Man has attempted to control unwanted vegetation, the weeds, with some kind of chemical since centuries. But a real breakthrough in the chemical control of weeds was achieved in 1945 with the announcement of 2,4-D and MCPA in the USA and England, respectively, as highly selective, growth regulator type of herbicides to control broadleaf weeds in small grains. It is believed that the two sister herbicides were developed much earlier to 1945, but their development was kept a wartime secret.
In the USA, 2,4- D was developed by P. W. Zimmerman and A. E. Hitchkock at the Boyce Thompson Institute, while in England MCPA and 2,4-D were developed at Jealotts Hills Research Station by W. G. Templeman and W. A. Sexton.
An important advantage of these compounds was that these formed effective herbicides at as low dose as 0.5 kg ha-1, besides being non-corrosive, non-inflammable, non-explosive, and comparatively unharmful to the worker. Therefore, 2.4-D and MCPA were fast accepted for weed control in wheat, barley and oat. Later, several other uses of 2.4-D and MCPA in agriculture were discovered.
After the profound success of 2,4-D and MCPA in agriculture, the scientists, mostly from private companies, have developed more than 300 herbicides by now to control different kinds of weeds in various crop and non-crop situations. The modern herbicides are so perfect that they can control weeds belonging to the same family as the crop, in a highly selective manner.
Herbicide is known by its three names viz. – the common name, chemical name, and the trade name. The common name of a herbicide is the technically accepted abbreviation of its chemical name. The chemical name of a herbicide reads out its molecular structure.
The trade name is the name given to the formulation of a herbicide by its manufacturer or formulator. There can be several trade names of the same herbicide. Sometimes, even there could be more than one common name of a herbicide, depending upon the authority for fixing nomenclature of the chemicals e.g. IUPAC (International Union of Pure and Applied Chemistry) and CAS (Chemical Abstracts Service).
In this article an attempt has been made to present important properties and uses of some more familiar herbicides in India.
1. Phenoxyalkanoic Acids:
2,4-D [(2,4-dichlorophenoxy) Acetic Acid]:
2,4-D is an excellent herbicide used round the world for obtaining highly selective postemergence control of common broadleaf weeds in small grains. It also finds its use in the control of broadleaf weeds and nutsedge in maize, millet, rice, and sugarcane. In transplanted rice, granular formulations of 2,4-D are widely used for obtaining preemergence control of annual weeds.
In the non-crop areas 2,4-D forms a component herbicide for obtaining wide-spectrum vegetation control. In waterbodies it is used largely for the control of water-hyacinth.
The effective rates of 2,4-D are 0.5-1.0 Kg ha-1 on crop lands, and up to 4 kg ha-1 on non-crop lands. It has been observed that certain annual weeds, both grasses and broadleafs, lose their seed viability significantly in response to their foliar treatment with 2,4-D.
The horticulturists use 2,4-D in very low concentrations of 100-1000 ppm as a growth substance, with varied objectives like improving fruit quality, hastening flowering, augmenting production of female flowers, etc.
2,4-D per se is only sparingly soluble in water; therefore, it is converted to either its sodium and amine salts, or different esters before formulating it for use by the farmers. In India, several 2,4-D formulations containing ethyl ester, dimethylamine, and sodium salt are manufactured, in liquid, powder, and granular forms.
In general, the ester formulations of 2,4-D are absorbed by plants the fastest, so much so that the susceptible weeds develop acute phytotoxicity symptoms within a couple of hours or even earlier. Therefore, against the more difficult weeds like Asphodelus tenuifolius, Convolvulus arvensis, and Cirsium arvense, the ester formulations of 2,4-D should be preferred over its amine and sodium salts.
In the susceptible plants, 2,4-D induces stunted growth and extensive epinasty. The plant slowly goes flat on the ground and dies (without loss of green color). The majority of the plant physiologists opine that 2,4-D kills plants by causing abrupt proliferation of its phloem, cambium and swelling of its leaf cells.
This chokes the translocation of food in the plants on one hand, and prevents CO2 intake for photosynthesis, on the other. The result is slow starvation and death of the plant. It may be noted here that though 2,4-D causes very quick abnormalities in the susceptible weeds to immediately stop their interference with the crop growth, yet the death to the affected plants comes only slowly.
MCPA [(4-choloro-2-methylphenoxy) Acetic Acid]:
MCPA has properties and uses similar to 2,4-D, although some plants exhibit higher levels of tolerance, for instance rice and oat, than 2,4-D. Likewise, amongst weeds, nutsedge (Cyperus rotundus) and Canada thistle (Cirsium arvense) succumb more rapidly to MCPA than to 2,4-D.
2,4-DB [4-(2,4-dichlorophenoxy) Butanoic Acid]:
2,4-DB differs from 2,4-D in being per se a non-phytotoxic compound. But several plants, largely the non-leguminous ones, possess certain oxidation enzymes which convert 2,4-DB to 2,4-D at cellular level. Thus, the 2,4-DB treated, susceptible plants actually die in response to 2,4-D.
Certain crop plants like lucerne, pea, linseed, and few clovers, happen not to possess these enzymes. As such, they fail to bring about biochemical conversion of 2,4-DB to 2,4-D and, thus prove tolerant. For these crops 2,4-DB forms a very good postemergence herbicide to control a variety of common broadleaf weeds. The common rates are 0.5-2.0 Kg ha-1.
MCPB [4-(4-chloro-2-methylphenoxy) Butanoic Acid]:
MCPB is very similar in its action and use to 2,4-DB. It is reported more effective on Canada thistle (Cirsium arvense) and more selective to pea and clovers than 2,4-DB.
Diclofop [(±)-2-[4(4-(2,4-dichloropehnoxy) phenoxy] Propanoic Acid]:
Diclofop is a very effective herbicide for the control of late stage wildoat seedlings. It can be applied with this objective in wheat and barley up to their fully tillered stage, as well as in established crop of linseed. Diclofop is also recommended for the control of grasses in soybean. The effective rates are 1-2 kg ha-1.
2. Substituted Alkanoic Acid Herbicides:
Dicamba [3,6-dichloro-2-methoxybenzoic acid]:
Dicamba is found very effective against the perennial, broadleaf weeds like Convolvulus arvensis and Cirsium arvense. At low doeses of up to 0.2 Kg ha-1 it is found selective to small grains and millets. It can also be applied mixed with 2,4-D.
On non-crop lands, including grasslands, at 1 Kg ha-1 dicamba forms a good brush killer. It can be applied either alone or as a component herbicide in any suitable herbicide mixture.
3. Chlorinated Aliphatic Acids:
Dalapon [2,2-dichloropropanoic acid]:
Dalapon is one of the earliest developed, potent grass herbicides. On croplands it finds its major use in killing young grasses by its semi-directed spray in jute, coffee, vineyards, and certain orchards, at rates of up to 7.5 kg ha-1.
On the non-crop lands, a 0.2% to 0.3% foliar spray of dalapon will control robust weeds like johnsongrass (Sorghum halepense), tigergrass (Saccharum spontaneum), and cattails (Typha spp.) Dalapon is manufactured in India as wettable powder of its Na salt. Coffee planters and jute growers are the major consumers of dalapon in India.
At low doses, dalapon causes leaf chlorosis but at higher rates it causes rapid scorching of plants. Against perennial grasses its split application is recommended to achieve its better translocation to the underground parts of the weed.
At cellular level dalapon is known to bring about certain conformational changes in specific enzyme proteins leading to loss of normal permeability controls in the cell membrane, and consequent leakage of cell content into the inter-cellular spaces. Other biochemical effects cannot be ruled out.
4. Substituted Triazines:
Simazine [6-chloro-N, N’-diethyl-1,3,5-triazme-2,4-diamine]:
Simazine is a soil active herbicide, effective against a variety of annual weeds. In India it is used as a preemergence herbicide in maize, sugarcane, and to some extent, in grain millets. Besides these field crops, simazine finds it’s more liberal use in apple orchards, tea, coffee, and pineapple plantations.
The selective rates are 0.5 to 2.0 kgha-1, depending upon the soil type and the proposed rotation crop. Maize and apple will tolerate simazine up to 6 kgha-1. High soil pH and moisture stress tend to reduce crop tolerance to simazine. At higher rates of up to 40 kg ha-1, simazine forms a good soil sterilant on pathways, fencelines, and other non- crop lands.
In the susceptible species, simazine interferes with photosynthesis in chloroplasts and blocks the electron transport system there. It eventually causes oxidation of the chlorophyll molecules. It is also postulated that some secondary phytotoxins may be formed to aid killing of the plants.
The affected plants develop severe leaf chlorosis, margin inward, and eventually dry out. At selective rates the apparent persistence of simazine in soils is 3-4 weeks. In India it is made available as wettable powder.
Atrazine [6-chloro-N-ethyl-N’-(l-methylethyl)-1,3,5- triazine-2,4-diamine]:
Atrazine is very similar in its action and use to simazine, with few differences. In variance with simazine it possesses also some shoot contact activity on juvenile weeds. It is so because atrazine is about six times more water soluble (33 ppm) than simazine. Its shoot activity can be greatly activated by adding to the spray tank a suitable non-phytotoxic oil whence it will also control the more grown-up weeds in maize, millets, sugarcane and apple.
In dryfarming areas atrazine usually provides better weed control than simazine, again perhaps because of its greater water solubility. Atrazine and simazine have been reported to enhance NO-3 reductase enzyme activity in maize and millets, resulting in greener foliage and more proteinaceous grains.
Terbutryn [N-(1,1-dimethylethyl)-N’-ethyl-6-(methylthio)- 1,3,-5-triazine-2,4-diamine]:
Terbutryn is both a soil and shoot-active herbicide. Its premergence application has been found selective to sunflower, pea, blackgram, and potato. In wheat it forms a very good postemergence herbicide to control broadleaf weeds, more effectively than 2,4-D. The recommended rates are 0.5 to 1.0 Kg ha-1. Terbutryn persists in soils for 3-10 weeks.
Metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)- 1,2,4-triazin-5(4H)-one]:
Metribuzin is a broad spectrum herbicide with both, soil and shoot activity. It is found specially useful in weeding potato, tomato, soybean, lucerne, sugarcane, and asparagus grown on medium to heavy texture soils. Rainfall (or irrigation) occurring within five days of the treatment will reduce selectivity to the crop. The effective rates are 0.2 to 1.0 kg ha-1, depending upon the soil type.
5. Substituted Triazoles:
Amitrole (1H-1,2,4-triazol-3-amine):
Amitrole is a highly shoot mobile herbicide. It translocates up to the rhizomes and roots of many perennial weeds, both grasses and broadleafs. Ammonium thiocyanate enhances mobility of amitrole in plants. The combination of the compounds is called amitrole-T. Amitrole-T finds its maximum use in the non-crop areas, including weedy irrigation and drainage channels in the canal command areas.
On crop lands it can be used during fallow seasons, about 2- 3 weeks before sowing a crop. Amitrole-T can be further usefully combined with dalapon against nutsedge and cattails. The effective rates of amitrole are 2-10 Kg ha-1 (or 0.2 to 1.0% spray), depending upon the weed species. Amitrole induces gradual bleaching in plants, turning their leaves milky white. The action is first seen on the younger leaves.
6. Substituted Diazines:
Pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone]:
Pyrazon is a root translocated and shoot contact herbicide, effective against several annual broadleaf weeds. Amongst the crop plants, sugarbeet and redbeet have been found particularly tolerant to this herbicide. Common rates are 2.5-4.0 Kg ha-1 and its persistence in soils is 4-8 weeks.
Pyrazon influences physiological disturbances in susceptible plants, similar to simazine.
Bentazon [3(1-methylethyl) (1H)-2,1,3-benzothia-diazin- (4)(3H)-one 2,2-dioxide]:
Bentazon provides selective, postemergence control of difficult broadleaf weeds in rice, wheat, maize, sorghum, groundnut, and soybean. Rice is very tolerant to the herbicide. The recommended rates are 1-2 Kg ha-1. Persistence in soils is not appreciable.
7. Pyridines and Pyridinols:
Picloram [4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid]:
Picloram is a very potent herbicide for the control of perennial, broadleaf weeds and brushes in grasslands, forests, and non-crop situations. It is both a root and shoot active herbicide. The common rates are 1-10 Kg ha-1 for the soil treatment and 0.25% spray for the foliar spray.
Picloram causes epinasty, split shoots, puckering of leaves, and root deterioration in susceptible plants. At cellular level it stimulates cell elongation and lowers the nuclease enzyme activity.
8. Quaternary Ammonium Compounds:
Paraquat [1,1′-dimethyl-4,4′-bipyridinium ion]:
Paraquat is a contact, non-selective herbicide with zero persistence in soils. It is adsorbed by the soil particles instantaneously. On crop lands it forms an excellent early postemergent herbicide for use in slow germinating crops like potato and sugarcane.
Besides, it is very useful for destroying weed foliage just before sowing of a crop as required in no-tillage and stale seedbed planting of crops. In orchards it is used for destroying weeds standing in the tree rings. In established lucerne (=alfalfa), paraquat is recommended to destroy thick mats of cuscuta.
On the non-crop lands paraquat can be used as a non-selective herbicide for obtaining short term, but immediate destruction of the unwanted vegetation.
Paraquat participates in photosynthesis, resulting in the production of H2O2, which proves phytotoxic to the plants. Paraquat (dichloride) is available in India in liquid form.
Diquat [6,7-dihydrodipyrido (1,2-α: 2′,1′-c) pyrazinediium ion]:
Diquat has properties and uses similar to paraquat. However, it is found somewhat more effective on dicot weeds and less so against grasses.
9. Substituted Uracils:
Bromacil [5-bromo-6-methyl-3-(1-methylpropyl)- 2,4(1H,3H) pyrimidinedione]:
Bromacil finds its largest use in the preemergence control of weeds in established citrus and pineapple orchards. The effective rates are 6.5 Kg ha-1. At higher rates of up to 25 kg ha-1, bromacil forms a good soil sterilant. Its persistence in soil is 5-12 months, depending upon the dose. The uracil herbicides kill plants by biochemical actions similar to triazines.
Terbacil [5-chloro-3-(1,1-dimethylethyl)-6-methyl- 2,4 (1H,3H-pyrimidinedione]:
Terbacil is a soil-active herbicide. It is particularly selective to mints (Mentha spp.) and sugarcane. Beside, it can be used for ring- weeding in three years’ or older apple, peach, and citrus or chards. Rates up to 3.5 Kg ha-1 control annual weeds while at higher rates of up to 9 Kg ha-1, perennial weeds are also controlled. Persistence in soils is 5-12 months.
10. Substituted Ureas:
Diuron [N’-(3,4-dichIorophenyl) -N,N’-dimethylurea]:
Diuron is primarily a soil-active herbicide to control annual grasses. On crop lands it has long been used as preemergence herbicide in cotton in many countries. Besides, it is recommended in asparagus, mint, sugarcane, pineapple, spinach, snapbeans, vineyards, and citrus orchards.
The common selective rates are 0.5- 2.0 Kg ha-1. At higher rates, diuron forms a good soil sterilant for drainage channels and like situations. Also, combined with a suitable wetting agent, diuron can be used to kill young grasses in cotton and orchards by its directed application.
Diuron is made available in India as wettable powder. The substituted herbicides are known to exert physiological disorders in susceptible plants similar to the ones described earlier for simazine. Persistence of diuron in soils is 1-4 months.
Fluometuron [N,N-dimethyl-N’-[3-(trifluoromethyl) phenyl] urea]:
Fluometuron acts much the same way as diuron but it is found more selective to cotton, both as pre-and directed postemergence treatment. It is also used as preemergence herbicide in grain sorghum, sunflower, sugarcane, pineapple, raspberry, cherry, and certain plantation crops. Common rates are 0.5-2.5 Kg ha-1. Persistence in soils is 8-20 weeks.
Isoproturon [N-N-dimethyl-N’ [4-(1-methylethyl) phenyl] urea]:
Isoproturon is the most commonly used substituted urea herbicide in wheat fields in India. Its major use is made for postemergence, selective control of annual grasses like canarygrass (Phalaris minor) and wildoat (Avena sp.), although it will simultaneously control some common broadleaf weeds. The common rates are 0.75-1.5 Kg ha-1, depending upon the weed intensity.
Metoxuron [N’-(3-chloro-4-methoxyphenyl)-N, N- dimethyl urea]:
Metoxuron is recommended for the control of annual grasses in wheat fields at 1-1.5 Kg ha-1.
11. Substituted Carbamates:
Thiobencarb [S-[(4-chIorophenyl)methyl)] diethylcarbamothioate]:
Thiobencarb = (Benthiocarb) was developed primarily for the control of barnyardgrass (Echinochloa spp.) in rice, although it simultaneously controls many other annual weeds. It is applied in lowland rice up to 4 days after transplanting and as preemergence treatment in upland rice. Besides rice, benthiocarb is claimed to be selective to several other field crops and vegetables. In rice the effective rates are 1-2 Kg ha-1.
EPTC [S-ethyl dipropylcarbamothioate]:
It is a preplant, soil incorporated herbicide found effective against annual weeds, particularly grasses. Potato, lucerne, and Egyptian clover (berseem) possess physiological tolerance to this herbicide. Besides, it has been successfully used in maize, safflower, certain beans, and ornamentals. The common rates are 2-4 Kg ha-1 and persistence in soils is 4-6 weeks in moist soils, and up to 16 weeks in the dry ones.
Pebulate [S-propyl butylethylcarbamothioate]:
Pebulate is a preplant, soil, incorporated herbicide, used primarily in transplanted tobacco. It has also shown acceptable selectivity to tomato, sugarbeet, and redbeet. Recommended rates are 3.5-6.0 Kg ha-1. The carbamate herbicides, in general, are mitotic poisons. They check elongation and/or expansion of plumules of germinating weed seedlings.
12. Substituted Acetamides:
Alachlor [2-chloro-N-(2,6-diethylphenyl)-N- (methoxymethyl) acetamide:
Alachlor is a very effective preemergence herbicide in soybean and maize. At common rates of 1-2 Kg ha-1, it will withhold annual weeds for 3-4 weeks. Alachlor has also shown acceptable selectivity to cowpea, sunflower, mustard and certain vegetables. Persistence in soils is 6-10 weeks.
Butachlor (N-(butoxymethyl)-2-chloro-N-(2,6- diethylphenyl) acetamide:
Butachlor has come to stay worldwide as the most important herbicide in rice crops. It is available in liquid form, as well as in granular formulations; the latter being preferred in transplanted rice. Butachlor controls barnyardgrass (Echinochloa spp.) and most other annual weeds effectively. The common rates are 1-2 Kg ha-1.
Propanil [N-(3,4-dichlorophenyl) propanamide]:
Propanil is a shoot contact herbicide, recommended for postemergence control of barnyardgrass and other weeds in rice. Rice plant possess physiological tolerance to propanil. However, certain insecticides have been found to antagonise propanil action and make rice susceptible to the herbicide. As such, no insecticide should be applied a few days before and after application of propanil in rice. The common rates of application are 2-3 Kg ha-1.
The soil-applied substituted acetamides inhibit radical growth in plants, while the shoot-active ones, cause leaf necrosis. At cellular level these herbicides are known to hinder protein synthesis. The shoot-active anilides also inhibit electron transport system in light reaction-II of photosynthesis.
13. Anilines and Toluidines:
Fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4- (trifluoromethyl) benzenamine]:
Fluchloralin is a preplant, soil-incorporated (PPI) herbicide, employed for the control of annual weeds, particularly grasses. It is found” selective to a number of crops, like groundnut, cotton, mustard, cumin, pepper, soybean, and chickpea. It has also been reported selective to sunflower, jute, onion, pea, and certain solaneceous vegetables. The common rates are 0.50-1.0 Kg ha-1. Sometimes fluchloralin is used as a preemergence herbicide at higher rates of 1-1.5 Kg ha-1.
Pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6- dinitrobenzertamine]:
Pendimethalin is primarily a preplant, soil-incorporated herbicide found effective against a number of annual weeds, particularly grasses in cotton, soybean, groundnut, and pea. These crops possess physiological tolerance to the herbicide. Pendimethalin can also be used as premergence herbicide in maize, rice, small grains, onion, and potato. Trianthema spp. are very susceptible to the soil applied Pendimethalin.
As a postemerngence herbicide Pendimethalin is used in transplanted onion and leek against young weeds in their 1.5-leaf stage. Pendimethalin is formulated as emulsifiable concentrate and granules. The selective rates are 1.0 to 1.5 Kg ha-1.
Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine]:
Trifluralin is used in the same manner as fluchloralin. It is found selective to .cotton, soybean, safflower, groundnut, pea, cucurbits, tomato, bellpepper, transplanted cole crops, citrus, and certain ornamentals.
The dinitroanilines cause mitotic aberrations and consequent inhibition of lateral root growth. The affected tissues show reduced levels of proteins, RNA, and of several enzymes.
14. Ether Compounds:
Oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4- (trifluoromethyl) benzene]:
Oxyfluorfen is both a pre-and postemergence herbicide. As a preemergence herbicide it is selective to potato, rice, maize, and certain pulses, oilseeds, and spices. In many transplanted vegetables it can be applied a couple of days before transplanting their seedling. The effective rates are 0.05-0.50 Kg ha-1.
As a postemergence herbicide, oxyfluorfen finds its place in orchards, plantation crops, no-till planting systems, and in non- crop areas. A dose as low as 0.1 Kg ha-1 is found effective in such situations.
Oxyfluorfen is made available in EC and G formulations. The ether herbicides are known to greatly enhance respiration in susceptible plants, damage their cell membranes, and disturb photosynthetic and phosphorylation mechanisms.
15. Miscellaneous Herbicides:
Fluazifop [(R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl) oxy] phenoxy] Propanoic Acid]:
It is a new postemregence herbicide found useful for the control of grasses, both annual and perennial, in crops like groundnut, soybean, sunflower, cotton, potato, jute, and sugarbeet. The effective rates are 0.25 to 0.5 Kg ha-1, depending upon the weed and the stage of crop growth.
Glyphosate [N-(phosphonomethyl) glycine]:
Glyphosate is a highly shoot mobile, weak residue herbicide. This makes it excellent for the control of perennial weeds, both grasses and broad leafs in fallow fields, certain orchards, plantation crops, roadsides, and some other non-crop areas. It can also be applied a few days before planting of crops to destroy any patches of perennial weeds.
Recommended rates are 2-6 Kg ha-1 or 0.2% to 0.6% spray. Glyphosate kills susceptible weeds slowly. It takes 10- 15 days to cause complete mortality of the weed. Being a highly translocated herbicide, it reaches considerable number of rhizomes and tubers of perennial weeds and damages these for a long time.
Flamprop [N-benzoyl-N-(3-chloro-4-fluorophenyl)-DL- alanine]:
Flamprop (methyl) has been developed to control wildoat up to its 3 to 5 leaf stage in wheat and barley. The late stage susceptibility of the weed to flamprop is advantageous since by this time most wildoat seeds have germinated in the field and, therefore, there are little chances of escapes. The common rates are 1-2 Kg ha-1.
Acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl) acetamide:
Acetochlor is a soil-active, preemergence herbicide recommended for the control of annual weeds in rice; both direct sown and transplanted crops. Its effective rates are 0.10-0.2 kg/ha.
Anilofos (S-4-chloro-N-isopropylcarbaniloylmethyl O, O-dimethyl phosphorodithioate]:
Anilofos is a root-active herbicide that is effective against primarily the annual grass weeds, besides some broadleaf species. It is selective to transplanted rice, when applied about one week after transplanting its seedlings. The effective rats are 0.25 – 1.0 kg/ha. It can be tank-mixed with 2.4-D for obtaining improved, simultaneous control of broadleaf weeds.
Chlorsulfuron [2-chloro-N-[[(4-methoxy-6-methyl-1,3,5- triazin-2-yl)amino] carbonyl] benzenesulfonamide]:
It is a shoot-active herbicide, recommended for postemergence application in wheat to control some difficult broadleaf weeds that do not fully succumb to 2,4-D or MCPA.
Clodinafop [(2R)-2-[4-[(5-chloro-3-fluro-2-pyridinyl)oxy] phenoxy] propanoic acid:
Clodinafop is used for the control of grass weeds like Phalaris minor and Avena fatua in wheat. It is applied about 40 days after sowing the crop, at 0.06 – 0.09 kg/ha.
Ethoxysulfuron [2-ethoxyphenyl [[4,6-dimethoxy-2- pyrimidinyl]amino]carbonyl]sulfamate]:
It is a preemergence herbicide developed for selective control of difficult broadleaf weeds and sedges in transplanted rice. It is applied 10 days after transplanting of the crop seedlings, at 0.01- 0.15 kg/ha.
Fenoxaprop [(+)-2-[4-[(6-chloro-2-benzoxazolyl)oxy] phenoxy] propanoic acid:
This herbicide is used to control weedy grasses, chiefly Phalaris minor and Avena fatua in wheat. It is applied 25 days after sowing of the crop, at 0.06-0.12 kg/ha.
Idosulfuron [4-iodo-2[[[[(4-methoxy-6-methyl-1,3,5-triazin- 2-yl) amino] carbonyl] amino] sulfonyl] benzoic acid]:
Idosulfuron is a shoot-active herbicide found effective in the control of difficult broadleaf weeds in wheat and maize. Its effective rates are 0.024-0.03 kg/ha.
Mesoulfuron [2-[3-(4,6-dimethyoxypyrimidin-2-yl) ureidosulfonyl]-4-methanesulfonamido- methybenzoate]:
At 0.012-0.019 kg/ha, mesosulfuron forms a very good postemergence herbicide in wheat and triticale for the control of grass weeds, particularly Phalaris minor and Avena fatua. It is applied 25-35 days after sowing the crop.
Metsulfuron [2-[[[[(4-methoxy-6-methyl-1,3/5-triazin-2-yl) amino] carbonyl] amino] sulfonyl] benzoic acid]:
Metsulfuron finds its major use in wheat for the control of difficult broadleaf weeds that are not controlled well with 2,4-D. Its effective rates are 0.003-0.008 kg/ha.
Oxadiargyl [5-tert-butyl-3-(2,4-dichloro-5-(prop-2- ynyloxy)phenyl]-1,3,4 oxadiazol-2-(3H)-one:
It forms a good wider spectrum, postemergence herbicide for the control of annual graces, broadleaf weeds, as well as sedges in rice. The recommended doses are 0.05-0.150 kg/ ha. It is applied 5-7 days after seeding or transplanting rice.
Sulfosulfuron [N-[[(4,6-dimethoxy-2-pyrimidinyl) amino] carbonyl]-2-(ethysulfonyl) imidazo [1,2-∝] pyridine-3-sulfonamide]:
Sulfosulfuron is found effective in controlling Phalaris minor and other annual grasses like P. minor and A. fatua in wheat, besides considerable broadleaf weeds. It can be applied 20-50 days after sowing of the crop, at recommended doses of 0.025-0.050 kg/ha.