Herbicide, commonly known as a weed killer, is a substance used to kill weed.
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A weed in a general sense is a plant that is considered as a plant in an undesired place, at an unwanted time. Weeds may be unwanted for a number of reasons : they might be unsightly, or crowd out or restrict light to more desirable plants or use limited nutrients from the soil. They can harbor and spread plant pathogens that infect and degrade the quality of crop or horticultural plants. Some weeds are nuisance because they have thorns or prickles, some have chemicals that cause skin irritation or tare hazardous if eaten, or have parts that come off and attach to fur or cloths
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Herbicides can be grouped by activity, use, chemical family, mode of action, or type of vegetation controlled.
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Classification by Activity :.
1.Contact
Destroy only the plant tissue in contact with the chemical. Generally, these are the fastest acting herbicides. They are less effective on perennial plants, which are able to regrow from rhizomes, roots or tubers.
2.Systemic
Translocated through the plant, either from foliar application down to the roots, or from soil application up to the leaves. They are capable of controlling perennial plants and may be slower acting but ultimately more effective than contact herbicides.
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Classification by Use :.
Soil-applied herbicides are applied to the soil and are taken up the roots of the target plant..
1.Pre-plant incorporated herbicides are applied prior to planting and mechanically incorporated into the soil. The objective for incorporation is to prevent dissipation through photodecomposition and/or volatility..
2.Pre-emergence herbicides are applied to the soil before crop emergence to prevent germination or early growth of weed seeds..
3.Post-emergence herbicides are applied after crop has emerged.
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Classification by Mechanism of Action.
The classification by mechanism of action ( MOA ) indicates the first enzyme, protein or biochemical step affected in the plant following application..
1.ACCase inhibitors
Acetyl coenzyme A carboxylase ( ACCase ) is part of the first step of lipid synthesis. Thus ACCase inhibitors affect cell membrane production in the meristems of grass plant. ACCase of dicot plants are not sensitive to these herbicides..
2.ALS inhibitors
Acetolactate synthase ( ALS ) enzyme, ( acetohydroxyacid synthase, AHAS ) is the first step in the synthesis of the branched-chain amino acid ( valine, leucine & isoleucine ). These herbicides slowly starve affected plants of these amino acids which eventually lead to inhibition of DNA synthesis.
ALS inhibitors affect both grasses and dicots.
ALS inhibitors includes : sulfonylureas ( SUs ), imidazolinones ( IMIs ), triazolopyrimidines ( TPs ), pyrimidinyl oxybnzoates ( POBs ), and sulfonyamino carbonyl triazolopyrimidines ( SCTs ).
ALS is a biological pathway that exists only in plants and not in animals, thus making the ALS inhibitors among the safest herbicides..
3.EPSPS inhibitors
The enolpyrubylshikimate 3-phosphate synthase enzyme ( EPSPS ) is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine.
EPSPS inhibitors affect both grasses and dicots.
Glyphosate is a systemic EPSPS inhibitors, but inactivated by soil contact..
4.Photosystem II inhibitors
Photosystem II inhibitors reduce electron for from water to NADPH+ at the photochemical step in photosynthesis. They bind to the Qb site on the D1 protein, and prevent quinine from binding to this site. Therefore this group of compounds cause electrons to accumulate on chlorophyll molecules. As a consequence, oxidation reaction in excess of those normally tolerated by the cell occure, and the plant dies.
Triazine and diuron are photosystem II inhibitors..
5.Photosystem I inhibitors
Photosystem I inhibitors steal electron from the normal pathway through FeS – Fdx – NADP leading to direct discharge of electron on oxygen. As result, reactive oxygen species ( ROS ) are produced and oxidation reaction in excess of those normally tolerated by the cell occure leading to plant death.
Diquat, paraquat hit Fe – S – Fdx step.
Nitrofen, nitrofluorfen, acifluoren hit Fdx – NADP step..
6.Synthetic auxin
Synthetic auxin is effective in the control of dicot plants.
2,4-D is a synthetic auxin herbicide.
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Classification by Type of Chemical
Anilides/Anilines
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Acetochlor · alachlor · asulam · butachlor · diethatyl · diflufenican · dimethenamid · flamprop · metazachlor · metolachlor · pendimethalin · pretilachlor · propachlor · propanil · trifluralin
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Aromatic acids
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Aminopyralid · chloramben · clopyralid · dicamba · picloram · pyrithiobac · quinclorac · quinmerac
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cacodylic acid · copper arsenate · DSMA · MSMA
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bensulide · bilanafos · ethephon · fosamine · glufosinate · glyphosate · piperophos
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2,4D · 2,4DB · dichlorprop · fenoprop · MCPA · MCPB · 2,4,5-T
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dithiopyr · fluroxypyr · imazapyr · thiazopyr · triclopyr
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diquat · MPP · paraquat
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ametryn · atrazine · cyanazine · hexazinone · prometon ·prometryn · propazine · simazine · simetryn · terbuthylazine · terbutryn
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chlortoluron · DCMU · metsulfuron-methyl · monolinuron
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Others
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3AT · bromoxynil · clomazone · DCBN · dinoseb · juglone · methazole · metham sodium · sulfentrazone
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Major Herbicides in Use
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2,4,-D
- broadleaf selective, systemic herbicide
- group : phenoxy, synthetic auxin
- MoA : absorbed through leaves, translocate to meristem, increase growth rate
- developed during WW2 by researchers from Rothamsted Experimental Station, lead by Judah Hirsch Quastel
- trade name : Amine®, Rumputox®, Weedtox®
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DCMU
- broad-spectrum herbicide
- group : Urea
- MoA : photosystem II inhibitor
- released in 1954 by Bayer
- trade name : Diuron®
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Dicamba
- broad-sprectrum herbicide
- group : benzoic
- MoA : increase growth rate
- toxic to human being by ingestion and inhalation
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Glyphosate isopropylamine
- broad-sprectrum, systemic herbicide
- group : organophosphorus
- MoA : absorbed through leaves, translocate to meristem, EPSPS inhibitor
- developed in 1970 by John E. Franz for Monsanto
- Trade name : Roundup®, Spark®, BM Glyphosate 41®
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Glufosinate ammonium
- broad-spectrum, contact herbicide
- group : organophosphorus
- trade name : Basta®, Hallmark 15®, Bufos 135®,
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Paraquat dichloride
- broad-sprectrum, contact herbicide
- group : pyridine
- MoA : inhibit photosystem I
- toxic to human being by ingestion
- produced in 1961 by Sinon Corporation for ICI ( now Syngenta )
- BM Parawin 130® , Paranox 13®, CSH-Paraquat®
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Fluazifop-butyl
- grass-selective, systemic herbicide
- group : phenoxyl
- MoA : absorbed through leaves, acetyl CoA carboxylase inhibitor
- trade name : Fusilade®
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