Weedkiller

Types of weedkillers

The work group “Terminology” of the Commission of the biological tests (CEB) of the French Association of protection of the plants, recommends to employ the following definitions for the various types of weedkillers:

According to the penetration of the weedkiller:

foliar Weedkiller : weedkiller pulverized on the Sheet S and absorptive by those.
Weedkiller racinaire : weedkiller applied to the ground and absorptive by the roots. The penetration is carried out by the underground bodies, between the germination of the Adventice and its lifting.

According to the migration of the weedkiller:

Weedkiller of contact : mobile weedkiller of post-lifting far from in the plant and which involves damage of fabrics only around the points of impact and penetration.
systemic Weedkiller : weedkiller of taken or post-lifting which migrates in the plant by wood or the liber, since the points of penetration ( root or breaks into leaf) until the site of action. This phrase is often used in a more restrictive direction to indicate the weedkillers of postlevée conveyed in the plant by the Sève.

According to its selectivity:

selective Weedkiller : weedkiller which a species cultivated under definite condition of uses can tolerate. If these condition of uses are not observed, it can become nonselective. A selective weedkiller is generally effective only on some Adventice S.
total Herbicide : effective weedkiller on the Adventitious whole of the S and also of the cultivated species.

Modes of action

The modes of action of the weedkillers are founded on:

the disturbance of the Photosynthesis,
the Inhibition of the synthesis of the Lipid S,
the inhibition of the synthesis of the amino-acid ,
the disturbance of the regulation of the Auxine,
the inhibition of the cellular Division with the Metaphase,
the inhibition of the synthesis of the Carotenoid (protective pigments of the chorophylles),
the inhibition of the synthesis of the Enzyme PO (protoporphyrinogene oxydase) leading to the synthesis of chlorophyls,
the deregulation of the pH enters the various cellular compartments or uncoupling,
the disturbance of the growth.

In France, water pollutions caused by the phytopharmacological Produits are mainly due to the weedkillers of synthesis.

Principal families of weedkillers

Mineral weedkillers

They were especially used at the beginning of the century. Currently the most used are:

the cyanide of calcium (Ca (CN) ₂), it returns by the roots and penetrates the crude sap for then accumulating in the sheets.

the ferrous sulfate (FeSO₄), weedkiller of contact used to fight against the foam S and which accelerates moreover humification of the vegetation wastes,

the Chlorate of sodium (Na Cl O ₃) which destroys the plants with strong rooting. Powerful oxidant, the soda chlorate penetrates mainly by the roots and is transported by the rough Sève towards the sheets. Its action is not selective and little perdurer up to six months in the ground. It is destroyed by limestone, the organic matters and the reducing bodies, it can be also washed by seepage waters. It is not very toxic for the man but it is a Comburant (which can enter the manufacture of explosives). It can be employed for the devitalisation of the stocks. This last product, because of its danger (risk of explosion) is replaced more and more by organic substances.

Organic weedkillers

They constitute the very vast majority of the weedkillers of the current market. By convenience, one gathers them according to their type of penetration in the plant: ----

the Glyphosate is one weeding total, i.e. a not-selective weedkiller, formerly produced under patent, exclusively by the company Monsanto as from 1974, under the mark Roundup. The Patent having expired, other companies produce from now on glyphosate. The mechanism of action of this Pesticide is systemic but it is not entirely included/understood at present.

Weedkillers racinaires

Dinitroanilines (toluidines)

Appeared in 1960, the dinitroanilines are far from water soluble, have a strong volatility and are often photodégradables: they are thus products has to incorporate in the ground, before with the installation of the culture.

They act by stopping the growth of the seedlings shortly after their germination. They are indicated under the term - unsuitable " of antigerminatif". It is more precisely of the Antimitotiques. They are used in taken against the graminaceous ones. Their toxicity is low and their persistence varies according to the amount employed (a few weeks at one year). Their name ends in the term " line".

Examples: benfluraline, Butraline, fluchloraline, nitraline, orysaline, pendiméthaline, Trifluraline

Substituted Ureas (NH₂-CO-NH₂)

They are exclusively weedkillers. Their absorption is primarily racinaire. Conveyed by the crude sap, they accumulate in the sheets where they inhibit the Photosynthèse. They have a very low solubility in water and present a rather long persistence of action in the ground (2 to 3 months) but variable according to the ecological conditions met (ground, rain, temperature). They have a good deed on the graminaceous ones and some Dicotylédones. They are used into pre or post-lifting. Their toxicity is almost null. Their name ends in the term " uron".

Examples: Chlortoluron, chloroxuron, Cycluron, Diuron, éthidimuron, Fénuron, isoproturon, linuron, monolinuron, méthabenzthiazuron, métobromuron, métoxuron, monuron, thiazafluron, tebuthiuron, thiazafluron, siduron, Néburon…

Triazines

This group has a cyclic structure. They act by blocking the Photosynthèse. They penetrate by absorption via the roots and are conveyed by the crude sap. They are applied directly to the ground. The Maïs is a very tolerant plant with these compounds, in particular with the Atrazine. The Sorgho is also tolerant but the Blé and the Soja are sensitive there. Their toxicity is low and their often good selectivity. Their solubility in water is reduced and thus are involved little in the ground. Their persistence can thus reach 6 to 12 months for some.

Examples: Atrazine, Cyanazine, méthoprotryne, propazine, terbuthylazine, Simazine, simétryne, secbumeton, secbumeton, Terbuméton, amétryne, desmétryne, prométryne, Terbutryne…

Weedkillers racinaires and foliar

Imidazolinones

Certain products of this family are total weedkillers, others are selective. Being absorbed by ways foliar and racinaire, they are independent of the climatic conditions. They act by blocking the activity of enzyme AHAS essential to the synthesis of 3 essential amino-acids: valin, leucine and isoleucine. This prevents the plant from growing and involves a premature senescence. This mode of action explains the little of toxicity of these substances with regard to the animals and the man, sight that the latter cannot synthesize these amino-acids, getting them through the plants. Used on cereals or in total weeding, they are very flexible with employment. Their persistence is several months.

Examples: Imazaméthabenz, Imazapyr…

the Dyphényls-ethers

Synthesized as from 1964, these molecules have 2 cores benzenes connected by an oxygen. They are absorbed by the sheets and the roots. Their transport in the plant is very limited, they have an action of contact. They have an inhibiting effect on the growth of the méristèmes and this fact generally used in are taken or early post-lifting against the graminaceous ones. They also inhibit breathing. Their solubility in water is low and they persist in the grounds from 2 to 4 months. Their toxicity with respect to the mammals is low. Their name generally ends in the term " fène"

Examples: acifluorfène-sodium, aclonifen, Bifénox, bromofénoxime, chlomethoxyfene, Diclofop-methyl, fluorodifene, fomesafen, Lactofène, Nitrofène, oxyfluorfèneé

Foliar weedkillers

Phytohormones de Synthèse

Known in 1942, they are absorbed by the foliage and are conveyed by the sap. Their causticity is null. There are 2 great groups:

first indolacetic drift of the acid § (AIA), growth hormone of the plants. They involve an abnormal growth of the plant (dycotylédone), leading to death.

Most known is the 2,4-D (acid dichloro 2,4 phenoxyacetic), very much used for the selective weeding of the monocotylédones which are not very sensitive there, unlike the dycotylédones. The 2,4,5-T is used as brushwood killer.

compounds deriving from the propionic acids and butyric. They are absorbed by the foliage and accumulate in the zones with intensive cellular divisions (Méristème, bud, root) where they cause an abnormal growth. Their persistence in the straws prohibits the use of these last in horticulture.

Examples: 1) 2,4-D, 2,4-MCPA, triclopyr, diclofop-methyl, 2,4,5-T, 2) 2,4-DP (Dichlorprop), MCPP (Mécoprop), 2,3,6-TBA, Dicamba, Piclorame, Clopyralid, Flurénol…

nitrated Dyes (derivatives of phenol, dinitrophenol)

Derived from the Benzene, this group includes/understands toxic molecules for the animals (insecticide) and the plants. They are of yellow color. They were very much used against a broad range of Dicotylédone S at the stage seedling, for the protection of cereals in treatment of post-lifting. They are weedkillers of contact to fast action involving of necrose on the fabrics which are desiccated and died. They operate the cellular membranes which they perméabilisent with the H+ ions, strongly lowering the pH of the cells. They do not move in the plant, only the touched parts will be assigned by the weedkiller by the appearance of burns to the point of impact.

They are dangerous for the man and the Environnement from their Toxicité high. The DNOC, at the dry state, presents moreover of the risks of explosion. The nitrated dyes are currently replaced by more selective products.

examples: DNBP (dinosèbe), DNOC (Dinitro-Ortho-Cresol), Dinoterbe, PCP (pentachlorophenol).

the Carbamates

Conceived in 1945 for the destruction of graminaceous, these weedkillers are subdivided in 4 categories: - 1) the derivatives of the carbamic acid (NH₂-COOH) which act on the cellular division. - the 2) derivatives of the acid thiocarbamic (NH₂-CO-SH) which inhibit the synthesis of the lipids with long chain and the gibbéréllines. - the 3) derivatives of the acid dithiocarbamic (NH₂-CS-SH) which prevent germination. - the 4) biscarbamates which prevent photosynthesis.

These weedkillers in common have their low toxicity and a more or less large volatility. They disturb the cellular division (antimitotic) and the general physiology of the plant, causing the phenomenon of handle out of basket, die with the sheets which cannot unfold itself.

They generally get busy in taken (thiocarbamates) or post-sowing, sometimes in post-lifting (phenmediphame, barbame). Except for the compounds allates, which persist several months in the ground, their persistence is almost null.

Examples: 1): asulame, barbame, chlorbufame, chlorprophame, prophame, carbétamideé 2) Thiocarbamate S: butilate, cycloate, diallate, triallate, EPTC, molinate, prosulfocarbe, vernolate, pédulate, Thiobencarbe 3) Dithiocarbamate S: métam-sodium, Nabame… 4) Biscarbamates: desmédiphame, phenmédiphame, karbutylate

Quaternary Ammoniums (Bipyridiles)

Synthesized in the Fifties, they are formed by the association of 2 cycles pyridilyques. They are acceptors of photosynthetic, active electrons on the luminous reactions of photosynthesis, causing the stop of the assimilation of CO₂. They also cause the degradation of the unsaturated fatty-acids, the whole of these actions leading to death. They are characterized by their speed of action and their absence of selectivity (weeding total), except for the difenzoquat. They penetrate in the air bodies but migrate little. It is above all the products of contact. They are very water soluble and do not have an effect by treatment of ground because they are strongly absorbed by clays where, so they are only degraded very slowly. They are very toxic for the man and the animals because of absence of antidote.

Examples: Diquat, Paraquat, Difenzoquat.

Controversies

Weedkillers and GMO

The British government and industries of biotechnology ordered a study undertaken by independent scientists during four years on beet and colza transgenic resistant to a weedkiller. In November 2004, the conclusions of this Bright study proves that these GMO do not have negative impact on the Environnement .

Military use of the weedkillers

The war of Vietnam revealed the harmful effects on the populations Vietnameses of " the agent orange" , defoliant formed of a mixture of 2,4-D and 2,4,5 T, used during this conflict by the American army. The 2,4,5-T showed its long remanence and the high toxicity of a dioxane, the 2,3,7,8-tétrachlorodibenzo-p-dioxane * (TCDD) contained with the state of traces in mixture with the active matter. This substance is a residue of synthesis of the 2,4,5-T, 100 times more toxic than the Strychnine. It has a pronounced teratogenic effect. It is estimated that 50 million liters of orange agent, is 20 ' 000 T of active matters containing 167 kg of dioxane were widespread in the jungle and the rice plantations of the South Vietnam of 1962 to 1971. It was recorded there of 1974 to 77 a clear increase in cancers of the liver in this area. This product is shown to generate a congenital disease, the Phocomélie (of the Greek " body of phoque") : Vietnamese children are born without arm and legs. Lastly, it involves serious cutaneous lesions (Chloracné). The TCDD remains a long time in the organization (30 years) where its solubility in oil supports its concentration and its storage of it. It remains a long time in the ground, because the dioxane is assimilated with difficulty by the plants. 2,4,5 T is interdict in 15 country whose USA and its use is severely restricted in 7 others.

Alternatives

The use of green Manure by avoiding the proliferation of adventitious makes it possible to limit the use of weedkillers.

References

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