroy, PhD
“There is nothing either good or bad–thinking makes it so.”
Adjuvants, or spray additives, are oft-misunderstood chemicals. End-users either expect too much from the addition of an adjuvant, do not fully respect the need for an adjuvant, or are duped into buying with unproven promises. All of these problems can be solved with a short review of what adjuvants are and how they work.
“To be or not to be that is the question.”
The first step in understanding adjuvants is clear definitions. First as stated previously, adjuvants are simply spray additives that have beneficial properties of enhancing performance of the active ingredient or improving the performance of the spray solution. From this definition we see that adjuvants really are the overarching term for all additives to an agrichemical spray solution.
Within the classification, there are essentially two groupings–activator and utility adjuvants. Activator adjuvants enhance the performance of active ingredients of pesticides while utility adjuvants have some beneficial effect on the spray solution, but do not directly affect active ingredients performance. Let’s first explain utility adjuvants.
Utility adjuvants have numerous uses, but they basically improve the ease of applying the spray solution. Examples of utility adjuvants includes compatibility agents, defoamers, drift control agents, deposition agents, water conditioning agents, acidifiers, buffers, and colorants. Definitions of utility adjuvants are presented in Table 1. If a herbicide does not mix well, if you have hard water, or if you have to spray at a high rate of speed, utility adjuvants can be added to the spray solution to improve performance.
Activator adjuvants have a beneficial effect on the active ingredient. If you are applying a herbicide, this is the most important group of adjuvants. Without this group of adjuvants many herbicides will simply not work. Types of adjuvants include wetting-spreading agents, sticking agents, humectants, absorption agents, safener, synergist, and extender. In order for an activator adjuvant to improve the activity of an active ingredient it must affect one of three areas–absorption, translocation, or metabolism (the exception is extender adjuvants). Absorption is the movement of the active ingredient through the plant cuticle, translocation is the movement of the active ingredient throughout the plant vascular system, and metabolism is the breakdown of the active ingredient. By increasing absorption or translocation, or decreasing metabolism, an adjuvant can increase the activity of an active ingredient. The opposite effect would occur if the opposite occurs.
“What’s in a name? That which we call a rose.”
One term that has not been mentioned thus far is surfactant. “Surfactant” is often used synonymously with the term adjuvant; however, these terms are not the same. Adjuvant refers to any additive to an agrochemical spray mixture; surfactant is a term describing a classification of chemicals.
Surfactants
Surfactants, shortened from surface-active agent, is a classification of chemicals that produce physical changes in the interface of two dissimilar liquids- primarily hydrophobic (water-hating) and hydrophilic (water-loving) liquids. Hydrophobic liquids such as oil are primarily non-polar molecules that do not mix with water or other polar, hydrophilic liquids. Surfactants bridge the polarity gap by virtue of their own chemical structure.
Surfactants have both hydrophilic and hydrophobic portions to their chemical structure that allow them to bridge between dissimilar liquids (Figure 1). The hydrophobic portion is a long, hydrocarbon chain often referred to as the tail. Whereas, the hydrophilic portion is a short, carboxylic group referred to as the head. The tails of the surfactant molecules imbeds itself into the oil droplet forming a micelle while the head interacts with the water allowing the oil molecule to dispense in the water.
Due to his chemical action, many surfactants function as adjuvants, primarily activator adjuvants. Surfactants can decrease the surface tension of water allowing the spray droplet to spread and wet. Surfactants can also promote interaction of the spray droplet with the hydrophobic leaf surface allowing for greater active ingredient absorption.
“There was never yet philosopher that could endure the toothache patiently.”
To completely complicate the situation, much of what we have covered about adjuvants thus far you will not find on the adjuvant label. Instead you will find terms like “non-ionic surfactant,” crop oil concentrate,” or “modified seed oil.” This is because many of the spray activator adjuvants you purchase are really what can be classified as emulsified oils (see common adjuvants listed in Table 3).
These emulsified oils are a mixture of a 50-90% oil base with a surfactant added. Oil-based adjuvants such as these are the most common adjuvant type and are often wetter-spreaders with some sticker properties as well. The oil portion of the adjuvant improves interaction of the plant cuticle, while the included surfactant aids in interaction of the spray solution with the oil. This interaction improves overall uptake of the active ingredient.
So which adjuvant should you buy? That question is not easily answered. First, when you read the active ingredient section of an adjuvant label two words often appear, “proprietary blend.” Companies simply do not divulge the contents of their adjuvant because they do not want competitors to have their information.
Second, the Compendium of Adjuvants lists 523 spray adjuvants available for purchase from 39 companies in 2006. Trying to sort through such a vast number of products is impossible.
“What is decreed must be.”
So the best answer to which adjuvant to buy is often the one that is recommended on the herbicide label. Herbicide companies want their product to work, so they are going to tell you exactly how to use it. Research is conducted to determine exactly how much herbicide needs to be applied for weed control and desirable plant/turf safety.
Another answer to which adjuvant you should buy is to simply understand how the active ingredient works. If you are applying a foliar absorbed active ingredient with effective translocation, you do not need an adjuvant that extends soil life or that improves translocation. Most likely, the herbicide label will recommend an adjuvant to improve foliar absorption.
“Et tu, Brute?”
As stated before, the best adjuvant to use is the one recommended by the pesticide manufacturer on the pesticide label. Manufacturers extensively research to determine the exact dose with the proper adjuvants to use. No more, no less. So beware of any salesman or adjuvant claim that says you can reduce the pesticide rate if you use a given adjuvant. Buyer beware. Adjuvants are helpful, but they are not magic potions.
Scott McElroy, PhD, is assistant professor, turfgrass weed science, at Auburn University. He also serves as this magazine’s Technical Editor. Our apologies to Billy Shakespeare.
Table 1. Definitions of common utility adjuvants.
Compatibility agents: Improves the mixture and uniformity of the application liquid.
Defoamers: Eliminates or reduces foam in the application liquid.
Drift control agents: Reduces the driftable portion of the application liquid once sprayed.
Deposition agents: Improves the ability of the applied liquid to apply to the target.
Water conditioning agents: Reduces the interaction of ions in the spray solution to interact with the active ingredient.
Acidifiers: Lowers the pH of the spray solution.
Buffers: Aids the spray solution by preventing change in pH when other chemicals are added to the mixture.
Colorants: Changes the color of the spray mixture. Also referred to as dyes or paints used for marking spray patterns and areas.
Table 2. Definitions of common activator adjuvants.
Wetting-spreading agents: Lowers surface tension of the spray droplet increasing droplet coverage of the leaf surface.
Sticking agents: Viscous materials that improve adhesion of spray droplets to leaf surface.
Humectants: Reduce evaporation speed of the spray droplet on the leaf surface potentially improving active ingredient absorption.
Absorption agents: Improve movement of the active ingredient through the leaf surface. Improves absorption by softening or dissolving epicuticular wax or improving stomatal infiltration.
Safener: Reduces phytoxicity to desirable plants. Potentially increases metabolism of the active ingredient, reduces translocation, or reduces absorption of the herbicide.
Synergist: Increases the activity of the active ingredient. Potentially decreases metabolism of the active ingredient, reduces translocation, or reduces absorption of the herbicide.
Extender: Increases the longevity of the herbicide in the soil often by decreasing microbial activity.
Table 3. Common terminology seen on adjuvant labels.
Non-Ionic Surfactant: Surface active agent having no polar end groups. These surfactants are beneficial because they are effective in hard water.
Crop Oil Concentrate: An emulsifiable petroleum-based adjuvant containing 80% phytobland oil and 5 to 20% surfactant.
Vegetable Oil Concentrate: An emulsifiable vegetable oil-based adjuvant containing 80% vegetable oil and 5 to 20% surfactant.
Modified Seed/Vegetable Oil: An emulsifiable oil-based adjuvant containing 5-20% surfactant and the remainder chemically modified vegetable oil.