Herbicides


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Herbicides

 

chemical substances that are used to destroy vegetation. Depending on the type of their action on plants, herbicides are divided into contact types, which kill all forms of plants, and selective types, which strike certain kinds of plants and do not harm others. Contact herbicides are used to destroy vegetation around industrial buildings, at felling areas, airports, railroads, and highways, under high-voltage power lines, and in drainage channels, ponds, and lakes; selective herbicides are used to protect cultivated plants from weeds (chemical weeding). The division is arbitrary, since in the majority of cases, depending on concentration, rate of distribution, and conditions of use, the same substance can be either contact or selective. For example, in doses of 1.2-1.6 kg of active substance per hectare (kg/ha), chlorphenydim and dichlorphenydim destroy annual weeds in cotton crops, whereas at higher doses they affect all plants.

The selectivity of the action of a herbicide is determined by its chemical composition, the form and doses of the preparation, the method and times of crop treatment, the growth phases and the anatomical and morphological structures of the plants, and soil and climatic conditions. A distinction is made between biochemical and topographical selectivity of herbicides. In biochemical selectivity the action of the herbicide is based on its interference with plant metabolism. In most cases biochemical selectivity is manifested as the nonuniform conversion of the herbicides. In resistant plants the herbicide is blocked by the cell components and is decomposed to nontoxic compounds or to toxic compounds, with their subsequent inactivation; in sensitive plants, it either depresses the plant (inhibiting action) or under the action of cellular components is decomposed to toxic compounds that kill the plant. For example, in resistant maize, simazin and atrazin are decomposed to nontoxic compounds; in plants that are sensitive (rootstock perennial weeds), they are not decomposed. The topographical selectivity of herbicides is associated with differences in anatomical and morphological structure and in the method of application. The use of 2M-4Kh in flax sowings is based on a sensitivity of this type. Flax, unlike many dicotyledonous weeds, has low sensitivity to 2M-4Kh when given large-drop spraying of this herbicide in the arborescent phase, since the solution drops run off the narrow leaves, which are covered with a waxy deposit and are placed at a sharp angle to the stem. Plants that are thickly covered with fine hairs, which do not become impregnated, and plants with thick cuticula of low permeability and a small number of stomata, are more resistant to herbicides. For example, varying sensitivity to herbicides

Table 1. Most important herbicides
Generic nameChemical nameLD50*for rats
(mg/kg)
* LD50 is the dosage at which 50% of the experimental animals die
Atrazin (gezaprim, zeazin, primatol, khungazin-PK)...............2-chlor-4-ethylamino-6-isopropylamino-symm-triazine3,000
Banvel-D (dikamba)...............2-methoxy-3,6-dichlorbenzoic acid2,900
Betanal...............3-methoxy-carbonylaminophenyl-N-3-methylphenyl carbamate5,000
2,4-D, amine salts (2,4-DA, komoks-D, dipal)...............Dimethyl- and triethanolamine salts of 2,4-dichlorophenoxyacetic acid1,150-1,200
2,4-D, butyl ester (2,4-DB)...............Butyl ester of 2,4-dichlorophenoxyacetic acid950-975
2,4-D, octyl ester...............Octyl ester of 2,4-dichlorophenoxyacetic acid710-730
2,4-D, γ-chlorcrotyl ester (krotilin)...............γ-chlorcrotyl ester of 2,4-dichlorophenoxyacetic acid500
Daktal...............Dimethyl-2,3,5,6-tetrachloroterephthalate> 3,000
Dalapon (agrapon, basfapon, daupon, propinate, radapon)...............Sodium salt of α,α-dichloropropionic acid7,500
Desmetrin...............2-methylmercapto-4-isopropylamino-6-methylamino-symm-triazine1,390
Diallat (avadeks)...............S-2,3-dichlorallyl-N,N-diisopropylthio carbamate395
Diuron (dichlorphenydim, DMI, DSMI, karmeks...............N-3,4-dichlorphenyl-N1.N1-dimethylurea3,000-3,400
Diphenamide (dimid)...............N1N-dimethyldiphenylacetamide1,000
Dichloralurea (DKhM)...............N,N1-di(2,2,2-trichlor-1-oxy-ethyl)-urea31,600
2,4-DM (butokson, legiumeks-D, embutoks)...............Sodium or diethanolamine salts of 2,4-dichlorophenoxy γ-butyric acid1,500
DNOC (dinitro-ortho-cresol, dinok, krezomon, krezonit, rafatoks, selinon, khedolit)...............Sodium or ammonium salts of 2-methyl-4,6-dinitrophenol85
2,4-DP (dichlorprop)...............Sodium salt of 2,4-dichlorophenoxypropionic acid800
Eptam (ERTS, EPTK)...............S-ethyl-N,N-di-n-propylthio carbamate1,630
Fenazon (pirazon, piramin)...............4-amino-5-chlor-1-phenylpyridazone-63,300
Fenuron (fenidim)...............N-phenyl-N1, N1-dimethylurea7,500
Gramokson (parakvat)...............1,11-dimethyl-4,41-dipyridyl chloride60
Ialan (gidram, ordram, R-4572, molinat)...............S-ethyl-1-hexamethylene-iminothio carbamate680
IFK(profam)...............Isopropyl-N-phenyl carbamate1,000
Ioksinil...............3,5-diiod-4-oxybenzonitrile120-190
Karbin (barban, S-847, chlorinate)...............4-chlorbutynyl-2-N-m-chlorophenyl carbamate600-1,000
Khlor-IFK (neksoval, prevenol!’, chlorprofam, el’bonil)...............Isopropyl-N-3-chlorophenyl carbamate1,500-3,800
Kotoran (I-2059, pakhtaran, ftormeturon)...............N-(3-trifluoromethylphenyl)-N,N-dimethylurea6,000
Lenatsil (venzar, geksilur)...............3-cyclohexyl-5,6-trimethylene uracil11,000
Linuron (aphalone, garnitan, loroks)...............N-3,4-dichlorophenyl-N1-methyl-N1-methoxyurea1,500
Meturin...............N-phenyl-N-oxy-N1-methylurea5,000
Monolinuron (arhezin)...............N1-4-chlorophenyl-N-methoxy-N-methylurea2,250
Monuron (tel’var, chlorphenydim)...............N-4-chlorophenyl-N1,N1-dimethylurea3,600
2M-4Kh (agrokson, dikoteks, leinam, metokson)...............Sodium salt of 2-methyl-4-chlorophenoxyacetic acid700
2M-4KhM (bekson, legumeks M, tropotoks, tropoton)...............Sodium salt of 4-chloro-2-methylphenoxy-y-butyric acid700
2M-4KhP (kombitoks, mekopon, mekoprop, rankoteks, khenodal)...............Sodium, potassium, or diethanolamine salts of 2-methyl-4-chlorophenoxy-α-propionic acid930
Nitrafen (preparation no. 125 VIZR)...............Sodium salts of the products of nitration of alkylphenols separated from resins produced by low-temperature carbonization of shales or coal900-1,300
Sodium pentachlorophenoxide (dautsid-7, dipentoks, penchlorol, premontoks, PKhF, santafen, santobrit)...............Sodium pentachlorophenoxide210-280
Pikloram (tordon, chloramp)...............Potassium or triisopropylamine salts of 3,5,6-trichloro-4-aminopicolinic acid8,200
Prometrin...............2-methylthio-4,6-bis-(isopropylamino)-symm-triazine2,500
Propazin (gezamil, primatol, siprazin, tsekateks)...............2-chloro-4,6-6- bis-(isopropylamino)-symm-triazine5,000
Propanid (DRA, DSRA, rog’iu STAM F-34, FW-734, propanil)...............3,4-dichloropropionanilide1,380
Region (dikvat, preglon)...............1,11-ethylene-2,21-dipyridyl-dibromide400
Simazin (bladeks, guezatop, G-27692, zaapur, khungazin-DT)...............2-chlor-4,6-bis-(ethylamino)-symm-triazine5,000
Solan (pentachlor)...............3-chlor-4-methylanilide-α-methylvaleric acid10,000
2,4,5-T...............Amine salts and esters of 2,4,5-trichlorophenoacetic acid300-500
2,3,6-TBK (benzak, 2-KF, trisben-200, TKhB)...............Sodium, potassium, or ammonium salts of 2,3,6-trichlorbenzoic acid750
Tillam (pebulat)...............S-propyl-N-ethyl-N-butylthio carbamate1,120
TKhA (TSA, sodium trichloroacetate)...............Sodium salt of trichloroacetic acid3,320
Treflan (nitrofor, trifloralin)...............2,6-dinitro-4-trifluoromethyl-N,N-dipropylaniline10,000
Triallat (avadeks BW, diptal)...............S-2,3,3-trichlorallyl-N,N-diisopropylthio carbamate1,340-1,810
Table 2. Use of herbicides in main agricultural crops
WeedHerbicide Dose (kg active substance per ha)Time of use of herbicide
  (Cereal and groat crops except maize and rice)  
Dicotyledons annual and certain perennials...............2,4-D, amine salts
2,4-D, esters
2M-4Kh
 0.7-1.0
0.25-0.4
0.8-1.2
During tillering phase of cereals (until appearance in the tube)
annuals and certain perennials resistant to 2,4-D and 2M-4Kh...............2M-4KhP
2,4-D, amide salts + Banvel-D
2,4-D, esters + Banvel-D
2M-4Kh + Banvel-D
 2.0-3.0
0.4-0.5 + 0.04-0.08
0.15-0.20 + 0.04-0.08
0.4-0.6 +0.04-0.08
annuals resistant to 2,4-D and 2M-4Kh (speedwells, bistorts, nettles, spurry)...............DNOC 2.0-4.0During phase of development of 2-4 leaves on weeds (at shoots of cereals)
perennials...............2,4-D, amine salts 1.4-2.0Over developing weeds after harvesting (on areas to be put under cereals) or over follow land
 2,4-D, amine salts + Banvel-D
2,4-D, esters + Banvel-D
 0.8-1.0 +0.08-0.16
0.6-0.8 +0.08-0.16
Monocotyledons (wild oats in wheat and barley crops)...............Karbit 0.4-0.6During the initial phase of development of the second leaf in wild oats; before the development of the third leaf
 Triallat 1.0-1.5Before planting wheat and barley, with immediate embedding
 Ialan 2.0-3.0
Dicotyledons and monocotyledonous annuals in millet and sorghum crops...............Propazin 1.5-3.0Immediately before preplanting treatment, simultaneously with sowing, or within three to four days after it
 Prometrin 1.5-3.0 
 Simazin 1.5-3.0 
  Rice  
Dicotyledonous perennials and annuals...............2M-4Kh 1.8-2.5 
 2,4-D, amine salts 1.5-2.0During phase of full tillering of rice
 2,4-D, esters 0.8-1.0 
Monocotyledons (velvet grasses) and certain dicotyledons...............Propanid 5.0-7.0Upon formation of 2-3 leaves on weeds
 Ialan 2.0-3.0Before sowing of rice with immediate embedding
  Maize  
Dicotyledons annuals...............2,4-D, amine salts 1.2-1.8Simultaneously with sowing or 3-4 days after it
 2,4-D, esters 0.8-1.0 
annuals and certain perennials...............2,4-D, amine salts 0.6-0.8During phase of development of 3-5 leaves on maize
WeedHerbicide Dose (kg active substance per ha)Time of use of herbicide
 2,4-D, esters 0.2-0.3 
 Simazin 4.0-8.0At time of fall plowing or after it (on plots to be planted with maize)
rootstock perennials...............Atrazin 4.0-8.0 
 Simazin 3.0-6.0In early spring, with embedding in zone of rhizomes
 Atrazin 3.0-6.0 
Dicotyledonous and monocotyledonous annuals...............DNOC 3.0-4.0Simultaneously with sowing or 3-4 days after it
 Sodium pentachlorophenoxide 6.0-8.0 
 Simazin 1.0-4.0 
 Atrazin 1.0-4.0During presowing cultivation, Simultaneously with sowing, or 3-4 days after it
 aphalone 2.0-3.0
  Cotton  
Dicotyledons (rootstock perennials-Johnson grass, Bermuda grass)...............TKhA 80-100Before or immediately after fall plowing (on fields to be planted with cotton)
 Dilapon 40-50
Dicotyledonous and monocotyledonous annuals...............chlorphenydim 1.2-1.6 
 dichlorphenydim 1.2-1.6 
 Kotoran 1.5-2.5Simultaneously with sowing or 3-4 days after it
 Meturin 2.0-3.0 
 Prometrin 2.0-3.0 
 Khlor-IFK 6.0-8.0Before sowing, with immediate embedding with a cultivator or harrow
 Treflan 1.0-1.5
  Flax  
Dicotyledons annuals...............2M-4Kh 0.8-1.0In arborescent phase of flax
annuals resistant to 2M-4Kh...............DNOC 1.5-2.0 
 2M-4Kh + DNOC 0.4-0.5 +0.8-1.5At flax shoots (during phase of development of 2-4 leaves on weeds); then during arborescent phase of flax
 2M-4Kh + Banvel-D 0.3-0.4 + 0.04-0.08
Monocotyledons creeping couch grass...............TKhA 20-30At fall plowing or after it (on fields to be planted with flax)
 propinate 12-16
hardy rye-grass...............Triallat 0.5-1.5At preplanting cultivation
 TKhA 8-12
  Sugar beet  
Dicotyledonous annuals...............Betanal 1.5-2.5During phase of development of 2-3 leaves on beets and weeds
 Fenazon 3.0-4.0During preplanting cultivation or before appearance of 2.0-3.0 beet shoots
 Lenatsil 2.0-3.0
Monocotyledonous and certain dicotyledonous annuals...............Eptam 2.0-4.0Before planting of beets, with immediate embedding
 Tillam 3.0-5.0
Monocotyledons annual (particularly wild oats)...............TKhA 5.0-10During preplanting cultivation
 Triallat1.0-1.5 Before planting or before appearance of beet shoots
annuals (particularly velvet grasses)...............Dichloralurea 5.0-10.0During preplanting cultivation
perennials (couch grass, Bermuda grass)...............TKhA 20.0-30.0At fall plowing or immediately after it
 Dichloralurea 20.0-30.0

even of plants of the same species is explained by topographical selectivity: plants that grow in the shade and on moist soil that is rich in nutrients, especially nitrogen, grow up more delicate and more sensitive to herbicides. Herbicides may have wide or narrow selectivity. For example, 2,4-D, which kills all dicotyledonous plants, has wide selectivity, whereas propanid, which kills Holcus mollis in rice crops, has narrow selectivity.

Depending on their capacity for migration within the plant, contact and selective herbicides are divided into local and systemic categories. Local herbicides (for example, dinitro-ortho-cresol [DNOC], propanid, gramokson, and sodium pentachlorophenoxide), upon coming in contact with the plant, cause local poisoning of parts of the tissue, which rapidly wilt, turn brown, and shrivel. Systemic herbicides (for example, 2,4-D, 2M-4Kh, simazin, atrazin, and chlor-phenydim) can move through the vascular system of plants along with nutrients and metabolic products, causing general poisoning (deformation of the stem and leaves, gradual depression of growth, chloroticity, brittleness of leaves and stems, and sterility), which is particularly valuable for combating perennials and weeds with a powerful root system. Local and systemic herbicides are applied to the leaf surface of plants (leaf herbicides) and to the soil (soil or root herbicides). Many herbicides can be used both to treat underground parts of weeds and for introduction into the soil.

Modern herbicides are organic compounds, which are divided into a few large groups: substituted phenols (DNOC, sodium pentachlorophenoxide), benzonitriles (ioksinil and others), quaternary ammonium compounds (region, ramok-son), chlorophenoxyalkylcarboxylic acid derivatives (2,4-D, 2,4-DM, 2,4-DP, 2M-4Kh, 2M-4KhM, 2M-4KhP, and 2,4,5-T), benzoic acids (2,3,6-TBK, Banvel-D), halogenated aliphatic acids (TKhA, propinate), carbamates (khlor-IFK, IFK, and karbin), thiocarbamates (diallat, eptam, tillam, triallat, and ialan), amides (solan, propanid, and diphena-mide), urea derivatives (dichloralurea, phenuron, meturin, and arhezine), uracil derivatives (lenatsil), triazines (simazin, atrazin, propazin, grometrin, and desmetrin), and herbicides of other groups (daktal, pikloram, and treflan). Some inorganic substances—ammonium sulfamate, potassium cyanate, sodium nitrate, and sodium, magnesium, and calcium chlorates—are also used as herbicides on a limited scale.

More than 1,000 compounds with herbicidal properties are known; about 140 are used to combat weeds (see Table 1). Herbicides are mainly prepared in the form of solutions, pastes, water-moistenable powders, and emulsion concentrates. The main method of use is by spraying, from the ground and from airplanes, using aqueous solutions, emulsions, and suspensions. Granulated herbicides are used, but so far their use has been insignificant. Herbicides are applied before or after fall plowing, at various lengths of time before sowing an agricultural crop, in the period between sowing and the appearance of shoots, and at various phases of development of the shoots.

The herbicide dose depends on the degree of contamination of the fields, the type of strain of the crop, the soil and climatic conditions, and the agrotechnical methods.

The selective action of a herbicide is manifested more strongly at low dosages; at very high dosages it vanishes completely. At identical dosages, a decrease in temperature to below 8°-12° C (except in the case of triazines) weakens the action of the herbicides, whereas an increase in temperature enhances it. Local herbicides act better in clear weather at 18°-22° C. On light soils the herbicide doses are usually less than on heavy, humus-rich soils, which retain the herbicide more strongly. Approximate doses and times and methods of application to plantings of main agricultural crops are given in Table 2.

The following herbicides are used against annual weeds of leguminous crops (doses in kg/ha are given in parentheses): prometrin (1.5-2.5), aphalone (2-3), DNOC (3-4), and sodium pentachlorophenoxide (6-8), which are applied simultaneously with planting or three to four days later; for soybeans, treflan (1-2) is applied before sowing; and for potatoes, amine salts (1-1.5) and esters (0.8-1), 2,4-D and 2M-4Kh (1-1.5), meturin (2-3), and arhezine (2-3), not later than five to six days before the shoots appear. To combat creeping couch grass, TKhA (20-30) is introduced under the spring plowing. The herbicide 2M-4KhM (2-3) is used against annual dicotyledonous weeds of clover; 2,4-DM (1.5-2.5) is used against alfalfa weeds during the development phase of its first triple leaf; to combat dodders, the stubble of perennial grasses is sprayed with DNOC (1.5), sodium pentachlorophenoxide (16-20), and region (1-1.5) one to three days after the first mowing. In gardens, vineyards, and berry patches, the most commonly used herbicides are simazin, atrazin, TKhA, propinate, and region; at mowing times and on pastures, amine salts and esters of 2,4-D are used.

Chemical methods of weed control are usually used in conjunction with agrotechnical methods.

The use of herbicides in agriculture aids in the improvement of agrotechnical methods. For example, chemical weeding made it possible to change over to close hole sowing of maize and cotton and ridge sowing of potatoes and to reduce the number of cultivations of the space between rows. The use of herbicides is very profitable and on the average raises grain harvests by 2.5 centners per ha (2,4-D), rice harvests by 4-7 (propanid), and harvests of maize for verdure by 50 and of corn by 7 (simazin and atrazin). In addition, the use of herbicides provides significant economization in manual labor.

Most herbicides have medium or low toxicity to humans and warm-blooded animals; only a few (DNOC and sodium pentachlorophenoxide) are highly toxic. Most are preserved unchanged in the soil for a maximum of a few weeks, and only some derivatives of triazines, urea, and trichlorobenzoic acid, introduced in large doses, may be preserved for a number of years. To prevent unfavorable action of herbicides (entry into reservoirs, accumulation in plant fodders and animal products, and so on), the rules given in the instructions for their use must be strictly observed; if there are effective biological methods of controlling weeds, preference is given to them. When working with herbicides, rubber gloves, protective clothing, gas masks, and goggles are worn to prevent the preparation from coming into contact with exposed parts of the body and from entering the mouth, nose, and eyes, and the rules of personal hygiene are observed.

REFERENCES

Mel’nikov, N. N., and Iu. A. Baskakov. Khimiia gerbitsidev i reguliatorov rosta rastenii. Moscow, 1962.
Prakticheskoe rukovodstvo po primeneniiu iadokhimikatov i gerbitsidov v rastenievodstve. Moscow, 1963.
Rakitin, Iu. V. “Biologicheski aktivnye veshchestva kak sredstva upravleniia zhiznennymi protsessami rastenii.” In the collection Nauchnye osnovy zashchity urozhaia. Moscow, 1963.
Spravochnik po primeneniiu gerbitsidov. [Moscow] 1964.
Crafts, A. S., and W. Robbins. Khimicheskaia bor’ba s sorniakami. Moscow, 1964. (Translated from English.)
Weed Control Handbook. Edited by J. D. Fryer and B. A. Evans. Oxford-Edinburgh, 1968.

N. N. MEL’NIKOV and L. D. STONOV

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