WHAT ARE HERBICIDES?
- Herbicides, also commonly known as weedkillers, are weedicides used to kill unwanted plants in the cropped areas.
- Herbicides are the substances that are toxic to plants, used to destroy unwanted vegetation.
- Herbicides used to clear waste ground, industrial sites, railways and railway embankments are not selective and kill all plant material with which they come into contact. Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat.
- Just as a pesticide is used to rid an area of unwanted pests, the herbicide is used to get rid of unwanted plant life, like weeds, brush, unproductive bushes or trees, and other growth that takes nutrients away from crops and other useful plants.
- Herbicides are available in two main types: nonselective and selective. The nonselective variety is used to kill all growth and is generally reserved for agricultural use or for clearing large or heavily overgrown areas. On the other hand, a selective herbicide is used to target certain types of plant life. This form works to curb growth, usually through some type of hormone disruption, and should not affect other vegetation.
WHAT IS HERBICIDE FORMULATION?
- The active ingredient in a herbicide is the chemical that controls the target weed. The herbicide product you purchase is rarely made up only of active ingredients. Often the herbicide is diluted in water or a petroleum solvent, and other chemicals are added before the product is offered for sale. These other chemicals may include wetting agents, spreaders, stickers, extenders, or diluents. They usually make the product easier to apply and more convenient to handle. This mixture of active and inert ingredients (inactive) is called a formulation of herbicide.
- An herbicide formulation is the total marketed product and is typically available in forms that can be sprayed on as liquids or applied as dry solids. It includes the active ingredient(s), any additives that enhance herbicide effectiveness, stability or ease of application such as surfactants and other adjuvants, and any other ingredients including solvents, carriers or dyes. The application method and species to be treated will determine which formulation is best to use.
FORMULATION OF HERBICIDES
The different formulations of herbicides are:
- Dry formulations
- Liquid formulations
- Salts & Esters
- Granules (G): Granules consist of the active ingredient absorbed onto coarse particles of clay or other substance, and are most often used in soil applications. These formulations can persist for some time and may need to be incorporated into the soil. The amount of a.i. ranges from 1 to 15%. Examples: Top-Site, Sprakill 13, Arsenal 0.5 G.
- Pellets (P) or tablets (TB) – Pellets are similar to granules but tend to be more uniform in size and shape. Pellets provide a high degree of applicator safety. Example: Spike 20P
- Dust (D) – A dust is a finely ground pesticide combined with an inert or inactive dry carrier. They can pose a drift or inhalation hazard.
- Wettable Powder (WP or W): Wettable powders are finely ground solids, typically mineral clays, to which an active ingredient is absorbed. They provide an effective way to apply an active ingredient in a water spray that is not readily soluble in water. These dry preparations look like dust, contain a high percent active ingredient (usually 50 percent or more) and are mixed with water for application. Wettable powders form a suspension rather than the true solution when added to water. Good agitation (mixing) is needed in the spray tank to maintain the suspension. Example: Spike 80W.
Soluble Powder (SP): This is a dry formulation that contains a high percent (usually above 50 percent) active ingredient. Soluble powders look like wettable powders but they form a true solution when added to water. Agitation in the spray tank will help this formulation to dissolve. After dissolving, no more agitation is usually needed. Few herbicides are available in this formulation because few active ingredients are soluble in water. Soluble powders are nonabrasive to equipment. Inhalation hazard is a characteristic of this formulation. Example: Solution
Water-Dispersible Granule or Dry Flowable (WDG or DF): Dry flowable are manufactured in the same way as wettable powders except that the powder is aggregated into granular particles. They are mixed with water and applied in a spray exactly like a wettable powder. This dry formulation usually contains 70 to 90 percent active ingredient. The formulation pours easily without the windblown dust associated with wettable powders and readily disperses in water to form a suspension. Constant agitation is required. Example: Diuron 80, Escort, Karmex IWC, Oust, Sahara, Velpar DF.
Liquid formulations do not exhibit the variety of physical forms possible with dry formulations. However, liquid formulations differ markedly in the nature of their characteristics that influence selection, rate and method of application, and environmental impact.
1. Water-Soluble Concentrate (WSC): Water-soluble concentrates to form a true solution when added to water and is applied with water as the carrier. These herbicides usually have an amine (ammonium salt) or mineral salt in the molecule that enables water solubility. These formulations are essentially nonvolatile. There are usually 2 to 6 pounds of active ingredient per gallon of the formulation. Agitation is not necessary to maintain the herbicide in solution. Example: Arsenal, Formula 40, Garlon 3A, Krenite, Roundup Pro, Tordon K, Vanquish, Veteran 720.
2. Emulsifiable Concentrate (E or EC): An emulsifiable concentrate formulation usually contains the active ingredient, one or more petroleum solvents, and an emulsifier that allows the formulation to be mixed with water. Emulsifiable concentrates usually contain 2 to 8 pounds of active ingredient per gallon. These concentrates are soluble in oil and form an emulsion in water. There usually is a dermal (skin contact) hazard associated with this formulation. Example: Garlon 4
3. Flowable or Aqueous Suspension (F, L or AS): In this formulation, very finely ground solid material is suspended in a liquid. Liquid flowable usually contain a high concentration (4 pounds or more) of active ingredient and are mixed with water for application. The formulation has the same major characteristics as a wettable powder; it forms a suspension when added to water, and it enables the application of water-insoluble herbicides in water. They seldom clog spray nozzles and they need only moderate agitation. Example: Diuron 4L
4. Liquid suspensions (L for liquid or F for flowable) that are dispersed in water include: suspension concentrates (SC), aqueous suspensions (AS), emulsions of water-dissolved herbicide in oil (EO), emulsions of an oil-dissolved herbicide in water (EW), micro-encapsulated formulations (ME), and capsule suspensions (CS). All these products consist of a particulate or liquid droplet active ingredient suspended in a liquid. They are easy to handle and apply, and rarely clog nozzles. However, they can require agitation to keep the active ingredients from separating out.
- An adjuvant is any material added to a herbicide mixture that facilitates mixing, application or herbicide efficacy. An adjuvant enables an applicator to customise a formulation to be most effective in a particular situation.
- Important types of activator adjuvants include:
1. Surfactants: Surfactants are the most important adjuvants. They are chemical compounds that facilitate the movement of the active herbicide ingredient into the plant. They may contain varying amounts of fatty acids that are capable of binding to two types of surfaces, such as oil and water. The most important group of surfactants is the nonionic type. Another group of surfactants is the organo-silicones.
2. Oils: 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, allowing more time for the herbicide to penetrate, thus increasing the amount that will enter the plant. Crop oil concentrates contains 80 to 83 percent oil and 17 to 20 percent surfactant and are used at rates similar to the nonionic surfactants.
3. Stickers: A sticker is an adjuvant that causes the herbicide to adhere to the plant foliage, thus reducing the possibility that rain will wash it off before the herbicide can penetrate. Many stickers are blended with wetting agents so that they both increase the spray coverage and provide better adhesion action. When combined, the product is often called a “spreader/sticker.”
4. Drift Inhibitors: Other adjuvants serve other specialised functions. 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; less subject to wind shear as it leaves the nozzles so as to reduce the amount of very small spray droplets.
5. Antifoaming Agents: Air gap filling or mechanical agitation in partially full tanks can cause excessive foaming. Antifoaming agents cut down on frothing so that the tank can be filled more easily. These are usually silicone-containing products that are used in relatively small amounts to breaking down the foam.
SALTS AND ESTERS
- Many herbicidally active compounds are acids that can be formulated as a salt or an ester for the application. Once the compound enters the plant, the salt or ester action is cleaved off allowing the parent acid (active ingredient) to be transported throughout the plant.
BENEFITS OF HERBICIDES
- They kill unwanted plants.
- They can be safely used whereas in some cases manually removing weeds can destroy the crop.
- Herbicides can be used on closely planted crops where other methods cannot be used.
- Most of the time one application of the herbicide is enough whereas other methods have to be continually used.
- They are easy to use
- They work fast. They can be removed quickly in critical situations.
- Herbicides are relatively cheap, and most of the time cheaper than manual weeding.
- Non-selective herbicides can effectively clear fields, where houses and roads can then be built.
- They can destroy plants bearing diseases.
- They help the crops grow by destroying the weed that is robbing the crops water, nutrients and sunlight.
- They are relatively safe on lands which may erode.
LIMITATIONS OF HERBICIDES
- Some herbicides are non-biodegradable and are harmful for a long period of time.
- They are all slightly toxic.
- They can cause illnesses. Glyphosate, a herbicide, can cause eye and skin problems and upper respiratory effects in the user. Paraquat can cause irritations to the skin and may also lead to death, accidental and even suicidal. Many other diseases and illness can be caused such as cancer, nausea, headaches, chest pains, and fatigue.
- The can be carried into streams by runoff rainwater or leached into underground water supplies polluting them.
- Herbivores may eat the plants treated with herbicides and then carnivores eat the herbivores. The toxic herbicide would be passed up the food chain increasing in concentration each time resulting in cancers and even deaths.