In this article, learn how adjuvants can be used in spray solutions to help improve performance of foliar-applied herbicides. The companion article on Factors Affecting Herbicide Performance discusses biological and environmental barriers that herbicides must overcome to control a target plant.
What are Adjuvants?
Adjuvants are compounds that can be added to a spray solution to improve herbicide performance. Adjuvant is a broad term and includes non-ionic surfactants, crop oils, organosilicones, anti-foaming agents, drift retardants, compatibility agents (used to aid mixing two or more herbicides in a common spray solution), spray buffers (used to change the pH of the spray solution) and more. Adjuvants can improve herbicide performance by altering the herbicide solution to facilitate more thorough and even coverage on the plant surface, increase herbicide penetration through the plant cuticle (link to cuticle photo), enhance absorption or translocation of the herbicide’s active ingredient, and/or minimize any problems associated with spray solution. Adjuvants are already included in some herbicide formulations (e.g., Milestone® herbicide), or they may be purchased separately and added into a spray solution (e.g., Opensight® herbicide) prior to use. Read the herbicide label to determine if addition of a surfactant is required or recommended.
Adjuvants may be classified in a variety of ways, such as by their:
function (activator or utility)
chemistry (such as organosilicones)
source (vegetable or petroleum oils)
This adds to confusion about which adjuvant to select in different situations. Figure 1 shows adjuvants listed by two primary functions, activator or utility. For a more complete listing of available adjuvants in the United States, see the Compendium of Herbicide Adjuvants at http://www.herbicide-adjuvants.com/index.html
The addition of adjuvants allows an applicator to customize the spray solution for a particular situation. However, there is no universal adjuvant that will improve performance of all herbicides for all weeds under any environmental condition.
Although adjuvants can help improve herbicide performance under adverse weather conditions, they are not a substitute for optimal weather or growing conditions. Moisture stress, and its impact on plant physiology (translocation) and metabolic processes within the plant, will significantly reduce herbicide efficacy regardless of the adjuvant system, and high winds can still cause off-site drift.
Adjuvants that Increase DROPLET COVERAGE AND SPRAY RETENTION
Surfactants, or surface-acting agents, are a broad category of adjuvants that facilitate and enhance the absorbing, dispersing, spreading, sticking, or penetrating properties of herbicides. Surfactants are most often used with herbicides to increase spray solution coverage on a leaf surface. Because of the high surface tension of water, spray mixture droplets can maintain their roundness and sit on the trichomes (leaf hairs) or waxy cuticle without much of the herbicide contacting the leaf. The pesticide mixture then becomes susceptible to run-off from the leaf surface or degradation by sunlight. Reducing the surface tension of the spray solution means that spray droplets will spread beyond their initial contact area. Increasing the coverage area increases herbicide absorption (See Uptake, Absorption, and Translocation). Stickers are materials that can increase droplet adherence to the leaf surface. Stickers are often water-soluble polymers, acrylic latex or resins. Wetting or spreading agents are often combined with stickers to improve coverage.
There are two types of surfactants based on their chemistry: Non-ionic surfactants and the silicone/organosilicone compounds.
1) NON-IONIC SURFACTANTS (NIS) are composed of alcohols and fatty acids, have no electrical charge, and are compatible with many herbicides. Non-ionic surfactants are all-purpose and most widely recommended with some herbicides used for weed control on natural areas, prairies, and rangeland.
Advantages of non-ionic surfactants (NIS):
Reduced surface tension of the spray solution and increased coverage and wetting ability of the applied spray
“Universal” fit with many herbicides
Less costly than other types of surfactants
Stable in cold water
A disadvantage of NIS:
May not correct problems associated with water quality such as pH or hard water minerals.
2) SILICONE-BASED SURFACTANTS, also known as organosilicones have superior spreading ability. Some of these surfactants are a blend of NIS or methylated seed oil (MSO) and an organosilicone, whereas others are only an organosilicone. The combination of a NIS or MSO and an organosilicone surfactant can increase absorption into a plant and may reduce the need for a long rainfree period following application of the herbicide solution (See Rain Following Application). Applicators are urged to exercise caution when applying silicones. The surfactant’s extreme spreading ability may lead to droplet coalescence and subsequent runoff. Even so, organosilicone surfactants have potential to give good coverage by chemical rather than by physical means of using high volumes of carrier. Organosilicone surfactants are weed and herbicide specific just like other adjuvants.
Advantages of organosilicone-based spray adjuvants include:
Low application use rates of 4 to 6 ounces per 100 gallons of water
Reduced spray droplet “bounce” from foliage
Enhanced coverage of low spray application volumes
Increased rate and amount of herbicide solution absorbed by plants
Some disadvantages of silicone-based adjuvants include:
Increased rate of spray droplet evaporation
Eye exposure hazards
High foaming potential
Poor stability in low pH spray mixes
Spray solution droplet coalescence and possible runoff from leaf surface
Adjuvants That Increase CUTICLE PENETRATION
Crop oil concentrates (COC) and methylated (and ethylated) seed oils (MSO) are considered penetrants. This type of surfactant can improve cuticular penetration by softening or dissolving cuticular waxes and allowing herbicide movement through the leaf surface. Penetrant adjuvants are often a complex mixture of surfactant and oil (paraffinic petroleum or modified vegetable oil).
Adjuvants That Increase SPRAY DROPLET DRYING TIME
Humectants are usually water-soluble materials that resist drying time of a spray droplet. Slowing the rate at which droplets dry will allow more time for absorption to occur. Glycerin, propylene glycol, diethylene glycol, polyethylene glycol, urea and ammonium sulfate are common humectants. Oil-based compounds, like COC or MSO also resist drying. Not all herbicides are compatible with oil-based humectants. Theoretically, adding a humectant should improve herbicide performance under hot, dry conditions; however, if weeds are under severe moisture stress herbicide performance rarely increases with the addition of any spray adjuvant.
Selecting an Adjuvant
Herbicides differ widely in chemical properties, therefore they differ in requirements for spray additives. The best source of information to determine the type of adjuvant to use is the herbicide label. Herbicide manufacturers conduct extensive research to determine how to achieve the most consistent performance with their products, so be sure to follow label recommendations. Some herbicides such as Milestone and Transline® herbicides, have adjuvants in the formulated product. Labels may also be very specific in their recommendation of adjuvants. For example, the label for Opensight® herbicide states "applications of Opensight must include either a crop oil concentrate or a non-ionic surfactant. Apply non-ionic surfactant at 0.25% v/v (1 quart per 100 gallons spray solution) or 0.5% under arid conditions."
Since a large number of adjuvants are available for use with herbicides, how do you know whether a specific product will perform adequately?If you need to add an adjuvant, chose one that will best complement the action of the active ingredient and formulation type chosen for the specific application conditions. Adjuvant selection should be based on label recommendations, basic herbicide manufacturers “Approved Adjuvant Lists”, and field research. Following are some general guidelines to consider when given a choice of adjuvants on the herbicide label.
If both crop oil concentrate and non-ionic surfactant are listed, then use a non-ionic surfactant under normal weather conditions when weeds are small and within label guidelines for application. Use crop oil concentrate if weeds are stressed from dry weather or when weeds are more mature.
If recommended on the herbicide label, include oil concentrate for control of grasses.
If the potential for desirable vegetation injury is a concern, then use non-ionic surfactant instead of oil concentrate if both are recommended on the label.
Include nitrogen fertilizer in the spray mixture only if it is recommended on the herbicide label.
Remember that adjuvants are not under the same registration guidelines as pesticides. The U.S. Environmental Protection Agency (EPA) does not register or approve spray adjuvants sold as separate products. However, there is a series of six acute toxicity tests that are required to test toxicity of the formulated pesticide product, which includes adjuvants and other inert ingredients if they are present in the formulated product. Various states Department of Agriculture (such as the Washington State Department of Agriculture) register adjuvants for use in their state.
Consider the following before adding an adjuvant to your spray solution:
Read the label and add the type and amount of adjuvant recommended for optimum herbicide performance.
What are environmental conditions prior to herbicide application?
Conditions that are not ideal for plant growth (hot and dry) do not favor optimum herbicidal activity. If an herbicide must be applied under less than ideal conditions, an adjuvant may improve performance.
What environmental conditions are likely to follow herbicide application?
Not all adjuvants are equal with respect to degree of rainfastness. Adjuvant classes providing good rainfastness include esterified seed oils, organosilicones and most nitrogen-surfactant blends.
What are leaf-surface characteristics of the plant(s) to be protected or controlled (barriers to herbicide penetration)?
Thick and waxy leaf surfaces, hairy or narrow-leafed (grasses) plants are generally less susceptible to herbicide phytotoxicity than broad-leafed plants with little or no hair. The addition of an adjuvant may improve herbicide performance where the leaf surface is thick and waxy, hairy, or narrow-leafed. In addition to improving the herbicidal activity on undesired plants, adjuvants can also enhance the potential for increased phytotoxicity on desirable plants.
What is the chemistry of the water you will be using to make the spray solution?
Hard or soft water, or low or high pH can influence herbicide performance and may require the use of an adjuvant(s).
What method will you be using to apply the herbicide?
Are you applying the herbicide as a foliar spot-spray, broadcast boom application, cut-stump or hack and squirt? For example, with foliar applications you need penetration of the herbicide solution through the leaf cuticle. This may require a very different adjuvant than mixing with bark oil for a cut stump or low volume basal bark application.
If you are mixing two different herbicides, what are the interactions of the herbicides and are they compatible?
Read the label. The compatibility of some herbicide combinations are listed on the label in addition to information on adding ‘compatibility’ adjuvants.
What cost concerns are there?
Adjuvant costs vary widely. In general, non-ionic surfactants and crop oil concentrates are the least expensive of the surfactant classes, followed by nitrogen-surfactant blends, esterified seed oils and organosilicones.
When purchasing agricultural adjuvants:
Purchase an adjuvant that is manufactured and marketed for agricultural use with herbicides.
Do not purchase products made for household use. Soaps and detergents are essentially surfactants. However, these are not generally effective because they contain low concentrations of surfactant (10 to 20%) compared to agricultural products (50 to 95%). They may also combine with hard water to form scum and precipitates that affect spray performance, and cause excess foaming in the tank.
Purchase adjuvants on the basis of percent active ingredient. Many pesticide labels call for the use of a surfactant with 80% or greater active ingredient. Read the label carefully to determine the active ingredients listed on the surfactant label. Do not consider isopropyl (isopropanol) and other alcohols or water as active ingredients. Some products list these solvents as part of the active ingredient or as functioning agents. Most spray adjuvant labels will clearly show active ingredients, inactive ingredients, and principal functioning agents as a percentage of the total.
Be wary of grandiose claims. Some adjuvants have limited field testing. There are no “miracle” adjuvants. Most adjuvants are good products and will increase the performance of foliar applied pesticides when used at the recommended rate suggested on the label.
The bottom line is that adjuvants can help improve herbicide performance under adverse weather conditions, and are required with some herbicide formulations. However, it is not possible to substitute adjuvants for good growing and application conditions. Adjuvants may improve herbicide movement into the cuticle or leaf, but the problem lies in the effects of moisture stress on physiology of the target plant. If the herbicide requires translocation to meristemic regions or requires active metabolic processes in the plant, moisture stress can significantly reduce herbicide efficacy regardless of the adjuvant system. (See Factors Affecting Herbicide Performance)
Adjuvants for Enhancing Herbicide Performance. Pennsylvania State University. Revised 2018 Online http://extension.psu.edu/pests/weeds/control/adjuvants-for-enhancing-herbicide-performance
Compendium of Herbicide Adjuvants 11th Edition. Online http://www.herbicide-adjuvants.com/
Dewey, Steve. Unpublished laboratory notes, Utah State University
Jordon TN. 2001. Adjuvant use with herbicides: Factors to Consider. Purdue Univ. Coop. Ext. Service IPMWS-7. Online https://www.extension.purdue.edu/extmedia/WS/WS-7.html
Plant and Soil Science eLibrary. Influence of Adjuvants on Absorption of Foliar-Applied Herbicides http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1056648673&topicorder=3&maxto=5
Tu M, Hurd C, Randall JM. 2001. The Nature Conservancy Weed Control Methods Handbook, Chapter 8. Adjuvants. Online http://www.invasive.org/gist/handbook.html
Zollinger R. 1999. North Dakota State University, Personal communication, and North Dakota Weed Control Guide.
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Published May, 2014; Updated June 2019