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Advances in Natural and Applied Sciences, 5(2): 201-213, 2011
ISSN 1995-0772
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLE
Herbicides and its Applications
1
1
2
Hamid, A.A., 2Aiyelaagbe, O.O. and 1Balogun, G.A.
Department of Chemistry, University of Ilorin, P.M.B. 1515, Ilorin-Nigeria.
Department of Chemistry, University of Ibadan, Ibadan-Nigeria.
Hamid, A.A., Aiyelaagbe, O.O. and Balogun, G.A.: Herbicides and its Applications
ABSTRACT
Herbicides are chemical substances used to destroy or inhibit the growth of plants, especially weeds. In
this study, the classifications of herbicides based on its activity, usage, mechanisms of action and chemical
characteristics were reviewed. The effects of herbicides which include environmental, health and ecological
effects were discussed. Further, the properties, chemistry, effects and applications of some named herbicides
were also treated in this study.
Key words: 2,4-dichlorophenoxyacetic acid, atrazine, glyphosate, organic herbicides, inorganic herbicides.
Introduction
The first widely used herbicide was 2,4-dichlorophenoxyacetic acid, often abbreviated 2,4-D. It was first
commercialized by the Sherwin-Williams Paint Company and saw use in the late 1940s. It is easy and
inexpensive to manufacture, and kills many broadleaf plants while leaving grasses largely unaffected (although
high doses of 2,4-D at crucial growth periods can harm grass crops such as maize or cereals). The low cost
of 2,4-D has led to continued usage today and it remains one of the most commonly used herbicides in the
world. Like other acid herbicides, current formulations utilize either an amine salt (usually trimethylamine) or
one of many esters of the parent compound. These are easier to handle than the acid.
2,4-D exhibits relatively good selectivity, meaning, in this case, that it controls a wide number of broadleaf
weeds while causing little to no injury to grass crops at normal use rates. A herbicide is termed selective if
it affects only certain types of plants, and nonselective if it inhibits a very broad range of plant types. Other
herbicides have been more recently developed that achieve higher levels of selectivity than 2,4-D (Kellogg,
200).
The 1950s saw the introduction of the triazine family of herbicides, which includes atrazine, which have
current distinction of being the herbicide family of greatest concern regarding groundwater contamination.
Atrazine does not break down readily (within a few weeks) after being applied to soils of above neutral pH.
Under alkaline soil conditions atrazine may be carried into the soil profile as far as the water table by soil
water following rainfall causing the aforementioned contamination. Atrazine is thus said to have carryover, a
generally undesirable property for herbicides.
Glyphosate, frequently sold under the brand name Roundup, was introduced in 1974 for non-selective weed
control. It is now a major herbicide in selective weed control in growing crop plants due to the development
of crop plants that are resistant to it. The pairing of the herbicide with the resistant seed contributed to the
consolidation of the seed and chemistry industry in the late 1990s.
Many modern chemical herbicides for agriculture are specifically formulated to decompose within a short
period after application. This is desirable as it allows crops which may be affected by the herbicide to be
grown on the land in future seasons. However, herbicides with low residual activity (i.e., that decompose
quickly) often do not provide season-long weed control (Kellogg, 2000).
Corresponding Author: Hamid, A.A., Department of Chemistry, University of Ilorin, P.M.B. 1515, Ilorin-Nigeria.
Phone: +2347035931646
E-mail: [email protected]; [email protected]; [email protected]
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Classification of Herbicides:
Herbicides can be classified by activity and uses.
Classification by Activity:
C
C
Contact herbicides 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.
Systemic herbicides are 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 (Howard, 1992).
Classification by Uses:
C
C
Pre-plant incorporated herbicides are soil applied prior to planting and mechanically incorporated into the
soil. The objective for incorporation is to prevent dissipation through photodecomposition and/or volatility.
Pre-emergent herbicides are applied to the soil before the crop emerges and prevent germination or early
growth of weed seeds.Post-emergent herbicides are applied after the crop has emerged (Manolis, 1997).
Classification by Mechanism of Actions [MOA]:
Their classification by mechanism of action (MOA) indicates the first enzyme, protein, or biochemical step
affected in the plant following application. The main mechanisms of action are:
C
C
C
C
ACCase inhibitors are compounds that kill grasses. 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 the grass plant. The ACCases of grasses are sensitive to these herbicides, whereas the ACCases of dicot
plants are not.
ALS inhibitors: the acetolactate synthase (ALS) enzyme (also known as acetohydroxyacid synthase, or
AHAS) is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine).
These herbicides slowly starve affected plants of these amino acids which eventually leads to inhibition
of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes sulfonylureas
(SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and
sulfonylamino carbonyl triazolinones (SCTs). ALS is a biological pathway that exists only in plants and
not in animals thus making the ALS-inhibitors among the safest herbicides.
EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme EPSPS is used in the synthesis
of the amino acids tryptophan, phenylalanine and tyrosine. They affect grasses and dicots alike. Glyphosate
(Roundup) is a systemic EPSPS inhibitor but inactivated by soil contact.
Synthetic auxin inaugurated the era of organic herbicides. They were discovered in the 1940s after a long
study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone. They have several
points of action on the cell membrane, and are effective in the control of dicot plants. 2,4-D is a synthetic
auxin herbicide (Stryer, 1995). Photosystem II inhibitors reduce electron flow from water to NADPH2+
at the photochemical step in photosynthesis. They bind to the Qb site on the D1 protein, and prevent
quinone from binding to this site. Therefore, this group of compounds causes electrons to accumulate on
chlorophyll molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the
cell occur, and the plant dies. The triazine herbicides (including atrazine) and urea derivatives (diuron) are
photosystem II inhibitors (Stryer, 1995).
Photosystem I inhibitors steal electrons from the normal pathway through FeS - Fdx - NADP leading to
direct discharge of electrons on Oxygen. As result ROS (reactive oxygen species) are produced and
oxidation reactions in excess of those normally tolerated by the cell occur leading to plant death. "Fe-S Fdx step". Diphenilethers herbicides (like Nitrofen , Nitrofluorfen, Acifluoren) hit "Fdx - NADP step.
ALS inhibitors: the acetolactate synthase (ALS) enzyme (also known as acetohydroxyacid synthase, or
AHAS) is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine).
These herbicides slowly starve affected plants of these amino acids which eventually leads to inhibition
of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes sulfonylureas
(SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and
sulfonylamino carbonyl triazolinones (SCTs).
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ALS is a biological pathway that exists only in plants and not in animals thus making the ALS-inhibitors
among the safest herbicides (Stryer, 1995).
Classifification by Chemical Characteristics:
Organic Herbicides:
Almost all herbicides in use today are considered "organic" herbicides in that they contain carbon as a
primary molecular component. A notable exception would be the arsenical class of herbicides. Sometimes they
are referred to as synthetic organic herbicides. Recently the term "organic" has come to imply products used
in organic farming. Under this definition an organic herbicide is one that can be used in a farming enterprise
that has been classified as organic. Organic herbicides are expensive and may not be affordable for commercial
production They are much less effective than synthetic herbicides and are generally used along with cultural
and mechanical weed control practices (Kolberg, 2002). Organic herbicides include:
C
C
C
C
C
Spices are now effectively used in patented herbicides.
Vinegar is effective for 5-20% solutions of acetic acid with higher concentrations most effective but
mainly destroys surface growth and so respraying to treat regrowth is needed. Resistant plants generally
succumb when weakened by respraying.
Steam has been applied commercially but is now considered uneconomic and inadequate. It kills surface
growth but not underground growth and so respraying to treat regrowth of perennials is needed (Kolberg,
2002).
Flame is considered more effective than steam but suffers from the same difficulties.
D-limonene (citrus oil). D-limonene (citrus oil) is a natural degreasing agent that strips the waxy skin or
cuticle from weeds, causing dehydration and ultimately death.
Inorganic Herbicides:
Early chemical herbicides were inorganic compound. Herbicides such as ashes, common salts and bittern
have been used in agriculture since ancient times.Inorganic herbicides is a chemical compound that kills plant
or inhibits their normal growth .A herbicides in a particular formulation and application can be describe as
selective or non selective. In agriculture, selective herbicides are often used instead of tillage, or in a
combination with tillage other agronomic practices, to control without damaging crops. For these low-till or
no-till systems, scientists are using biotechnology to develop crop varieties with increased tolerance for
herbicides .Non selective herbicides, toxic to all plants are used where complete control of plant growth is
required.Other popular inorganic herbicides includes; ammonium sulfamate, carbon bisulfide, sodium chlorate,
sulfuric acid solutions and solutions and formulations containing borate.
Health and Enviromental Effects:
Herbicides have widely variable toxicity. In addition to acute toxicity from high exposures there is concern
of possible carcinogenicity as well as other long-term problems such as contributing to Parkinson's disease.
Parkinson's disease (also known as Parkinson's, Parkinson disease, or PD) is a degenerative disorder of the
central nervous system that impairs motor skills, cognitive processes, and other functions. The most obvious
symptoms are motor-related, including tremor, rigidity, slowness of movement, and postural instability. Among
non-motor symptoms are autonomic dysfunction and sensory and sleep difficulties. Cognitive and behavioral
problems, including dementia, are common in the advanced stages of the disease. PD usually appears around
the age of 60, although there are young-onset cases (Howard, 1992).
Some herbicides cause a range of health effects ranging from skin rashes to death. The pathway of attack
can arise from intentional or unintentional direct consumption, improper application resulting in the herbicide
coming into direct contact with people or wildlife, inhalation of aerial sprays, or food consumption prior to
the labeled pre-harvest interval. Under extreme conditions herbicides can also be transported via surface runoff
to contaminate distant water sources. Most herbicides decompose rapidly in soils via soil microbial
decomposition, hydrolysis, or photolysis. Phenoxy herbicides are often contaminated with dioxins such as
TCDD; research suggested that such contamination results in a small rise in cancer risk after exposure to these
herbicides. Triazine exposure has been implicated in a likely relationship to increased risk of breast cancer,
although a causal relationship remains unclear (Manolis, 1997; Stryer, 1995). Herbicide manufacturers have
made bold and false or misleading claims about the safety of their products.
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Chemical manufacturer Monsanto Company agreed to change its advertising after pressure from New York
attorney general Dennis Vacco; Vacco complained about misleading claims that its spray-on glyphosate based
herbicides, including Roundup, were safer than table salt and "practically non-toxic" to mammals, birds, and
fish (Kolberg, 2002).
Roundup is toxic and has resulted in death after being ingested in quantities ranging from 85-200 ml,
although it has also been ingested in quantities as large as 500ml with only mild or moderate symptoms. The
manfucturer of Tordon 101 (Dow AgroSciences, owned by the Dow Chemical Company) has claimed that
Tordon 101 has no effects on animals and insects, in spite of evidence of strong carcinogenic activity of the
active ingredient Picloram in studies on rats (Kolberg, 2002).
The risk of Parkinson's disease has been shown to increase with occupational exposure to herbicides and
pesticides. The herbicide paraquat is suspected to be one environmental factor causing Parkinson's disease
(Gorell, 1998; Dinis-Oliveira, 2006).
All organic and non-organic herbicides must be extensively tested prior to approval for commercial sale
and labeling by the Environmental Protection Agency. However, because of the large number of herbicides
in use, there is significant concern regarding health effects. In addition to health effects caused by herbicides
themselves, commercial herbicide mixtures often contain other chemicals, including inactive ingredients, which
have negative impacts on human health. For example, Roundup contains adjuvants which, even in low
concentrations, were found to kill human embryonic, placental, and umbilical cells in vitro (Benachour, 2008).
One study also found that Roundup caused genetic damage, but found that the damage was not caused by the
active ingredient.
Some herbicides may have therapeutic uses. There is current research into the use of herbicides as an antimalaria drug that targets the plant-like apicoplast plastid in the malaria-causing parasite Plasmodium falciparum
(Peluso, 1998).
Ecological Effects:
Herbicide use generally has negative impacts on bird populations, although the impacts are highly variable
and often require field studies to predict accurately. Laboratory studies have at times overestimated negative
impacts on birds due to toxicity, predicting serious problems that were not observed in the field (Lawrence).
Most observed effects are due not to toxicity but to habitat changes and the decrease in abundance of species
birds rely on for food or shelter. Herbicide use in silviculture, used to favor certain types of growth following
clearcutting, can cause significant drops in bird populations. Even when herbicides are used which have low
toxicity to birds, the herbicides decrease the abundance of many types of vegetation which the birds rely on.
Herbicide use in agriculture in Britain has been linked to a decline in seed-eating bird species which rely on
the weeds killed by the herbicides. Heavy use of herbicides in neotropical agricultural areas has been one of
many factors implicated in limiting the usefulness of such agricultural land for wintering migratory birds
(MacKinnon, 1993; Ian Newton,; Robbins).
Scientific Uncertainty:
The health and environmental effects of many herbicides is unknown, and even within the scientific
community there is often disagreement on the risk. For example, a 1995 panel of 13 scientists reviewing
studies on the carcinogenicity of 2,4-D had divided opinions on the likelihood that 2,4-D causes cancer in
humans (Ibrahim, 1991). In 1992 there were too few studies on phenoxy herbicides to accurately assess the
risk of many types of cancer from these herbicides, even though evidence was stronger that exposure to these
herbicides is associated with increased risk of soft tissue sarcoma and non-Hodgkin's Lymphoma (Howard,
1992).
Classes of Some Common Herbicides:
Glyphosate:
Glyphosate is an aminophosphonic analogue of the natural amino acid glycine, and the name is a
contraction of gly(cine) phos(phon)ate. The molecule has several dissociable hydrogens, especially the first
hydrogen of the phosphate group. The molecule tends to exist as a zwitterion where a phosphonic hydrogen
dissociates and joins the amine group. Glyphosate is soluble in water to 12 g/L at room temperature.
Glyphosate was first discovered to have herbicidal activity in 1970 by John E. Franz, while working for
Monsanto. Franz received the National Medal of Technology in 1987, and the Perkin Medal for Applied
Chemistry in 1990 for his discoveries.
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In an editorial on May 17, 2010, the New York Times noted the appearance of glyphosate-resistant weeds
(Alibhai, 2001; Richard, 2005; People, 1990).
Uses of Glyphosate:
Glyphosate is effective in killing a wide variety of plants, including grasses, broadleaf, and woody plants.
It has a relatively small effect on some clover species. By volume, it is one of the most widely used
herbicides. It is commonly used for agriculture, horticulture, and silviculture purposes, as well as garden
maintenance (including home use) (Caviness, 1971; Shipitalo, 2008).
Glyphosate is supplied in several formulations for different uses:
Ammonium salt,
Isopropyl amine salt.
Glyphosate acid - standalone, as ammonium salt or as isopropyl salt.
Potassium salt
Products are supplied most commonly in formulations of 120, 240, 360, 480 and 680 g active ingredient
per litre. The most common formulation in agriculture is 360 g, either alone or with added cationic surfactants.
For 360 g formulations, European regulations allow applications of up to 12 litres per hectare for control of
perennial weeds such as couch grass. More commonly, rates of 3 litres per hectare are practiced for control
of annual weeds between crops (Johanns, 2005).
Effects of Glyphosate on Humans:
Glyphosate is rated least dangerous in comparison to other herbicides, such as those from the
organochlorine family. Roundup has a United States Environmental Protection Agency (EPA) Toxicity Class
of III (on a I to IV scale, where IV is least dangerous) for oral and inhalation exposure. It does not
bioaccumulate, and breaks down rapidly in the environment (Chivian, 2008; U.S. EPA, 1993; Wojtaszek,
2004).
The United States Environmental Protection Agency considers glyphosate to be relatively low in toxicity,
and without carcinogenic or teratogenic effects. The EPA considered a "worst case" dietary risk model of an
individual eating a lifetime of food entirely from glyphosate-sprayed fields, and with residue levels remaining
at their maximum levels, and concluded that no adverse effects would exist under these conditions In 2007,
the EPA selected glyphosate for further screening for endocrinal disruptor effects (Balthazor, 1986; Fernandeza,
2005; Albers, 2009).
Laboratory toxicology studies suggest that other ingredients combined with glyphosate may have greater
toxicity than glyphosate alone. For example, a study comparing glyphosate and Roundup found that Roundup
had a greater effect on aromatase than glyphosate alone. Another study has shown that Roundup formulations
and metabolic products cause the death of human embryonic, placental, and umbilical cells in vitro even at
low concentrations. The effects are not proportional to Glyphosate concentrations but dependent on the nature
of the adjuvants used in the formulation (Benachour, 2008).
Structure of Glyphosate:
2,4-diclorophenoxyacetic Acid:
2,4-Dichlorophenoxyacetic acid (2,4-D) is a common systemic herbicide used in the control of broadleaf
weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America.
2,4-D is a synthetic auxin (plant hormone), and as such it is often used in laboratories for plant research and
as a supplement in plant cell culture media such as MS medium (Dichlorophenoxyacetic acid, 2,4- (2,4-D):
1989.).
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Uses of 2,4-diclorophenoxyacetic Acid:
2,4-D is primarily used as a herbicide. It is sold in various formulations under a wide variety of brand
names. 2,4-D can be found in lawn herbicide mixtures such as "Weed B Gon MAX", "PAR III", "Trillion",
"Tri-Kil", "Killex" and "Weedaway Premium 3-Way XP Turf Herbicide". All of these mixtures typically
contain three active ingredients: 2,4-D, mecoprop and dicamba. Over 1,500 herbicide products contain 2,4-D
as an active ingredient (Suwa , 1996). 2,4-D is most commonly used for weed control in lawns and other turf,
No2-till burndown, Control of weeds and brush along fences and highway and railroad rights of way, Conifer
release (control of broad-leaf trees in conifer plantings), Grass hayfields and pastures, Cereal grains, Corn and
sorghum (occasionally), and as a synthetic auxin analogue
2,4-D continues to be used, where legal, for its low cost. However, where municipal lawn pesticide bylaws
exist, such as in Canada, alternatives such as corn gluten meal and vinegar-based products are increasingly
being used to combat weeds (Jones, 2008).
Effects of 2,4-Diclorophenoxyacetic Acid:
Different organizations have taken different stances on 2,4-D's cancer risk. On August 8, 2007, the United
States Environmental Protection Agency issued a ruling that stated that existing data does not support a
conclusion that links human cancer to 2,4-D exposure. However, the International Agency for Research on
Cancer (IARC) has classified 2,4-D among the phenoxy acid herbicides MCPA and 2,4,5-T as a class 2B
carcinogen - possibly carcinogenic to humans. A 1995 panel of 13 scientists reviewing studies on the
carcinogenicity of 2,4-D had divided opinions, but the predominant opinion was that it is possible that 2,4-D
causes cancer in humans (Suwa, 1996; Jones, 2008; Ibrahim, 1991).
A 1990 study of farmers in Nebraska, even when adjusting for exposure to other chemicals, found that
2,4-D exposure substantially increased the risk of Non-Hodgkin's lymphoma (NHL). A 2000 study of 1517
former employees of Dow Chemical Company who had been exposed to the chemical in manufacturing or
formulating 2,4-D found no significant increase in risk of mortality due to NHL following 2,4-D exposure, but
did find an increase in risk of mortality due to amyotrophic lateral sclerosis (Zahm, 1990; Burnsa, 2001).
Structure of 2,4-dichlorophenoxyacetic Acid:
Atrazine:
Atrazine, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine, an organic compound consisting of an striazine-ring is a widely used herbicide. Its use is controversial due to widespread contamination in drinking
water and its associations with birth defects, menstrual problems, and cancer when consumed by humans at
concentrations below government standards. Although it has been excluded from a re-registration process in
the European Union, it is still one of the most widely used herbicides in the world (Duhigg, 2009; "Chemicals,
2010).
Uses of Atrazine:
Atrazine is used to stop pre- and post-emergence broadleaf and grassy weeds in major crops. The
compound is both effective and inexpensive, and thus is well-suited to production systems with very narrow
profit margins, as is often the case with maize. Atrazine is the most widely used herbicide in conservation
tillage systems, which are designed to prevent soil erosion.Its effect on yields has been estimated from 6% to
1%, with 3-4% being the conclusion of one review. In another study looking at combined data from 236
university corn field trials from 1986–2005, atrazine treatments showed an average of 5.7 bushels more per
acre than alternative herbicide treatments (Ackerman, 2007; Fawcett, 2008).
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Effects of Atrazine on Amphibians:
Atrazine is a suspected teratogen, causing demasculinization in male northern leopard frog even at low
concentrations, and an estrogen disruptor. A 2010 study found that atrazine rendered 75 percent of male frogs
sterile and turned one in 10 into females. A 2002 study found that exposure to atrazine caused male tadpoles
to turn into hermaphrodites - frogs with both male and female sexual characteristics. But another study,
requested by EPA and funded by Syngenta, was unable to reproduce these results (Fawcett, 2008; Jennifer,
2003; Tyrone, 2003; Mizota, 2006; Pesticide, 2010; Jooste et al., 2005).
Tyrone Hayes, Department of Integrative Biology, University of California, notes that all of the studies
that failed to conclude that atrazine caused hermaphroditism were plagued by poor experimental controls and
were funded by Syngenta, one of the companies that produce the chemical (Jooste et al., 2005). The U.S.
Environmental Protection Agency (EPA) and its independent Scientific Advisory Panel (SAP) examined all
available studies on this topic — including Hayes' work — and concluded that there are "currently insufficient
data" to determine if Atrazine affects amphibian development. Hayes, formerly part of the SAP panel, resigned
in 2000 to continue studies independently (Hayes, 2004). The EPA and its SAP made recommendations
concerning proper study design needed for further investigation into this issue. As required by the EPA,
Syngenta conducted two experiments under Good Laboratory Practices (GLP) and inspection by the EPA and
German regulatory authorities. The paper concluded "These studies demonstrate that long-term exposure of
larval X. laevis to atrazine at concentrations ranging from 0.01 to 100 microg/l does not affect growth, larval
development, or sexual differentiation" (Bichat, 1999). Another independent study in 2008 determined that "the
failure of recent studies to find that atrazine feminizes X. laevis calls into question the herbicide's role in that
decline." A report written in Environmental Science and Technology (May 15, 2008) cites the independent
work of researchers in Japan, who were unable to replicate Hayes' work. "The scientists found no
hermaphrodite frogs; no increase in aromatase as measured by aromatase mRNA induction; and no increase
in vitellogenin, another marker of feminization" (Briggs, 2002).
Structure of Atrazine:
Dicamba:
Dicamba controls annual and perennial rose weeds in grain crops and highlands, and it is used to control
brush and bracken in pastures, as well as legumes and cacti. It kills broadleaf weeds before and after they
sprout. In combination with a phenoxyalkanoic acid or another herbicide, dicamba is used in pastures, range
land, and noncrop areas (fence rows, roadways and wastage) to control weeds. Dicamba is toxic to conifer
species but is generally less toxic to grasses (Crawford, 1998).
Dicamba functions by increasing plant growth rate. At sufficient concentrations, the plant outgrows its
nutrient supplies, and dies.
The enzymes responsible for this first breakdown step is a three-component system called dicamba Odemethylase. One component of the three has recently been incorporated into the genome of a variety of crop
plants making them resistant to dicamba (Mark, 2007).
Uses of Dicamba:
Dicamba is notorious for its ability to persist in dead plants and compost, and has accumulated to
phytotoxic levels in finished compost in a few highly publicized cases. In Seattle, Washington, Dicamba was
widely used for weed control in lawns until prohibited in 1999. There, a city-mandated curbside grass clipping
collection and composting program produced compost with measurable levels of dicamba. Subsequently,
DowAgro, the manufacturer of dicamba, voluntarily deregistered it for lawn uses.
Dicamba is still licensed for lawn use in France and is available under the following names: Bayer Jardin:
Désherbant jeune gazon and Scanner Sélectif gazon Vilmorin: désherbant Gazon LONPAR.
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Effect of Dicamba:
Dicamba does not present unusual handling hazards. It is moderately toxic by ingestion and slightly toxic
by inhalation or dermal exposure (oral LD50 in rats: 757 mg/kg body weight, dermal LD502,000 mg/kg,
inhalation LC50200 mg/L) (Arnold, 2002). In a 3-generation study, dicamba did not affect the reproductive
capacity of rats. When rabbits were given doses of 0, 0.5, 1, 3, 10 or 20 (mg/kg)/day of technical dicamba
from days 6 through 18 of pregnancy, toxic effects on the mothers, slightly reduced fetal body weights, and
increased loss of fetuses occurred at the 10 mg/kg dose. U.S. Environmental Protection Agency (EPA) has
set the NOAEL for this study at 3 (mg/kg)/day. In dog tests, some enlargement of liver cells has occurred,
but a similar effect has not been shown in man (Merck Index, 11th Edition, 3026).
Structure of Dicamba:
Triazine:
A Triazine is one of three organic chemicals, isomeric with each other, whose molecular formula is
C3H3N3 and whose empirical formula is CHN.
Uses of Triazine:
The best known 1,3,5-triazine derivative is melamine with three amino substituents used in the manufacture
of resins. Another triazine extensively used in resins is benzoguanamine. Triazine compounds are often used
as the basis for various herbicides such as cyanuric chloride (2,4,6-trichloro-1,3,5-triazine). Chlorine-substituted
triazines are also used as reactive dyes.
These compounds react through a chlorine group with hydroxyl groups present in cellulose fibres in
nucleophilic substitution, the other triazine positions contain chromophores. Mixtures of Triazines and water
are also used to remove H2S from natural gas.
A series of 1,2,4-triazine derivatives known as BTPs have been considered in the liquid-liquid extraction
community as possible extractants for use in the advanced nuclear reprocessing of used fuel. BTPs are
molecules containing a pyridine ring bonded to two 1,2,4-triazin8-3-yl groups (Benzoguanamine, 2000;
Hansjuergen, 1956).
Effects of Triazine:
Although triazines are aromatic compounds, the resonance energy is much lower than in benzene and
electrophilic aromatic substitution is difficult but nucleophilic aromatic substitution more frequent. 2,4,6Trichloro-1,3,5-triazine is easily hydrolyzed to cyanuric acid by heating with water at elevated temperatures.
2,4,6-Tris(phenoxy)-1,3,5-triazine reacts with aliphatic amines in aminolysis, and this reaction can be used to
give dendrimers.
Pyrolysis of melamine under expulsion of ammonia gives the tri-s-triazine melem. Cyanuric chloride assists
in the amidation of carboxylic acids (Christian, 2000).
The 1,2,4-triazines can react with electron-rich dienophiles in an inverse electron demand Diels-Alder
reaction. This forms a bicyclic intermediate which normally then extrudes a molecule of nitrogen gas to form
an aromatic ring again.
In this way the 1,2,4-triazines can be reacted with alkynes to form pyridine rings. An alternative to using
an alkyne is to use norbornadiene which can be thought of as a masked alkyne (Barbara, 2003).
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Structure of Triazine:
DL-Phosphinothricin:
DL-Phosphinothricin is an herbicide commonly used in plant molecular biology and plant tissue culture.
It is the ammonium salt of glufosinate. It is also known as Glufosinate or Glufosinate ammonium. The
herbicide is a broad spectrum herbicide which is comparatively bio-degradable similar to glyphosate (Mayer,
2004). Glyphosate was first sold by Monsanto Company under the tradename Roundup, but is no longer under
patent. Glufosinate is sold under trade names Basta, Buster and Liberty. Crops have been developed
(genetically engineered) which are resistant to this herbicide through the insertion of the bar gene into plants.
Sructure of DL-Phosphinothricin:
Applications and Formulations of Herbicides:
Most herbicides are applied as water-based sprays using ground equipment. Ground equipment varies in
design, but large areas can be sprayed using self-propelled sprayers equipped with a long boom, of 100 to 50
feet (30 to 15 m) with flat fan nozzles spaced about every 20 inches (510 mm). Towed, handheld, and even
horse-drawn sprayers are also used. Synthetic organic herbicides can generally be applied aerially using
helicopters or airplanes, and can be applied through irrigation systems (chemigation).
A new method of herbicide application involves ridding the soil of its active weed seed bank rather than
just killing the weed. Researchers at the Agricultural Research Service have found that applying herbicides to
fields late in the weed’s growing season greatly reduces its seed production, and therefore fewer weeds will
return the following season. If herbicides are applied at the correct stage in the weed’s growing season, then
the weed’s presence in the soil seed bank will greatly be reduced. Because most weeds are annual grasses,
their seeds will only survive in soil for a year or two, so this method will be able to “weed out” the weed
with only a few years of herbicide application (Mayer, 2004). Weed-wiping may also be used, where a wick
wetted with herbicide is suspended from a boom and dragged or rolled across the tops of the taller plants. This
allows treatment of taller grassland weeds by direct contact without affecting related but desirable plants in
the grassland sward beneath.
Herbicides in Modern Usage:
C
C
C
2, 4-D, a broadleaf herbicide in the phenoxy group used in turf and in no-till field crop production. Now
mainly used in a blend with other herbicides that allow lower rates of herbicides to be used, it is the most
widely used herbicide in the world, third most commonly used in the United States. It is an example of
synthetic auxin (plant hormone).
Aminopyralid is a broadleaf herbicide in the pyridine group, used to control broadleaf weeds on grassland,
such as docks, thistles and nettles. Notorious for its ability to persist in compost.
Atrazine, a triazine herbicide used in corn and sorghum for control of broadleaf weeds and grasses. Still
used because of its low cost and because it works extrodinarily well on a broad spectrum of weeds
common in the U.S. corn belt, Atrazine is commonly used with other herbicides to reduce the over-all rate
of atrazine and to lower the potential for groundwater contamination, it is a photosystem II inhibitor
(Ackerman, 2007).
Adv. in Nat. Appl. Sci., C(C): CC-CC, 2011
C
C
C
C
C
C
C
C
210
Clopyralid is a broadleaf herbicide in the pyridine group, used mainly in turf, rangeland, and for control
of noxious thistles. Notorious for its ability to persist in compost. It is another example of synthetic auxin.
Dicamba, a post-emergent broadleaf herbicide with some soil activity, used on turf and field corn. It is
another example of a synthetic auxin (Arnold, 2002).
Glufosinate ammonium, a broad-spectrum contact herbicide and is used to control weeds after the crop
emerges or for total vegetation control on land not used for cultivation.
Fluroxypyr, a systemic, selective herbicide used for the control of broad-leaved weeds in small grain
cereals, maize, pastures, range land and turf. It is a synthetic auxin. In cereal growing, fluroxypyr's key
importance is control of cleavers, Galium aparine. Other key broad-leaved weeds are also controlled.
Glyphosate, a systemic non-selective (it kills any type of plant) herbicide used in no-till burndown and
for weed control in crops that are genetically modified to resist its effects. It is an example of an EPSPs
inhibitor (Shipitalo, 2008).
Imazapyr a non-selective herbicide used for the control of a broad range of weeds including terrestrial
annual and perennial grasses and broadleaved herbs, woody species, and riparian and emergent aquatic
species.
Imazapic, a selective herbicide for both the pre- and post-emergent control of some annual and perennial
grasses and some broadleaf weeds. Imazapic kills plants by inhibiting the production of branched chain
amino acids (valine, leucine, and isoleucine), which are necessary for protein synthesis and cell growth.
Linuron is a non-selective herbicide used in the control of grasses and broadleaf weeds. It works by
inhibiting photosynthesis.
Historical Interest of 2,4,5-Trichlorophenoxyacetic Acid:
2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) was a widely used broadleaf herbicide until being phased out
starting in the late 1970s. While 2,4,5-T itself is of only moderate toxicity, the manufacturing process for 2,4,5T contaminates this chemical with trace amounts of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). TCDD is
extremely toxic to humans. With proper temperature control during production of 2,4,5-T, TCDD levels can
be held to about .005 ppm. Before the TCDD risk was well understood, early production facilities lacked
proper temperature controls. Individual batches tested later were found to have as much as 60 ppm of TCDD.
2,4,5-T was withdrawn from use in the USA in 2011, at a time of heightened public sensitivity about
chemical hazards in the environment. Public concern about dioxins was high, and production and use of other
(non-herbicide) chemicals potentially containing TCDD contamination was also withdrawn. These included
pentachlorophenol (a wood preservative) and PCBs (mainly used as stabilizing agents in transformer oil). Some
feel that the 2,4,5-T withdrawal was not based on sound science. 2,4,5-T has since largely been replaced by
dicamba and triclopyr.
Agent Orange was a herbicide blend used by the U.S. military in Vietnam between January 1965 and April
1970 as a defoliant. It was a 50/50 mixture of the n-butyl esters of 2,4,5-T and 2,4-D. Because of TCDD
contamination in the 2,4,5-T component, it has been blamed for serious illnesses in many veterans and
Vietnamese people who were exposed to it. However, research on populations exposed to its dioxin
contaminant has been inconsistent and inconclusive. Agent Orange often had much higher levels of TCDD than
2,4,5-T used in the US. The name Agent Orange is derived from the orange color-coded stripe used by the
Army on barrels containing the product. It is worth noting that there were other blends of synthetic auxins at
the time of the Vietnam War whose containers were recognized by their colors, such as Agent Purple and
Agent Pink.
Control of Weeds with Specific Herbicides:
Weeds Controlled with Velpar
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Grasses
Bushes and Srubs
Other Weeds
Kentucky bluegrass
Bristly sarsaparilla Sheep laurel (lambkill) Birch
Tall white aster
Soft rush
Willow*
Mouse-eared hawkweed
Black sedge
Hardhack
Rough goldenrod
Witchgrass
Poplar
Large-leaved goldenrod
Browntop
Trailing blackberry*
Five-fingered cinquefoil*
Creeping bentgrass
Rhodora
Stitchwort
Wood rush*
Wild Raspberry
Sheep-sorrel
Poverty oat grass
Meadow-sweet
Cow wheat*
Sweet vernal grass
Yarrow
Hair fescue
Purple aster
King devil hawkweed
Canada goldenrod
Fireweed
Wild strawberry
Orange hawkweed
Narrow-leaved goldenrodPearly everlasting
Adv. in Nat. Appl. Sci., C(C): CC-CC, 2011
211
WEEDS CONTROLLED WITH ATRAZINE
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------GRASSES
OTHER WEEDS
Creeping bentgrass
Large-leaved goldenrod*
Sweet vernal grass
Mouse-eared hawkweed
Kentucky bluegrass
King devil hawkweed
Witchgrass
Five-fingered cinquefoil*
Hair fescue
Wild strawberry
Poverty oatgrass
Orange hawkweed
*Browntop
Canada goldenrod*
Stitchwort
Rough goldenrod
Purple aster
Yarrow*
Narrow-leaved goldenrod
These tables are provided as a guide only. Control will vary depending on the conditions in the field. This
table is not an intended as an endorsement of any specific product. Not all weeds are listed in the table. For
a more complete list of weeds controlled with these products, see "Weeds of Eastern Canadian Blueberry
Fields".
Conclusion:
This study has shown that herbicides are chemical substances because of their chemical composition and
are also referred to as economic substances because they unite and are majorly used on agricultural farm.
While some herbicides are naturally supplied (walnut), others are artificial such as 2,4-dichlorophenoxyacetic
acid and Atrazine. These various classes of Herbicides have different physical and chemical properties that
gives them value and usefulness under their own class.
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