Magazine: Adjuvants Part.1
By Jerry Spencer in Consultancy on 20th Nov 2007 10:30
Adjuvants make many commonly used pesticides work better. However, with the wide range of these available it is difficult to make an informed decision. A pesticide adjuvant is broadly defined as any substance added to the spray tank, separate from the pesticide formulation that will improve the performance of the pesticide.
Adjuvants should do at least one of the following for the end user to help Turf managers be more efficient and ultimately more environmentally sound:
1. Stick to the leaf
2. Prevent drifting
3. Spread evenly to ensure even coverage
4. Penetrate the leaf
Adjuvants can act in a number of ways. The range of functions attributed to an adjuvant includes the following: Acidifier, Activator, Anti-evaporant, Anti-foaming agent, Buffer, Compatibility agent, De-foaming agent, Deposition agent, Drift control agent, Emulsifier, Extender, Foaming agent, Humectant, Mineral oil, Penetrating agent, Spreader, Sticker,
Surfactant, Vegetable oil, Water conditioner, Wetter.
The diagram below shows how the uptake of a commonly used herbicide, Imazethapyr, is increased dependant upon which particular adjuvant is used.
Of the major uses, all but foliar Nutrients, Buffers, Drift Retardants, and possibly Oils usually depend on surfactants to perform their major function. It is, therefore, of some value to understand the role of the surfactant chemical or principal active ingredients in performing the functions expected of an adjuvant and to be able to read the active ingredient statement on the label when comparing adjuvants and selecting the one right for your situation.
A surfactant is a "surface active agent" and is the active ingredient in most adjuvants. Surfactants are nonionic (do not ionize, but will have a slight electrostatic charge due to the polarity of dissimilar atoms in the molecule), anionic (ionized, have a strong negative charge), or cationic (ionized, have a strong positive charge).
When applying pesticides to turf there are multiple barriers to entry that can effect their success or failure following application. These include leaf angle, leaf hairs or pubescence, leaf surface or cuticle, cell wall or membrane. The pesticide must quite often pass through these barriers and translocate to be effective.
Leaf angle
In turf as the diagram above shows grasses offer a smaller target than for example herbicides and so this can directly effect pesticide efficacy
Leaf hairs
Leaf surface or cuticle
Made up of many parts; waxes - oil soluble or lipophilic, cutin - water soluble or hydophilic, pectin - water soluble pathways for entry.
Surfactants, also called wetting agents and spreaders, physically alter the surface tension of a spray droplet. For a pesticide to perform its function properly, a spray droplet must be able to wet the foliage and spread out evenly over a leaf.
Surfactants enlarge the area of pesticide coverage, thereby increasing the pest’s exposure to the chemical. Surfactants are particularly important when applying a pesticide to waxy or hairy leaves. Without proper wetting and spreading, spray droplets often run off or fail to adequately
cover these surfaces. Too much surfactant, however, can cause excessive runoff or deposit loss, thus reducing pesticide efficacy.
Surfactants are classified by the way they ionize, or split apart into electrically charged atoms or molecules called ions. Pesticide activity in the presence of a nonionic surfactant can be quite different from activity in the presence of a cationic or anionic surfactant.
Selecting the wrong surfactant can reduce the efficacy of a pesticide product and injure the target plant. Anionic surfactants are most effective when used with contact pesticides.
Cationic surfactants should never be used as stand-alone surfactants because they usually are phytotoxic.
Nonionic surfactants, often used with systemic pesticides, help pesticide sprays penetrate plant cuticles and are compatible with most pesticides.
Surfactants drastically reduce surface tension to the point where the herbicide droplets thin and coalesce to form a thin layer on the leaf surface (known as “superspreading”). They can even reduce surface tension to the point that some of the formulation may be able to slide through the microscopic stomatal openings on leaf surfaces. Once through the stomates however, the herbicide formulation must still penetrate the thin cuticle and cell membranes of the cells that line the cavity below the stomates.
Silicone surfactants also decrease surface tension and may allow spray solutions to penetrate the stomates. They can also make the formulation nearly impossible to wash off (rainfast) even if it rains shortly after they are applied. Silicone surfactants can also influence the amount/rate of herbicide that is absorbed through the cuticle.
Surfactants contain both hydrophilic and lipophilic components (this is called amphiphatic). The hydrophiliclipophilic balance (HLB) is a measure of the molecular balance of the hydrophilic and lipophilic portions of the compound. Many herbicides have an optimum surfactant HLB, and surfactants that most closely match a particular herbicide’s optimum HLB will optimize the formulation’s spread on and penetration into plants. Unfortunately, information about the HLB of most surfactant products is not available or hard to find, and so matching them appropriately is difficult.
For nonionic surfactants, the optimum surfactant HLB for a herbicide can be predicted based on the solubility of the herbicide in water. For ionic surfactants, the HLB can be estimated by observing their dispersablity in water (with no dispersion = 1 to 3; poor dispersion = 3 to 6; unstable milky dispersion = 6 to 8; stable milky dispersion = 8 to 10; translucent to clear dispersion = 10 to 13; and clear solution = 13+).
Typically, low HLB surfactants work best with water insoluble herbicides, while high (>12) HLB surfactants work best for water-soluble herbicides. For example, surfactants with a high HLB are more active with the hydrophilic herbicide Glyphosate.
Pesticide formulations often contain surfactants to improve the suspension of the pesticide’s active ingredient. This is especially true for emulsifiable concentrate (EC) formulations.
Pesticide Emulsifiable Concentrates, Wettable Powders, and Flowables will contain 2 or 3 surfactants, usually nonionic and anionic It is reasonable to assume that products that contain similar surfactants will perform in a similar manner and in proportion to the amount of active ingredient they contain. A factor to also consider is that certain adjuvants or adjuvant mixes may sometimes be more toxic to certain non-target organisms than the herbicide itself. For example, the surfactant included in RoundUp® is more toxic to fish than the active ingredient Glyphosate, and hence ‘bio-active’ formulations are now on the market.
The following brief descriptions of some of the categories of adjuvants may be helpful in clarifying the many functions adjuvants can perform.
Part 2- click on the link
http://www.pitchcare.com.au/magazine/article/239
| There are 2 comments on this article |
20 Nov 07 by KeepitSharp\\\
Might be a silly question here but is wetting agent a Adjuvants?
I was always told applying some wetting agent in with your mixes (boom or hang gun) can help improve your results?
Or was my old boss way off the mark?
20 Nov 07 by Mr--slipper
I wouldnt think it would make a difference would it? As like the article is saying Adjuvants are in there anyway? maybe it would improve it a little more im not sure. Sounds like you are just throwing money away to me however
Dont slip use safemate antislip
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