Adjuvants are chemicals employed to improve the herbicidal effects, sometimes making a difference between satisfactory and unsatisfactory weed control. Mode of Action: Adjuvants aid the herbicide availability at the action site in plants.
Kinds of adjuvants
1. Surfactant (Surface active agents)
• Aid in wetting the waxy leaf surface with aqueous herbicide sprays (wetting agents)
• In spreading the hydrophilic herbicides uniformly over the foliage (spreaders)
• In the penetration of herbicide into the target leaves and stems (penetrates) A water drop is held as a ball on a waxy leaf surface. (Take water in a beaker, if you dip a leaf of Cynodon dactylon and pull it back, you can see the leaf without wetting. But if you add a drop of surfactant you can readily wet the foliage.). With the addition of surfactant, the water drop flattens down to wet the leaf surface and let the herbicide act properly.
2. Stabilizing agents – These include emulsifiers and dispersing agents.
(i) Emulsifiers – A substance which stabilizes (reduces the tendency to separate) a suspension of droplets of one liquid which otherwise would not mix with the first one. It substitutes for constant agitation of spray liquids during field operation. E.g., ABS, Solved, 15-5-3, 15-5-9.
(ii) Dispersing agents – They stabilize suspensions. They keep fine parricides of wettable powder in suspension in water even after initial vigorous agitation has been withdrawn. They act by increasing the hydration of fine particles of WP laden with the herbicides.
3. Coupling agents (Solvents and co-solvents) – Chemical that is used to solubilize a herbicide in a concentrated form; the resulting solution is soluble with water in all proportions. e.g., 2,4-D is insoluble in water, but it can be dissolved in polyethylene glycol to make it water soluble. Common solvents: Benzene, acetone, petroleum ether, carbon tetrachloride.
4. Humicants (Hygroscopic agents) – Humicants prevent rapid drying of herbicide sprays on the foliage, thus providing an extended opportunity of herbicide absorption e.g., glycerol.
5. Deposit builders (Stickers or filming agents) – Chemicals added to herbicide concentrates to hold the toxicant in intimate contact with the plant surface. They also reduce washing off of the toxicant from the treated foliage by rain. e.g., several petroleum oils, Dupont spreader sticker, Citowett.
6. Compatibility agents – Used to intimately mix fertilizers and pesticides in spray liquids e.g., Compex.
7. Activators (Synergists) – Chemicals having cooperative action with herbicides that the resultant phytotoxicity is more than the effect of the two working independently. e.g., Paraffinic oils, Ammonium thiocyanate, Urea and Ammonium chloride to enhance 2,4–D phytotoxicity.
8. Drift control agents – Herbicide spray drifts may pose serious hazards to non-target plants. e.g., 2,4–D on cotton. Solution is to spray herbicide liquids in large droplets.
9. Thickening agents e.g., (Decagon, Sodium alginate).
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