AGR 3102 Principles of Weed Science Herbicide Muhammad Saiful Ahmad Hamdani

Unit 6 – Topics Covered Herbicides: Adjuvants

Why do not all agrochemicals go in? Well some do quite easily, others not at all.

1. Differences in plant leaf shape & structure Can be flat, thin, long, round, or needles. Can also be horizontal, vertical or intermediate angle. All these shapes and structures can affect herbicide penetration into leaves.

2. Existence of leaf cuticle and wax Thickness of wax and cuticle affect the entrance of a herbicide into a leaf. A leaf with a thin cuticle allows the spray solution good contact with the leaf surface. On a leaf with a thick waxy surface, the spray solution tends to stand up in droplets.

3. Existence of trichomes/hairs Hairs on the leaf surface tend to keep the spray solution from entering. The herbicide droplets stand up on the hair and do not contact the leaf surface.

Also other deposits like dust, inorganic and organic compounds, water, microorganisms All these things affect herbicides retention & absorption. How to overcome this problem??? How to break/reduce all these barriers/obstacles??? How to make herbicides work efficiently??? WITH ADJUVANTS

Adjuvants Definition: “any substance in other formulation (in our case, it’s herbicide) or added to the spray tank to enhance or modify that formulation performance or application characteristics” Some people call spray additives. Main functions: 1) improve ease of application/handling 2) enhance product efficacy (increase foliar adhesion and cover, uptake or translocation of herbicides)

Formulation vs. Spray Adjuvants: Formulation adjuvants Those included in the formulated herbicide product by the herbicide manufacturer. Spray adjuvants Added to the mixing tank primarily to improve herbicide performance, aid in mixing, reduce drift, reduce foaming, etc. Commonly used with post-emergence herbicides. Our focus will be on spray adjuvants.

Numerous types (or functions) of adjuvants: Acidifiers Humectants Buffering agents Nitrogen fertilizers Colorants, dyes Penetrants Compatibility agents Spreaders Crop oils Stabilizers Dispersing agents Stickers Drift inhibitors Surfactants Emulsifiers UV absorbents Antifoaming agents Water conditioners Wetting agents

For simplicity, categorized into 2 basic groups: 1. Activator adjuvants: Spray adjuvants used to enhance the biological efficacy of herbicides. What they do: result in more herbicide being deposited, adhered, retained, penetrated & absorbed into the weeds. What they overdo: can, and often do, result in more herbicide also being absorbed by the crop. Consequence: while increasing weed control, can also increase crop injury by the herbicide. Examples:

Surfactants Derived from surface active agent Reduce water surface tension and improve dispersion of the spray 2 groups: non-ionic type; organo-silicones. They are good dispersing agents, stable in cold water, and have low toxicity to both plants and mammals. Surface tension is so reduced that the spray solution can penetrate the stomata on the leaf surface.

Surfactants affect droplet spread This has implications for uptake.

This discovery revolutionised the weed control sector When the surface tension of a liquid is reduced by adding other chemicals, it can flow through very tiny spaces, such as the stomata on leaf surfaces. Final slide…video clip??? This discovery revolutionised the weed control sector

Stickers Cause the herbicide to adhere to the plant foliage. Reduces the possibility that rain will wash it off before the herbicide can penetrate. Many blended with wetting agents so that they both increase the spray coverage and provide better adhesion action.

Crop Oils Crop oil concentrates, and methylated seed oils, like surfactants, improve the spreading of the herbicide solution. Being oil instead of water, they keep the leaf surface moist longer than water. Allow more time for the herbicide to been absorbed, thus increasing the amount that will enter the plant.

Penetrants In general, penetrants make herbicides “hotter” than it normal condition. Improve cuticular penetration by softening, plasticizing, or dissolving cuticular waxes; allowing herbicide to move underneath. Penetrants are often a complex mixture of surfactant and oils (paraffinic petroleum or modified vegetable).

- penetrant + penetrant

2. Utility/Spray Modifier Adjuvants: Spray adjuvants used to modify the physical characteristics of the spray mixture. Long story short: they improve ease of application/handling of the herbicides. Examples:

Drift Inhibitors Drift inhibitors or thickeners are used to control drift. These may be powders, granules, or liquids that cause the spray solution to be more cohesive/thickened; less subject to wind shear as it leaves the nozzles Also reduce the amount of very small spray droplets.

Antifoaming Agents Air gap filling or mechanical agitation in partially full tanks = excessive foaming. Foaming: interfere with herbicide flow and spray Antifoaming agents: silicone containing products. They cause rupture of the air bubbles and breakdown of surface foam.