Adjuvants with Herbicides. When and Why They Are Needed.

Herbicides used for weed control in agriculture are applied over a wide range of environments, geographies, times, spray carriers, application equipment, pesticide tank mixes, and target species. They often need assistance to maximize weed control and/or minimize risks during the application. Only a few herbicides are formulated with an adjuvant package that can optimize the application in every scenario. Therefore, the applicator often needs additional adjuvants to optimize the herbicide application for the specific conditions of the area being treated.   

Selecting the correct adjuvant and adjuvant rate can be critical to the success or failure of a herbicide application. Not using an adjuvant, using the wrong adjuvant, or using an unnecessary adjuvant can be costly due to reduced weed control, crop damage, and/or extra expense over many acres. There are many adjuvants on the market with proprietary ingredients and advertisements that can quickly become overwhelming and confusing to those purchasing and/or applying them. In this publication, we discuss what adjuvants are, adjuvant categories, when and why adjuvants should be used, and recommendations for selecting an adjuvant. This publication provides an overview of adjuvants and their use with foliar herbicides across field crops in the United States and Canada. Specific adjuvant usage and rates vary across geographies and environments. Always read the herbicide and adjuvant labels and consult your local crop consultant or Extension specialist if you have questions about specific uses and rates.

What are Adjuvants?

An adjuvant is any material added to a herbicide spray solution that modifies the biological activity or physical characteristics of the spray solution. Adjuvants can be divided into the following categories: activators, water conditioners, spray solution modifiers, and products that are a combination of these categories. Water conditioners and spray solution modifiers may also be considered utility adjuvants. Each category is defined along with the definitions of the different types of adjuvants within each category.

An adjuvant can also be classified as a spreader, a sticker, or both. Spreaders are materials that increase the surface area of a leaf covered by droplets, while stickers are materials that increase the adherence of a droplet to its target surface.

1. Activator Adjuvants

Activator adjuvants increase herbicide activity through increased spray retention, droplet spreading, and/or absorption. The following are types of activator adjuvants.

a. Surfactants (Surface Active Adjuvants)

Surfactants is a shortened form of the phrase surface active adjuvants or surface active agents. Surfactants are materials that modify the interaction of the liquid and the surfaces with which they interact. This often refers to the interaction between a waxy or oily leaf surface and a water-based droplet of a spray solution. Adding a surfactant reduces the surface tension between the leaf surface and the spray droplet (Figure 1). The reduced surface tension increases the area of droplets contacting the leaf surface, thus increasing the potential surface area for herbicide penetration. Surfactants can also modify the interaction of water-based and oil-based liquids within the spray tank. Two common types of surfactants are non-ionic and organo-silicone. Organo-silicone surfactants typically increase droplet spreading and surface coverage further than non-ionic surfactants.

b. Crop Oil Concentrate (COC)

Crop oil concentrates are petroleum oil-based products with emulsifiers to allow dispersion in a water-based tank mix. COC formulations contain a minimum of 80% oil mixed with 15 to 20% surfactant. COC adjuvants reduce the surface tension of the spray droplet on the leaf surface, increase retention, and increase the penetration of herbicides through waxy leaf cuticles.

c. Methylated Vegetable or Seed Oil (MSO)

MSO adjuvants consist of a methylated vegetable or seed oil (i.e., canola, cotton, linseed, soybean) mixed with an emulsifier to allow for dispersion in water. MSO adjuvants reduce the surface tension of the spray droplet, increase retention, and increase the speed of herbicide absorption. In general, MSOs are more aggressive in dissolving the leaf cuticle than COCs and result in greater and faster herbicide absorption into the plant. This can also result in greater crop injury compared to other adjuvant types.

d. High Surfactant Oil Concentrates (HSOC)

A high surfactant oil concentrate contains at least 50% of either petroleum oil or a methylated seed oil combined with 25 to 50% surfactant. Thus, HSOC products have properties of a surfactant and an MSO or COC that are desired for tank mixes that contain a herbicide requiring an oil concentrate and herbicides such as glyphosate that can be antagonized by an oil concentrate. Where the herbicide label does not provide specific guidelines for use, HSOC adjuvants should be used at half the rate (typically 0.5%v/v) of COC or MSO that is on the herbicide label to reduce the possibility of crop injury.

2. Water Conditioners

Water sources used for spray carriers will almost always contain contaminants that may interfere with herbicide solubility or activity. If the level of hard water cations or pH exceeds acceptable levels, remediation is needed to maximize herbicide activity.  Water conditioners are adjuvants that modify the water carrier properties to enhance the effectiveness of herbicides. The following are different types of water conditioners.

a. Ammonium Sulfate

Ammonium sulfate (AMS) is added to carrier water to reduce the interaction of hard water cations, such as calcium, magnesium, iron, and sodium, with weak acid herbicides such as glyphosate. When these cations bind with weak acid herbicides, the absorption and, thus, effectiveness of the herbicide can be reduced, especially under stressful environmental conditions. AMS dissociates into ammonium and sulfate ions when added to carrier water. The sulfate ions bind with hard water cations, preventing cation binding with the weak acid herbicide. Additionally, the ammonium ions bind with the weak acid herbicide, which is beneficial since this ammonium salt of the weak acid herbicide can increase foliar absorption. The addition of AMS will also reduce water pH. However, AMS does not reduce the pH as much as other adjuvants designed specifically for pH modification. 

b. Ammonium Sulfate Replacements

Many adjuvants or additives are sold as AMS replacements, such as citric acid, phosphoric acid, or the product of reacting urea with sulfuric acid. Many of these adjuvants tie up or bind hard water cations similar to the activity of AMS, while others effectively lower the water pH to a level in which weak acid herbicides remain in the acidic state and avoid binding with hard water cations. While many of these products can be equally effective in neutralizing hard water cations, they often lack the ammonium component of AMS that increases herbicide uptake and translocation in the plant.

c. pH Buffers

pH buffers are added to a spray solution when herbicides or other products are known to increase or decrease the pH of the carrier water. pH buffers help maintain the desired pH of the spray solution.  Generally, most herbicides are more effective in acidic solutions (~pH 5-7); however, acidic solutions can sometimes cause other problems such as increased volatility or herbicide degradation in the spray tank. Some herbicides, like sulfonylureas, are more water soluble in basic solutions (pH 7-9), allowing greater foliar absorption.

d. Volatility Reduction Agents (VRA)

Volatility reduction agents are required by EPA for all dicamba applications over the top of dicamba-resistant crops to reduce the risk of dicamba volatility. These products stabilize or increase solution pH to reduce dicamba volatility potential.  VRA products that are approved for use with dicamba can be found on the dicamba product label and/or website. 

3. Spray Solution Modifiers

Spray solution modifiers are products that alter the characteristics of the final spray solution mixture for enhanced ease of handling, product compatibility, or mitigation of off-target movement. The following are types of spray solution modifiers.

a. Antifoam Agents

Antifoam agents reduce or prevent the formation of excessive foam during mixing and agitation.  

b. Compatibility Agents

Compatibility agents allow the mixture of agrochemicals into a consistent solution, where the agrochemical formulations would otherwise not mix consistently. Compatibility agents can also increase compatibility in mixtures of fertilizers and other agrochemicals.

c. Drift Reduction Agents (DRA)

Drift reduction agents modify the spray solution to reduce driftable fine droplet formation during spray applications. Most drift reduction agents increase the viscosity (thickness) of the spray solution to produce relatively larger spray droplets. However, not all drift reduction agents are the same, as some can interact with certain low-drift nozzle types to create smaller droplets rather than larger droplets. Before using a new drift reduction agent and nozzle combination, consult with the manufacturers to ensure the combinations are compatible.

4. Adjuvant Mixtures or Combinations

Adjuvant mixtures are combinations of two or more adjuvant types in a single adjuvant product to increase the number of functions provided to aid in the herbicide application. Combination products is another term to describe adjuvant mixtures. Numerous adjuvant mixtures are marketed as premixed adjuvant packages. Examples include a mixture of ammonium sulfate and a surfactant or ammonium sulfate with a surfactant and a drift reduction agent.  

When to Use Adjuvants

Correctly using adjuvants is important because using an incorrect type or rate can result in poor weed control and/or crop injury. The appropriate use of an adjuvant, including adjuvant selection and the amount required, may vary by herbicide and environment. The herbicide label should be the first place to look to determine if an adjuvant is required, the correct type needed, and the recommended rate. If multiple herbicides are being applied, consult the label of each herbicide paying particular attention to any guidance on adjuvant and tank mix partners and the “do not” statements. Many Land Grant University Weed Control Guides list labeled adjuvants for each herbicide. The following variables should also be considered when selecting the appropriate adjuvant for an application.

Understand Carrier Water Properties

Knowing and understanding the initial carrier water properties is important. In known instances of hard water (presence of calcium, magnesium, iron, and/or sodium), water conditioning adjuvants have been shown to improve glyphosate effectiveness (Figure 2). However, selecting the most appropriate and economical rate of AMS can be challenging. For example, the AMS rate recommended for most glyphosate formulations is 8.5 to 17 pounds AMS per 100 gal of carrier water (or 2 to 3 lb/acre in some geographies). It is easy to assume that if 8.5 is good, 17 is better. But that is not necessarily true. The amount of AMS needed depends on the cation concentration of the carrier water. A greater rate is needed for water with a higher concentration of cations (harder water); thus, knowing your water’s specific hardness/cation concentration is necessary to determine the appropriate AMS rate. The same goes for using pH buffers and acidifiers - knowing the initial pH of carrier water is necessary to understand if a pH buffer or acidifier is needed, which one, and how much. Therefore, it is recommended that carrier water sources be tested at least once a year. Numerous private and public labs will test water for hardness and pH. Consult your local crop consultant or Extension specialist for a laboratory near you that offers spray water quality testing.

Assess Environmental Conditions and Application Scenarios

Beyond the herbicide label, it is also important to assess environmental conditions to determine the appropriate need for an adjuvant. Activator adjuvants such as NIS, COC, or MSO are often critical for foliar applications during drought conditions. Plants that have experienced drought can have either a thicker cuticle (waxy layer on leaf surface) or a cuticle that has a greater resistance to water that herbicides must penetrate, making the use of the correct activator adjuvant critical for adequate penetration of this waxy layer. In general, the order of effectiveness for dissolving the cuticle is MSO > COC > NIS.

Environmental conditions such as temperature and relative humidity can also dictate the need for an adjuvant. In low relative humidity environments, droplets on the leaf surface can dry more rapidly than in high relative humidity environments. Using an oil-based adjuvant in a low relative humidity environment can decrease the rate of droplet drying and thus increase potential absorption into the target leaf.

The use of drift reduction agents and volatility reduction agents also depends on the herbicides used, environmental conditions, and vegetation adjacent to the application site. When applying a herbicide near sensitive vegetation, it may be desirable to use a drift reduction agent to minimize driftable fines. In other scenarios, a drift reduction agent may be undesirable such as when using a contact herbicide where coverage is essential. Some herbicides, such as certain dicamba formulations, require a drift reduction agent when applied with certain tank mix partners. Refer to the herbicide label or its associated label website for the appropriate use of drift reduction agents and volatility reduction agents. Drift reduction agents alone do not eliminate drift. Applicators should select the proper nozzles, operating pressures, and environmental conditions in combination with a drift reduction agent to reduce drift potential. If winds are blowing towards a sensitive crop, the application should not occur. 

Applicators should be aware that adjuvants can interact with different herbicides and nozzle types resulting in incompatibility issues (Figure 3) or modifying droplet size (Figure 4). Using a drift reduction agent and certain low-drift nozzles can result in less desirable spray patterns and reduced coverage. In contrast, some adjuvants and nozzle combinations can result in a smaller droplet spectrum and increased driftable fines. Unfortunately, due to the millions of potential adjuvant, herbicide, and nozzle combinations, experience is often the best indicator of an appropriate combination. If an applicator is trying a new adjuvant and nozzle combination, it may be wise to try the combination on a small area to observe spray patterns and weed control.

The Correct Adjuvant Rate

Using the incorrect rate of an adjuvant can lead to less than desirable results. A rate that is too low may not positively influence weed control, while too high of a rate may result in crop injury and/or unnecessary expense. Surfactants such as NIS are typically applied at 0.25% to 0.5% v/v, while COC and MSO are typically used at 0.5 to 1% v/v.  Herbicide or adjuvant labels are the best resource for finding the correct adjuvant rate. If there is an adjuvant rate range on the label, use information such as what is provided in “Assess Environmental Conditions and Application Scenarios” above to determine the appropriate rate to use within the label range.

Special attention is necessary when using COC or MSO for postemergence herbicide applications. Most herbicide labels recommend 1% v/v of COC or MSO, although this recommendation is typically based on a spray volume of 15 gallons per acre or greater. When applying at a lower spray volume, oil volumes may need to be adjusted on a per acre basis. The recommended rate of COC or MSO is at least 1.25 pt/A when applying at low spray volumes (10 gallons per acre or less).

When using a HSOC, the rate of adjuvant should be half of the recommended MSO or COC label rate. HSOC products have a high surfactant ratio, hence the name, and if applied at the full MSO or COC label rate can cause significant crop injury.

Adjuvant rate listings on labels and weed control guides vary greatly. The most common units on a pesticide or adjuvant label include percent volume per volume (%v/v), quarts or pints per 100 gallons, and quarts or pints per acre. While there is no right or wrong unit of adjuvant measurement, the different units can become confusing. Table 1 is included for the ease of quickly converting adjuvant rate between the differing unit forms.

Table 1. Adjuvant rate unit conversions between percent volume per volume, pints per 100 gallon, quarts per 100 gallon, and gallon per 100 gallon. Rate units are converted to pints per acre at a carrier volume of 5, 10, and 15 gallons per acre.

¹ %V/V = Percent volume of additive per volume of spray mixture; ² pt/100gal = Pints of additive per 100 gallons of spray mixture; ³ qt/100gal = Quarts of additive per 100 gallons of spray mixture; ⁴ gal/100gal = Gallons of additive per 100 gallons of spray mixture; ⁵ pt/a – 5 GPA = Pints of additive per acre at a carrier volume of 5 gallons per acre; ⁶ pt/a – 10 GPA = Pints of additive per acre at a carrier volume of 10 gallons per acre; ⁷ pt/a – 15 GPA = Pints of additive per acre at a carrier volume of 15 gallons per acre

Conduct a Jar Test on New Adjuvant Combinations

When using a new adjuvant or agrochemical tank mix, a jar test is advised before mixing a full-size spray tank. If a herbicide tank mix and adjuvant combination results in an unsprayable mixture or precipitates that settle out of solution, it is much easier to clean up a small jar of mixture than a large spray tank. Jar tests can be conducted in a small jar or bottle, hence the name “Jar Test.” However, it should be noted that in a small-scale jar test, an appropriate concentration of herbicide and adjuvant should be mixed according to the label, similar to mixing a full-scale spray tank load. Additionally, make sure that the water source in the jar test is the same as the water source that will be used to fill the tank for the actual spray application. Conducting the jar test with water at the same temperature as the predicted water temperature at the time of the spray application will also ensure accurate results.

When to Add Adjuvants to the Spray Tank

The appropriate order of mixing herbicides and adjuvants can dictate the effectiveness of both the adjuvant and the herbicide. The following adjuvants should specifically be added in the following order. Always refer to the labels of all agrochemicals used in the spray tank for specific directions for mixing order for each unique tank mix.

1. Water Conditioning Agents Are Recommended to Go in First

Water conditioning agents, such as AMS, are used to modify the properties of the carrier water. This is most effectively accomplished if the water conditioner is added before any herbicide or other adjuvant.

2. Spray Solution Modifiers & Mixtures

Spray solution modifying adjuvants can typically be added initially with water conditioning adjuvants. This order reduces foam and enhances mixture compatibility compared to adding them later in the mixing process. The exception would be drift reduction agents. In general, single-function drift reduction agents are typically added last; however, many drift reduction agents today are in a mixture with other adjuvant types. As a result, it is best to read the adjuvant label to identify the manufacturer’s recommended mixing timing.

3. Activator Adjuvants

Activator adjuvants such as NIS, COC, and MSO should go into the spray tank after adding all herbicides. This order is especially important when using dry herbicide formulations and COC or MSO. Adding a dry or granular herbicide after an oil-based adjuvant can cause the dry product to become coated in oil and not effectively disperse into the spray water carrier.

Which Adjuvant Should You Use?

The ultimate question for farmers and applicators is which adjuvant should be chosen from a saturated market. Having multiple options is a great scenario for an open market, but it can overwhelm the consumer. Here are a few tips for selecting an adjuvant.

All-in-One Adjuvants are Not Necessarily the Best

While it may be tempting to buy an adjuvant mixture with multiple adjuvant types to cover all the bases, this is not always the best option. There are so many different adjuvant types because there is not a one-size-fits-all adjuvant for all application scenarios. Adding multiple adjuvant types into one jug does not make an adjuvant one-size-fits-all either. In some instances, the farmer may be applying unnecessary adjuvant thus wasting financial resources, causing incompatibility, or causing crop injury.

Use Past Experience and Trusted Colleagues

Experience with an adjuvant can go a long way. If you have had good experiences with an adjuvant, stick with that product. When changing adjuvants, consult with experienced farmers, crop advisers, adjuvant experts, Extension specialists, herbicide manufacturers, and/or applicators. 

Use Certified Adjuvants

Unlike pesticides, adjuvants are not regulated by the United States Environmental Protection Agency (EPA) and do not require certification. While most adjuvant manufacturers produce quality products, the lack of regulation contributes to the confusion around adjuvant selection. When there is a desire for an adjuvant with certification, look for adjuvants certified by the Council of Producers and Distributors of Agrotechnology (CPDA) Adjuvant Certification Program. Adjuvants with this certification have met 17 benchmark qualifications of a consistent and quality adjuvant. A complete list of CPDA-certified adjuvants can be found at https://cpda.com/cpda-certified-product/.

If It Sounds Too Good to be True, It Probably Is!

While it is great to have many adjuvant options, it is important to remember the role of advertising in the marketplace. While most adjuvant manufacturers have honest advertising, some claims seem too good to be true. For example, adjuvants cannot reverse herbicide resistance. By definition, a product that can modify a plant to overcome resistance would not be an adjuvant. If an adjuvant claims to do things beyond the functions described above, use caution as you consider purchasing. If in doubt, consult with Extension specialists in your area.

Acknowledgements

Authors

Travis Legleiter, University of Kentucky; Thomas (Tommy) Butts, Purdue University; Alyssa Essman, The Ohio State University; Joe Ikley, North Dakota State University; Sarah Lancaster, Kansas State University; and Rodrigo Werle, University of Wisconsin.

Reviewers

Meaghan Anderson, Iowa State University; Todd Baughman, Oklahoma State University; Jason Bond, Mississippi State University; Michael Flessner, Virginia Tech University; Dwight Lingenfelter, Penn State University; Mike Marshall, Clemson University; Scott Nolte, Texas A&M University; Eric Prostko, University of Georgia; David Russell, Auburn University; Debalin Sarangi, University of Minnesota; Connor Webster, Louisiana State University; and Bryan Young, Purdue University.

Additional Reviewers

Daren Mueller, Iowa State University; Albert Tenuta, Ontario Ministry of Food, Agriculture and Rural Affairs; and Kiersten Wise, University of Kentucky.