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Choosing the Right Surfactant for Agriculture

Feb 12, 2014

In recent years, sulfonylurea herbicides have become popular with farmers as crop protection manufacturers continue to develop new uses for this family of compounds. Their low application rates, broad spectrum weed control and favorable toxicological properties have contributed to the success of this group of herbicides.

Surfactants are critical to the performance of these post-emergence herbicides. Many labels recommend the addition of surfactants with agricultural sprays because they are proven to enhance performance and maintain effectiveness across a variety of spray conditions and water types.

In addition to enhancing weed control, surfactants can also improve the window of application and lower herbicide use rates. Surfactants do not act as a pesticide and there is no biological activity through their use. Rather, they optimize the activity that already exists in a herbicide.

A surfactant is a surface acting agent that reduces the surface tension of a liquid, thereby increasing the spreading, dispersing and wettability of the spray solution on plant leaf surfaces. Because of the high surface tension of water, spray mixture droplets can maintain their 'roundness' and sit on the leaf hairs or waxy surface without much of the herbicide actually contacting the leaf. The crop chemical mixture then becomes susceptible to degradation by sunlight or run-off from the leaf surface.

By reducing the surface tension of the spray solution, surfactants flatten the water droplets, thus spreading the herbicide on the leaf surface. This allows more surface area for the chemical to come in contact with the leaf, resulting in more chemical uptake and better performance. This is especially important in situations where weeds' leaves have a waxy protective cover or fine leaf hairs.

There are many types of surfactants that perform different functions in spray solutions. Confusion frequently occurs concerning the proper selection and use of surfactants with herbicides. It is wrong to assume that any product that lowers the surface tension of water or increases the wettability of a spray solution can be used as a surfactant.

For example, products like household soaps and detergents can combine with hard water to form residues that will interfere with the performance of spray equipment. Also, many liquid detergents have a fairly low concentration of surfactants (10 to 20 percent) compared with a 50 to 90 percent concentration usually found in agricultural surfactants. Agricultural surfactants are equally effective in hard and soft water as well as cold or warm water.
There are four basic groups of agricultural surfactants: anionic, cationic, amphoteric and non-ionic.

Anionic and cationic surfactants form electrical charges in water (negative and positive, respectively). Cationic surfactants can be toxic to plants and are not generally used with selective herbicides. It is often used with non-selective herbicides, like glyphosate at a rate of one litre per acre in 150L/ac of water. However, if the product is used with a lower volume of water, say 75L/ac, there may be sufficient surfactant in the product formulation that additional surfactant is not needed.

Anionic surfactants are sometimes blended with non-ionic surfactants to provide the wetting and emulsifying properties of a herbicide formulation. Amphoteric surfactants may or may not form a charge depending on the acidity of the spray solution.

Non-ionic surfactants do not form an electrical charge. That means there is no positive or negative ions to compete or react with the active chemical with which it is tank-mixed. Non-ionic surfactants are the type usually used in most herbicide spray solutions. These surfactants are good dispersing agents, stable in cold water and have low toxicity to both plants and animals.

Specialty surfactants include a silicone-based compound, also known as organo-silicone, which is increasing in popularity due to its superior spreading ability. This is a blend of a non-ionic surfactant and silicone. This combination can increase the absorption into a plant so the time between application and rainfall can be shortened. This is known as rain fastness. An organo-silicone surfactant has the potential of giving good coverage by chemical rather than by the physical means of using high volumes of carrier. Thus, farmers can reduce the water volume — but not below label rates — and gain the same effect.

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