Surfactants are adjuvants that are added to the spray solution to help increase movement of the herbicide from the leaf surface to the interior of the cell. They are commonly used in postemergence herbicide applications on cereal crops. Some herbicides are formulated with adjuvants included and the user usually does not need to add a surfactant to the tank, whereas other products require the addition of adjuvant to ensure proper performance.1

Are all spray adjuvants surfactants?

No, spray adjuvants are divided into two broad categories:

  1. Activators and spray modifiers
    • Surfactants (surface active agents)
    • Oils
  2. Utility modifiers
    • Compatibility agents improve mixing, especially when using a liquid fertilizer carrier.
    • Drift control agents increase the droplet size to reduce drift.
    • Anti-foaming/defoaming agents are used to reduce and prevent foaming in the spray tank.
    • Foaming agents are used with specialized equipment to produce and apply foam.
    • Buffering agents can be used to enhance solubility or adjust pH.
    • Dyes are used in some instances to enhance visibility of spray foam solutions.

Why are oils not considered surfactants?

Oils solubilize the water-impervious (waxy) protective layer covering the surface cells of leaves (cuticle). By dissolving this cuticle layer on a weed leaf surface, it allows the herbicide to penetrate the leaf cells and travel to its target site and enabling the herbicide to kill the weed. Oils are refined mineral oils (petroleum based) or seed oils. Seed oils are categorized as triglycerides, methylated seed oils (MSO) or crop oil concentrates (COC). Crop oil concentrates are a combination of seed oil and surfactants.

What surfactants are most often used on postemergence small grain herbicides?

The most common surfactant used in small grain herbicide applications are non-ionic surfactants (NIS). NIS are water soluble chemical and lipid compounds that are not molecularly charged (positive or negative). Surfactants reduce the surface tension of the water molecule enabling the water droplet to cover a greater leaf surface area; essentially the water droplet spreads out across a larger area. NIS are often referred to as wetting agents or spreading agents. NIS are commonly used under “average” growing conditions. NIS typically causes less injury than other adjuvants; however, if applied at higher than labeled rates, crop injury can occur.2

A second surfactant commonly used with postemergence herbicides on cereal crops is ammonium sulfate (AMS). AMS helps the herbicide penetrate the surface of the weed, reduce surface tension and improve leaf surface spreading and uptake.

When should a producer add a surfactant to the herbicide solution?

Surfactants should be included to a herbicide solution when the herbicide label recommends a surfactant to be added. Surfactants are not regulated but herbicide labels often state which surfactants are recommended and the rate to be used. Not including a recommended surfactant to the spray solution can negatively affect the degree of weed control achieved, the ability of the herbicide to stay in solution, or cause the herbicide to be tied up in parts of the sprayer. Some products have surfactants formulated into the product while other products require that the user add the surfactant. The selection of surfactant is key to obtaining the right balance between maximizing weed control and minimizing crop injury.

How much surfactant is recommended to be added to the spray solution?

Most adjuvants are listed as the amount (in litres) added to 1,000 L (L/1,000 L) of spray solution. To convert to % volume/volume (v/v) use the following conversion table:

5L/1,000 L = .5% v/v

Table 1. Adjuvant rates per sprayer tank volume.
% Adjuvant / Water 0.1% v/v 0.2% v/v 0.25% v/v 0.5% v/v 1.25% v/v
L Adjuvant / L Water 1 L/1,000 L 2 L/1,000 L 2.5 L/1,000 L 5 L/1,000 L 12.5 L/1,000
L Adjuvant / U.S. gal. Water 0.38 L /100 U.S. gal. 0.76 L /100 U.S. gal. 0.95 L /100 U.S. gal. 1.9 L/100 U.S. gal. 4.75 L/100 U.S. gal.

Can the amount of surfactant added vary depending on environmental factors?

In some cases, the presence, or absences and type of a surfactant varies depending on conditions of weather, crop stage, crop type (i.e. spring wheat vs. durum) weed species, water quality, etc. The three “Ls” describe some of the most common challenging conditions that can often reduce herbicide performance:

  • Larger weeds
  • Lots of weeds
  • Low temperatures

Other stress environmental conditions like hot, dry weather hardens plants off and reduces herbicide absorption and under these conditions higher rates of surfactant would be beneficial. When herbicides are applied under adverse conditions adding the correct type and amount of surfactant, to the spray solution, can be critical step to achieve the expected degree of weed control. When considering when and how much surfactant to use always consult and follow the herbicide label.

Complete information on each adjuvant is available on the product label located on the product container. Pesticide labels can also be found online from the federal Pest Management Regulatory Agency website and many pesticide manufacturers websites have product labels and/or Material Safety Data Sheets (MSDS) listed on their websites.3