What to Know: Herbicide Adjuvants

Here’s What You’ll Learn...

How adjuvants can help to improve post-emergence herbicide performance and enhance spray solution characteristics.
How herbicide product labels outline specific instructions for the use of adjuvants.
How adjuvants must be properly matched to each herbicide formulation and tank mixture for optimum effectiveness and crop safety.

The Purpose of Adjuvants

Adjuvants play a key role in herbicide formulations and spray mixtures to help herbicide performance either by improving herbicide activity or increasing ease of application. Adjuvants are also specific for each product as researched and developed by the herbicide manufacturers. That’s why it’s so crucial to remember that herbicide labels are the most important source of information for adjuvant recommendations that cover diverse use situations and tank mixtures.

Adjuvants include penetrants, wetting agents, stabilizing agents, spreaders, compatibility agents, stickers, antifoam agents and others.¹
Adjuvants can be classified into two main groups: activators and utility modifiers (or special purpose adjuvants).
Activator adjuvants, such as surfactants, crop oil concentrates (COC) and nitrogen (N) are normally used to help improve the performance of herbicides by increasing herbicide retention or penetration into leaf surfaces, improving rainfastness, or decreasing photodegradation of herbicides.²
Utility modifiers, such as buffering, antifoam and drift control agents, typically modify the characteristics of the spray solution and product compatibility.

Since herbicide spray adjuvants impact how much herbicide enters the plant, they can impact weed control and crop safety. That is why it’s important to follow the label requirements for each herbicide. Each herbicide product has unique adjuvant requirements that are specified on the product label. The label will provide guidance and adjuvant options to address tank mixtures, environmental conditions or weed species characteristics.

Some herbicide products, including Roundup PowerMAX^® 3 Herbicide, are formulated with sufficient adjuvants in the herbicide formulation and may not need additional adjuvants added to the spray mixture. However, some products have specific recommendations for spray adjuvants that the applicator must add to the spray mixture. Applicators should always pay particular attention to label instructions for each tank-mix product because label recommendations for adjuvants may differ for each product.

There is a diverse array of adjuvant products and brands. Different herbicide manufacturers may have supplemental labels or fact sheets that provide additional guidance for the use of adjuvants for specific weed species, crops, tank mixtures or application situations. Applicators must understand the composition and function of each product — while considering expected environmental conditions at the time of herbicide application — to properly match the adjuvant to individual herbicides or tank mixtures. Comprehensive information on commercially available adjuvants can be found at http://siu-weeds.com/adjuvants/index-adj.html.

Know the Types of Adjuvants

Varieties of adjuvants that are used with herbicides include nonionic surfactants, oil concentrates, ammonium N fertilizers, spreader-stickers, wetting agents and penetrants.^1,2,3,4 Here are some of the most common adjuvants used today:

Nonionic surfactants are dispersing agents that can help improve plant coverage and penetration of foliar-applied herbicides with low toxicity to the crop.
Oil concentrates help improve herbicide penetration into leaf surfaces and reduce surface tension. COCs are derived from petroleum. Methylated seed oils (MSO) function like other oil concentrates but are derived from seed oils.2 High-surfactant oil concentrates (HSOC) are emulsifiable oil-based products containing 25%-50% surfactant (wt/wt) in a minimum of 50% oil (wt/wt). HSOC oil concentrates may be MSO- or COC-based. COC, MSO and HSOC may increase the risk of crop injury more than surfactants.³
Ammonium N fertilizer products, when used at recommended rates, can act as adjuvants to help improve the performance of certain herbicides, especially under hard water conditions, drought or in tank mixtures. Spray-grade ammonium sulfate or urea ammonium nitrate are common N fertilizer adjuvants. N fertilizer solutions are generally recommended in conjunction with NIS or COC.
Blended adjuvants contain specific combinations of special purpose and/or activator adjuvants that serve multiple functions.²

Table 1. Adjuvant recommendations for Bayer herbicides.

Assumptions unless otherwise stated: AMS and UAN are spray-grade quality, NIS at 80% concentration, MSO at least 80% MSO and 10% emulsifier.

For additional information, contact your local Bayer representative.

Sources

¹Jordan, T, Johnson, B., and Nice, G. 2011. Adjuvants used with herbicides: Factors to consider. Purdue University. https://extension.entm.purdue.edu/pestcrop/2011/issue25/index.html#adjuvant.

²Curran, W.S. and Lingenfelter, D.D. 2009. Adjuvants for enhancing herbicide performance. Agronomy Facts 37. Penn State Extension. https://extension.psu.edu/.

³Hartzler, R. 2013. (Re) learning to accept herbicide injury to crops. Iowa State University, https://www.extension.iastate.edu/.

⁴Hartzler, B. 2001. Role of spray adjuvants with postemergence herbicides. Iowa State University Weed Science Online https://crops.extension.iastate.edu/encyclopedia/role-spray-adjuvants-postemergence-herbicides.

Web sources verified 6/29/21. 1034_S3