Does paraquat need adjuvants?

Wax crystals on the surface of leaves of yellow foxtail (Setaria lutescens) seen under an electron microscopeThe labels of paraquat products must be consulted to determine whether an adjuvant should be added to the spray tank, or whether their use should be avoided. Surfactants (wetters) are often included in the formulated product and any additional adjuvants are unnecessary and may cause problems. On the other hand, some products may benefit from a tank-mix adjuvant and options will be listed on the label. Foliar herbicides like paraquat can face challenges on the surface of leaves and in reaching places where they will be most effective. Leaves are covered with a waxy cuticle, which forms a considerable barrier, especially to water-based sprays. Quite simply, wax repels water. Crystalline wax as shown in the image, is particularly repellent. Spray droplets are prone to bouncing-off leaves or sitting on the top of wax crystals and not wetting the surface - as essential for good uptake. Once inside a leaf, there are further barriers to movement. Surfactants and other adjuvants are used to help overcome these barriers to effective weed control.1

What are adjuvants?

Adjuvants are products with biological, chemical or physical effects, which aim to improve herbicide performance and are increasingly used with other crop protection products too. Sometimes they are built-in to a complete product and sometimes they are added as a tank-mix. Surfactants are the most widely used type of adjuvant. Crop oil concentrates and methylated seed oils are other important categories. These each work in different ways, but are ultimately designed to increase the amount of active ingredient reaching the parts of weeds where they exert their effects.

Label instructions

Adjuvants should only be used with paraquat herbicides as instructed on the product label. For example, the US label for the herbicide Gramoxone SL 2.0 states that there are two basic options: Non-ionic surfactant: for ground and aerial application, a non-ionic surfactant containing at least 80% actual surfactant should be added at a minimum of 0.25% of the final spray volume, i.e. 0.5 litres in 200 litres of water per hectare (2 pints in 100 US gallons per acre). Crop Oil Concentrate: a non-phytotoxic crop oil concentrate or methylated seed oil containing 15 to 20% of approved emulsifier (i.e. a surfactant) should be added at 1% of the final spray volume, i.e. 2 litres in 200 litres of water per hectare (1 US gallon in 100 US gallons). In the US, adjuvants used should meet the requirements set by the Council of Producers and Distributors of Agrotechnology (CDPA).2 Note that these are specific recommendations for one product in the US only. Always read the product label.

What is a non-ionic surfactant?

Surfactants (surface active agents) have molecules with one end attracted to water and the other attracted to waxy or oily materials. This means that they reduce the surface tension of spray droplets, allowing them to wet leaves properly rather than being repelled by wax crystals or hairs covering the epidermis. Surfactants also help spray deposits to spread, so giving improved contact. They may also help active ingredients to penetrate and move within plants. Non-ionic surfactants are the most versatile class.

What is a crop oil concentrate?

Crop oil concentrates are blends of oil with 5-20% surfactant, which functions as an emulsifier to allow oil and water to mix, and acts on the surface tension of spray droplets. Both oil and surfactant may interact with leaf waxes to enhance penetration of the active ingredient. Strictly speaking, the oils used are of mineral origin, but those from oily seeds, such as canola (rape), soybean and sunflower are also used, and are usually called vegetable oil concentrates.

What is a methylated seed oil?

Seed oils comprise fatty acids and glycerol. It was discovered that improved adjuvant effects could sometimes be achieved if a seed oil was chemically modified with an alcohol, for example methanol, to create a methylated seed oil. These adjuvants contain 5 – 15% surfactant.

Incompatibility issues

If label instructions for adjuvant use are not complied with, apart from unnecessary expenditure, there may be other problems including reduced levels of weed control and incompatibility with the paraquat herbicide or with other products used in tank-mixes. Charged molecules like paraquat are attracted by oppositely charged molecules, resulting in the formation of a sludge, which will be difficult to remove from the spray tank.

Read the label

Following the label instructions on the correct use of adjuvants will result in the best weed control and avoid any mixing problems.

References

  1. Bland P and Brian R C (1975). Surfactants and the uptake and movement of paraquat in plants. Pesticide Science6, (4), 419-427
  2. Council of Producers and Distributors of Agrotechnology (CDPA)

Notes

The brand name for the leading paraquat product is Gramoxone.