Improving herbicide performance with adjuvants

Landscape and turfgrass managers use herbicides to control weeds quickly and effectively. To increase the effectiveness of herbicides, they sometimes use adjuvants, which are non-herbicidal additives. An adjuvant's primary function is to modify the physical nature of herbicides or herbicide spray solutions for the purpose of increasing a herbicide's safety, range of usage and, in many cases, increasing weed control.

You always should use adjuvants according to the herbicide and adjuvant label directions. Some manufacturers add adjuvants to the formulated herbicide product. In such cases, more adjuvant probably will be wasteful and unnecessary. Likewise, do not haphazardly decide the amount of adjuvant to use. A “dab” or a “glug” is not an effective way to measure adjuvants. Excessive amounts of an adjuvant can injure plants; insufficient amounts may not achieve the desired result.

Because adjuvants can increase efficacy, some applicators may be tempted to use them as a tool to “cheat” on herbicide rates. However, while it may be possible in some circumstances to do this, we do not advise reducing herbicide rates. To understand why cutting rates is not advisable, let's look at the types, uses and functions of adjuvants, as well as how they are marketed and manufactured.

Defining adjuvants

“Adjuvant” is a broad term and includes surfactants, spreaders, stickers, penetrants, drift-control agents, crop oils, emulsifiers and compatibility agents. They may be formulation adjuvants or spray adjuvants.

Formulation adjuvants are those the manufacturer incorporates into pre-packaged herbicide products. Formulation adjuvants are not specifically intended to improve the efficacy of a herbicide. Rather, they modify the physical nature of a herbicide to improve its storage characteristics or to make it easier to use. They may increase shelf life or herbicide compatibility, improve solubility, decrease volatility or in some other way improve storage, handling or mixing characteristics. Types of formulation adjuvants include emulsifiers, dispersants, stabilizers, compatibility agents, buffering agents and antifoaming agents.

Spray adjuvants may be present in a preformulated herbicide or you may add them to the herbicide during tank mixing. If you add an adjuvant during mixing, it most likely is one that is categorized as a spray adjuvant. These adjuvants are defined by the direct effect they have on weed-herbicide interaction. They modify the applied herbicide to increase its efficacy.

Many herbicides are pre-formulated by the manufacturer with spray adjuvants. Sometimes, these adjuvants are the secret inert compounds that differentiate one manufacturer's formulation from another's using the same active ingredient.

Surfactants are the most common types of spray adjuvants. The term “surfactant” is an acronym for “surface-active agent.” Surfactants reduce the surface tension between a spray droplet and the waxy surface of a leaf, which allows the droplet to cover more leaf surface area (see figure above).

The reduction in surface tension is possible because each molecule of surfactant has a lipophilic (fat-loving) and a hydrophilic (water-loving) portion. Because “like dissolves like,” the lipophilic portion is attracted to the waxy surface of the leaf, and the hydrophilic portion is attracted to the herbicide spray droplet. Thus, the surfactant allows the herbicide spray droplet to spread evenly over a surface that would normally repel it.

Greater coverage of the leaf surface offers important advantages: it decreases the chance that the spray droplet will roll off the leaf, and it increases the amount of herbicide absorbed by the leaf. Given these effects, it's not surprising that the most common use for surfactants is for post-emergence, foliar-absorbed herbicides, which depend heavily on plant uptake for their effectiveness.

The four main types of surfactants are: spreaders, stickers, spreader-stickers and wetting agents.

  • Spreaders increase spray coverage of the target plant surface by decreasing the surface tension of a spray droplet (as described above), thus decreasing the angle of adhesion (see figure at right).

  • Stickers increase the adhesive nature of the spray solution, resulting in greater adherence to the plant surface.

  • Spreader-stickers are essentially combinations of stickers and spreaders. Stickers, spreaders and spreader-stickers are used with contact insecticides and fungicides for which complete coverage is critical. In addition, insecticides and fungicides often serve as protectants, and the longer they remain on the leaf surface, the longer they can protect.

  • Wetting agents promote greater spray solution coverage of the leaf surface by decreasing the repelling nature of the water-based spray solution and the oil-based waxy leaf surface. Wetting agents come in three types: anionic, cationic and nonionic. Anionic and cationic wetting agents separate in water to form negative and positive ions, respectively. They are used only occasionally and have little value in turf and ornamental pest management. Nonionic wetting agents, however, do not form charged particles in spray solutions. This allows them to be compatible with many herbicides. They are especially good at helping herbicides penetrate the waxy cuticle of plant leaves. Thus, they are excellent aids to systemic herbicides that need good plant absorption and translocation to maximize effectiveness.

  • Crop oils, another group of spray adjuvants, are non-phytotoxic, light oils combined with a surfactant. Crop oils can increase herbicide absorption into the leaf cuticle layer and slow down drying. Increasing drying time allows the herbicide to be in the solution longer, which can lead to greater absorption. Crop oils are generally applied at 1 gallon per acre.

Crop-oil concentrates contain 80 to 90 percent petroleum-based oil (and surfactant) and have begun to replace traditional crop-oil formulations because they can be used at lower rates, usually around 1 quart per acre. Vegetable-oil concentrates are related products and typically contain 80 to 85 percent cotton or soybean seed oil, plus surfactant. They have more or less the same use and mode of action as crop oils and crop-oil concentrates, but be sure to consult the herbicide label to determine which oil product (petroleum or vegetable) you should use.

Can you decrease herbicide rates by using adjuvants?

There's no question that adjuvants are valuable tools for pest-control applications. But this fact has led some applicators to see if they can “cheat” on rates to save on pesticide costs. Is this possible? Maybe. In fact, you sometimes can achieve acceptable control with reduced rates and adjuvants. But we do not recommend that applicators try this, for several reasons.

First, the pesticide manufacturer has probably determined the best application rate already. Manufacturers must ensure that rates are high enough for effective control of target weeds. However, they also must keep rates as low as possible to minimize the potential for phytotoxicity to desirable plants (such as turf) and allay environmental concerns. To achieve this goal, manufacturers take into account possible herbicide mixtures and product formulations that include adjuvants as well as recommendations for adding adjuvants during mixing.

Also keep in mind that label rates are designed to ensure consistent control. Manufacturers don't want slight changes in application rates or small environmental variations to result in poor control. In other words, there is somewhat of a margin for error. Cutting rates, even with adjuvants, may reduce the margin for error and may result in control failures, despite prior successes.

Developing a herbicide costs between $35 and $50 million (with an additional $50 to $100 million to build a plant to manufacture the herbicide, in some cases). Much of this cost is devoted to research for creating an effective formulation and determining a label rate recommendation for the herbicide. If there is a way to reduce rates and retain effectiveness, it's likely the manufacturer has found it and will either incorporate an adjuvant into the formulated product, or include the use of adjuvants in their label recommendations.

Manufacturers want their products to work well and to be cost-effective. If they aren't, people won't use them. Thus, it is in their best interest to recommend the lowest effective rate. If achieving that necessitates that the user add an adjuvant, the label will say so.

Another precaution involves using adjuvant products that are new to the market. Trying to achieve acceptable control by using a lower herbicide rate and a new adjuvant is a gamble. It is possible that it could work. However, you must acknowledge the risk. Ask yourself the following questions: Has adequate research been conducted to determine if the adjuvant and herbicide are compatible? Will the combination bring about acceptable control? If it does not, can you afford the additional costs to retreat? Will the combination damage the turf? If it does, can you afford to replace the damaged areas?

The reason this is especially risky with new products is that adjuvants do not have to undergo strict scrutiny by the Environmental Protection Agency the way herbicides do. The result of this relative lack of regulation means that producing, registering and selling an adjuvant costs a fraction of that for a herbicide. There are hundreds of adjuvants on the market, and many have little to no scientific basis to validate the manufacturer's claims. Therefore, when a herbicide label suggests an adjuvant, a recognized brand may be the best choice. At the very least, if you wish to try a new adjuvant, do your homework and ask for independent research that backs up performance claims.

When deciding whether to use an adjuvant, always consult the herbicide label; it is the best source of information concerning the consistent and effective use of the herbicide. For example, an adjuvant may be needed only if a certain weed with a waxy leaf is present.

When a manufacturer claims that you can reduce rates if you use its adjuvant, be wary, and carefully assess the risk you are taking. You may want to treat only a portion of the area and evaluate the results. Evaluating a new treatment on a small area may take more time initially, but not having to reapply or recover from injury will save you time and money in the long run and will give you peace of mind as well.

Finally, remember that adjuvants are not the only way to improve control. Make sure you treat weeds at a susceptible stage, that environmental conditions are favorable for control, that you've properly calibrated your equipment and you're using it correctly, and that you're using the best herbicide for your needs and following the label closely.

Scott McElroy and Hennen Cummings are doctoral candidates, and Dr. Fred Yelverton is a professor of turfgrass science, all at North Carolina State University (Raleigh, N.C.).


The four main types of spray adjuvants are:

  • Stickers, which help spray droplets stick to plant surfaces.

  • Spreaders, which increase spray coverage of the target plant surface by decreasing the surface tension of a spray droplet, allowing it to spread out over a larger area.

  • Spreader-stickers, which are essentially combinations of stickers and spreaders.

  • Wetting agents, which promote greater spray solution coverage of the leaf surface by decreasing the repelling nature of the water-based spray solution and the oil-based waxy leaf surface. They help herbicides penetrate the waxy cuticle of plant leaves, and so are excellent aids to systemic herbicides that need good plant absorption and translocation to maximize effectiveness.

  • Crop oils, another group of spray adjuvants that are non-phytotoxic, light oils combined with a surfactant.

Formulation adjuvants modify the physical nature of a herbicide to improve its storage characteristics or to make it easier to use. Types of formulation adjuvants include:

  • Emulsifiers, which enable tank mixing of products that may be water-insoluble.

  • Dispersants, which allow for viscous liquids to move through small openings in spray nozzles.

  • Stabilizers, which reduce chemical degradation that could occur in the spray mix.

  • Compatibility agents, which enable you to mix chemicals that might otherwise be incompatible.

  • Buffering agents, which resist large changes in pH in the spray mix that might affect chemical stability.

  • Antifoaming agents, which reduce foaming that can result from mixing and agitation in the tank.

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