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Herbicide

The chemicals or biological agents used to protect crop from weeds, diseases or insects, plant pathogens, weeds are known as pesticides.

On the basis of target pesticides can be classified in following ways.

  1. Herbicides: Chemicals are used to control unwanted plants or weeds.
  2. Insecticides: Chemicals are used to control insects.
  3. Fungicides: Chemicals are used to control fungal diseases.
  4. Miticides: Chemicals used to control ticks and mites.
  5. Rodenticides: Chemicals are used to control rodents like rats and mice.
  6. Nematocides: Chemicals are used to control hair-like worms that feed on plant roots.
  7. Molluscicides: Chemicals are used to control slugs & snails.

On the basis of chemical composition of pesticides, they can be classified in three types.
  1. Inorganic pesticides: Molecules are composed of inorganic groups like heavy metals such as lead, arsenic, copper and sulfur.
  2. Organic pesticides: Molecules are composed of open of cyclic carbon chain.
  3. Biological pesticides: Viruses, bacteria, fungi, plants, nematodes, insects and other parasites or predators are also act as pesticides.
Pesticides are highly toxic and can affect skin (dermal), eyes (ocular) and mouth (oral). Due to toxic effects of, pesticides must be store and dispose properly. Pesticides are used as per recommended doses with rubber chemical gloves or respirator.

 

What is Herbicide?

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Some plants which grow and reproduce aggressively and invasively in human-controlled settings like farm fields, parks, gardens, lawns, woods, and other areas are called as ‘weeds’. Some common weeds with their biological names are as follow.

Common Name
Botanical Name
Acacia Species A. melanoxylon, bailyana
African Love Grass
Eragrostis curvula
Agapanthus
Agapanthus praecox
Arum Lily Zantedeschia aethiopica
Blackberry
Rubus fruticosus
Blue Gurrs
Eucalyptus globulus
Coton easter
Cotoneaster pannosus
Dock Rumex spp
Dolichos Dipogon lignosus
Easter LilyAmaryllis belladonna
Evening Prirrose Oenothera spp
Fleabane Conyzaa spp
Garlic weed
Alium triquetrum
Gladiola
Gladiola undulatum
Honeysuckle
Lonicera spp
Hop Bush
Dodonea spp
Inkweed Phytolacca octandra
Kikuyu Pennisetum clandestinum
Pasture Grasses
Dactylis glomerata
Pelargonium
Pelargonium capitatum
Pine trees
Pinus spp
Pittosporum
Pittosporum undulatum
Polygala Polygala myrtifolia
Rose
Rosa spp
Solanum speciesSolanum
Soursob Oxalis incamate
Sydney Golden Wattle
Acacia longifolia
Tagasaste
Chamaecytisus palmensis
Taylorina Psoralea pinnata
Vinca
Vinca major
Watsonia
Watsonia bulbilifera


These unwanted plants compete with the desired plants for typical needs like direct sunlight, soil nutrients, water, and space. They also act as hosts and vectors for plant pathogen and degrade the quality of the desired plants. Weeds also provide food or shelter for animal pests which are otherwise could hardly survive seasonal shortages.

The chemicals or biochemical agents used to control the growth of weeds are called as herbicides or weedkiller. Because of monoculture and mechanization of agricultural processes, the uses of herbicides increase 1.9% per year between 1980-1985. The selectivity of herbicides vary, as some are quite selective, and act only on certain types of plants, while others are non-selective and kill almost any type of plant.

Herbicides are generally applied as water-based sprays by using ground equipments like towed, handheld, and horse-drawn sprayers. Some herbicides like synthetic organic herbicides can applied aerially using helicopters or airplanes or through chemigation.

Some common examples of herbicides with their trade name and way of applying are as follows.

Herbicide
Trade name
Way of use
Amitrole
Arnitol-T
Foliar spray
Brorracil
Hyvar
Broadcast treatment
2,4-D
Weedtrine-ii, Aqua-Kleen,
Maler bane, Plantgard, Lawn- keep, Planotox and Barrage
Foliar spary
Dichloro prop 2,4-DP, weedone 2,4-DP Foliar spary
Dicamba Banvel
Cut surface
Glyphosate Roundup
Foliar spary
Fosamine
Krenite
Foligate treatment
Metsulfuron Escort
Foliar spary
Imazapvr Arsenal
Frill, Foliar, stump treatment
Hexazinone
Velpar Soil uptake
TriclopyrGarlon
Spray
Picloram Tordon
Frill treatment, Broadcast, foliar
Tebuthiuron Spike
Broadcast treatment

Types of Herbicides

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Herbicides are chemical compounds used to control unwanted plants called as weeds. These chemicals are effective and quite reactive. These are one of the easier and cheaper options to control weeds. They do not affect the soil texture and speared over weed parts. However, herbicides can kill beneficial plants and contaminate the environment also. Some of the plants have resistance over herbicides and no effected by them.

Herbicides can be classified in various ways on the basis of different concepts.

1. On the basis of selectivity


Herbicides can be two types; Non-selective herbicides: These herbicides have ability to kill most of the plant part and also known as broad-spectrum herbicides.

Selective herbicides: These herbicides affect only certain types of plants and also called as narrow spectrum herbicides.

2. On the basis of contact

  • Contact herbicides: These herbicides are unable to reach in underground parts of plant and affect only those parts in which they come in contact with plant. Therefore they are useful on annual weeds. For example, a broadleaf weeds, can affect the above ground leafy part of the plants, not the below ground plants parts like roots, bulbs, tubers, or rhizomes. These herbicides are very popular due to their high reactivity over tissues. The combination of these herbicides with systemic chemicals makes them more reactive over weeds.
  • Translocated herbicides: These herbicides can transported through the plant and kill all parts of weeds. Therefore they use for perennial weeds. These herbicides are also known as Systemic Herbicides. As they follow a systematic path, absorbs through the leaves or stems and transports internally throughout the plant. Since they transport through sap which is quite slow, therefore they do not show a quick "knockdown" effect.
The main advantage of a systemic herbicide is that it will kill the entire plant, so there is no possibility to regrow the weed. The speed of herbicide to affect the plant depends on soil and air temperature, the mode of action of herbicide and the
reactivity of herbicide on particular weed.

3. On the basis of uses

  • Pre-emergent Herbicide: These herbicides are used directly to the soil at 50°C temperature of internal soil for several consecutive days. This temperature is common in the early spring; therefore this is the correct time of germination of first weeds like crabgrass. These herbicides prevent the contact between newly-sprouting weeds and the surface of the ground which forcing them to die upon contact.
  • Post-emergent Herbicide: These herbicides can be used if we have missed the time to use a pre-emergent herbicide, or with new weeds which have already begun to sprout. Post-emergent herbicides usually found in liquid and granular forms and require multiple applications throughout the season and possibly into the fall.
  • Organic Herbicides: These herbicides are batter option for killing stubborn weeds. These herbicides are more natural and healthy, but may not be as effective in many cases.

4. On the basis of molecular structure

  • Chlorophenoxy acid herbicides: These herbicides behave as growth hormones of plants like auxin and causes hypertrophy by causing lethal growth abnormalities. They are selective for broad-leaved or angiosperm plants, and not effective over monocots and conifers. These herbicides are moderately persistent in the environment, with a half-life in soil. The most common examples of Chlorophenoxy acid herbicides are
  1. 2,4-D (2,4-Dichlorophenoxyacetic acid)
  2. 2,4,5-T (2,4,5-Trichlorophenoxyacetic acid)
  3. MCPA (2-Methyl-4-chlorophenoxyacetic acid)
  4. Silvex [2-(2,4,5-Trichlorophenoxy)-propionic acid]
  • Triazine herbicides: These herbicides are mostly used as soil sterilants in corn agriculture. Since these chemicals are not very persistent in surface soils and extreme mobile in nature, therefore cause contamination of groundwater. Some important examples of this class are:
  1. Cynazine or 2-(4-Chloro-6-ethylamino -5-triazin-2-ylamino)-2-methylpropionitrile
  2. Atrazine or 2-Chloro-4-(ethylamino)-6-(isopropylamino)s-triazine
  3. Metribuzin or 4-Amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one
  4. Hexazinone or 3-Cyclohexyl-6-(dimethyl-amino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione
  5. Simazine or 2-chloro-4,6-bis-(ethyl-amino)-s-triazine
  • Organic phosphorus herbicides: A very few organic phosphorus herbicides are known and mainly include glyphosate (N-phosphonomethyl-glycine). Glycophosate transported to perennating tissues like roots and rhizomes, where it interferes with the synthesis of certain amino acids. It is immobile in soil, less toxic and has a moderate persistence.

Natural Herbicide

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Instead of using chemical herbicides there are many natural ways and products which batter act as herbicide with no adverse effect compare to chemical herbicides. Some common natural ways to prevent the growth of herbicides are as follows.

  1. Disturb the soil: Little disturbance in the soil by raking, tilling, or turning it over uncovers the dormant weeds seeds which are lie dormant in the soil for many years.
  2. Destroy weeds: If we destroy the weeds before their flowering process, there will be little chance of their cultivation.
  3. No bare soil: Don’t leave areas of bare soil which invites weed and hindered the growth of other plants.
  4. Crop rotation: Rotation in crop helps to prevent the growth of weeds in the vegetable garden as some crops provide natural weed control which help for next crop also.
  5. Formation of mulch: Formation of organic mulch of one to three inches deep around plants prevents the growth of weeds.
  6. Plant a cover crop: Cover crops like buckwheat or winter rye prevent weed growth in vegetable gardens. These cover crops contain natural growth inhibitors which release toxins and suppress the growth of certain weeds.
  7. Correct watering system: The presence of soaker hoses, watering cans, container watering systems and drip irrigation make all space occupied and decrease the possibility of weeds growth.
  • Use of Ace Scuffle Stirrup Hoe: It has a sharp; double-edge blade which can cuts on both push and pull motions to remove weeds.
  • Use of Burnout 2 Weed & Grass Killer Concentrate: It is pet safe, nontoxic substance which can be used for playgrounds. It is composed form clove oil, lemon juices and vinegar. Burnout 2 is a broad-spectrum, natural herbicide which effects on weeds in eight hours or less.
The use of natural herbicides eliminates the need for toxic chemical herbicides and keeps herbicides out of water system like streams, rivers, and other waterways. They also prevent the exposure to herbicides which have been caused serious genetic damage and health problems.

Organic Herbicide

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Organic herbicides are environmentally friendly option for lawn and garden stores. These herbicides are affective on stubborn weeds. Compare to other abrasive chemicals, these herbicides are not so affective but much more natural and healthy option. Organic herbicides are non-systematic and non selective in nature, therefore only affect the above surface foliage of weeds.

Some common examples of organic herbicides are d- limonene, clove oil, cinnamon oil, citric acid, corn gluten and vinegar. The trade names of some organic herbicides are as follows.
  • Weed Pharm (20% acetic acid)
  • C-Cide (5% citric acid)
  • GreenMatch (55% d-limonene),
  • Matratec (50% clove oil)
  • WeedZap (45% clove oil + 45% cinnamon oil)
  • GreenMatch EX (50% lemongrass oil)
Corn gluten is a pre-emergent herbicide and has a tendency to keep good plants from germinating as well. On the other hand, horticultural vinegar is a post-emergent herbicide and used to kill newly-sprouting plants.

1. Acetic Acid (Vinegar): Vinegar is a 5-10% aqueous solution of acetic acid which can burn the leaves and stems. Vinegar solution which is available in grocery store contains only about 5% acetic acid which is not sufficient to kill weeds. Therefore vinegar used as herbicide is a concentrated solution as high as 50% acetic acid solution. For different weeds, the concentration of solution varies. For examples,
  • Nature’s Glory - Grass and Weed Killer-6.25% acetic acid
  • Greenergy's Blackberry and
  • Brush Block-7.0% acetic acid
  • St. Gabriel Labs Fast Acting
  • Burnout RTU-6.25% acetic acid
It is a non-selective, contact weed killer which will kill the above ground plant tissue and will not translocate in internal parts of weeds. Therefore those weeds which are regenerate from roots, rhizomes or tubers will not be controlled by using this herbicide.

2. Corn Gluten Meal (CGM): It is a side product of corn processing and milling and abbreviated as CGM. It has a natural pre emergent effect and first discovered by Dr. Nick Christians while using corn gluten meal in an unrelated experiment. It acts as a fertilizer containing 9% slow release Nitrogen and also a natural pre emergent for grassy and broad leaf weeds. It absorbs the surface moisture which is needed by tiny germinating seeds. In the lacking of water, weeds are not affected as they can reach water below the effects of the CGM.

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