The way we approach weed management has been greatly influenced by the introduction of Nonionic surfactants (NIS). Adjuvants have been around for over 200 years, but it wasn’t until the 1960s that research and implementation of nonionic surfactants began. Initially, petroleum-based oil was used in conjunction with surfactants to create an emulsion for herbicide application. Only a few surfactants were commercially available at that time; however, research into nonionic surfactants continued into the 1970s and 1980s. It was found that not all surfactants enhanced the efficacy of herbicide applications. In the 1990s, there was a collective effort to significantly understand the relationship between surfactant structures and herbicide uptake enhancement. Since then, there has been a growing understanding of how surfactants, especially nonionic surfactants, work to enhance herbicide uptake through spray applications. Nonionic surfactants are now included as at least one component within many adjuvant formulations. An adjuvant is referred to as a Nonionic Surfactant (NIS) when nonionic surfactants provide the dominant features of the adjuvant formulation.
How do Nonionic surfactants work in weed control?
Surfactant can be broken down into three words – surface, active, agent – surfactants are compounds that lower the surface tension between two liquids or between a liquid and a solid. When herbicide mixtures are applied to plants as a spray, the herbicide solution spreads over the leaf surface, wets it, and penetrates the cuticle to reach the target site. Nonionic surfactants, when added to herbicide mixtures, can affect several aspects of the uptake process of the herbicide within the plant leaf.
Adding nonionic surfactants to the herbicide mixture has three main effects:
- It enhances the contact between the liquid solution and the solid leaf surface by spreading the droplet over the leaf surface, which is called spreading or wettability.
- It prevents or delays the formation of crystal residues from droplets. Without the presence of the surfactant, droplets can form crystalline solids, while with the surfactant, they form amorphous solids. Amorphous solids are easier to transport into the leaf because they are less ordered than crystals.
- It increases the probability of the herbicide active ingredient’s retention on the leaf surface and, therefore, the probability of diffusion through the leaf cuticle. This probability increases due to an increase in the surface area coverage of the droplet, which gives the herbicide active ingredient more chances to diffuse into the leaf. Without the surfactant, the herbicide mixture droplet has a significantly higher surface tension at the contact point with the leaf surface and will ‘bead-up’ like a ball instead of spreading out, which reduces the wetting of the leaf surface. (See image)
In summary, adding nonionic surfactants reduces the surface tension of the solution, resulting in better spread, coverage, and uptake of the herbicide by the plant.
Droplets containing a surfactant deposited on a waxy geranium leaf surface have a much larger area of coverage than droplets without the surfactant. (Photo courtesy of https://www.ars.usda.gov/midwest-area/wooster-oh/application-technology-research/engineering/evaporation-and-spread-of-surfactant-amended-droplets-on-leaves/)
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What are the benefits of nonionic surfactants and why are they important?
- They increase the spreading and retention of the herbicide solution spray droplet, which reduces the gallons per acre (GPA) usage of the herbicide.
- They increase the efficiency of herbicide active ingredients by providing the highest probability of herbicides diffusing into the plant leaf. Additionally, they reduce the spray droplet bounce and run off when spraying the herbicide mixture. This saves time for applicators and minimizes the risk of potential respraying.
As a result, using nonionic surfactants can lead to more efficient herbicide application, reducing the amount of herbicide needed and minimizing the need for additional applications.
Why are nonionic surfactants preferred over ionic surfactants in weed control?
Both types of surfactants have benefits, but nonionic surfactants are generally preferred due to superior performance and lower risks of phytotoxicity. Ionic surfactants can be categorized as cationic, anionic, and amphoteric surfactants, and carry either an overall negative or positive charge or a potential for both charges depending on the pH. These charges can interact with the herbicide active ingredient. Although in some cases this interaction can result in an enhancement of herbicide activity, it can also increase the risk of phytotoxicity and reduce the effectiveness of herbicides in hard water conditions. Nonionic surfactants do not carry an overall charge, allowing for greater versatility in a wider range of conditions. In addition, nonionic surfactants are generally less toxic and have a lower environmental impact than ionic surfactants. While both nonionic and ionic surfactants reduce the surface tension between two liquids, the reduced risk of phytotoxicity and versatility of nonionic surfactants make them the preferred choice for herbicide application.
What are the widely used chemical types of nonionic surfactants?
The most widely used chemical types of nonionic surfactants include:
- Alcohol ethoxylates
- Alkylphenol ethoxylates
- Ethoxylated sorbitan esters
- Trisiloxane ethoxylates
- Alkyl polyglucosides
- Alkylamine ethoxylates
A simple way to check if an adjuvant contains nonionic surfactants is to look for the terms “Ethoxylates” or “polyethylene oxides” in the ingredient list on the label. These terms represent the water-loving part of the surfactant molecule.
The versatility of nonionic surfactants in adjuvants:
Nonionic surfactants are versatile adjuvants that are not only used on their own but also in the formulation of other types of adjuvants such as Methylated Seed Oils (MSO), Crop Oil Concentrates (COC), and High Surfactant Oil Concentrates (HSOC). Nonionic surfactants are preferred in these formulations due to their ability to function as wetters/spreaders, emulsifiers, dispersants, compatibility agents, and more. This versatility has made nonionic surfactants a vital component of adjuvants for weed control.
In conclusion, the use of nonionic surfactants has revolutionized the chemical control of weeds, making herbicides more effective, efficient, and environmentally friendly.