Sustainable agrochemistry from beginning to advance.

It is about use of chemistry in field of agriculture.

1. Roll No : 364746 submitted to : Dr Muhammad shoaib The Battle for a Sustainable Food Supply

2. Fungicides : Prior to the development and adoption of effective fungicides, uncontrolled plantdisease epidemics significantly reduced crop yields in years favourable for infection. Amongst the many examples is the loss of 38% of wheat production in the USA dueto stem rust in 1916, a reduction of 10-70% in Florida cucumber production dueto downy mildew in the 1920s and a 20% loss of potato production in Maine in the 1930s due to pink rot ( Gianessi and Reigner 2005).

3. Sulphur compounds Lime sulphur , also known as sulphuret of lime, Grison’s compound or hydrosulphate of lime.
The use of sulphur as a fungicide was important, and in California, for example, there was the possibility that complete loss of the grape crop due to powdery mildew could occur without sulphur sprays

Apart from grapes, sulphur preparations were used in the USA on a wide varietvof fruits, such as treatment for brown rot of peaches, powdery mildew of apples gooseberries, hops, omamentals , peaches and strawberries However it was recognised that the spray caused more injury to the fruit and foliage than was desired, and studies were carried out to find ways of reducing phytotoxicity without reducing its effectiveness as a fungicide. Modifications to the lime sulphur solution by adding chemicals having an acidic reaction so that the polysulpide sulphur present was precipitated as elemental sulphur did not seem to have anvsignificant beneficial effect when compared to a weaker lime sulphur solution orthe finely divided elemental sulphur sprays that were available.

4. Lime sulphur and sulphur remained the standard fungicides for the control of such diseases as apple seab ( Venturia inaequalis ) and the powder mildews ( Erysiphaceae ) affecting many species of plants. Sulphur was used for the control of brown sot ( Sclerotinia fructicola ) and seab ( Cladosporium carpophilum )in peaches.
For example, lime Sulphur, containing elemental sulphur as the active ingredient, which was the first successful fungicide for the control of brown rot ( Sclerotinia fructicola ) in peaches was almost completely superseded by sprays containing finely divided sulphur as a suspension in water.

However, by the 1940s, lime sulphur began to be replaced by synthetic organic fungicides which caused less damage to the crop’s foliage.

5. Mercury compounds: Mercury-containing chemicals were also still being widely used as fungicides, particularly meceutic chloride (corrosive sublimate) which was untroduced in 1910 as the first remedy against scow mold ( Fusanom nivale ) in cereals. Meccucic chloride also had a good efficiency against corn smut ( Ustilago maydis ) andelhizoctonia in potatoes.
In 1914, methylmercury became commercially important as a crop fungicide, and there was a shift towards the less phytotoxic organ mercury seed dressings with the launch of chlorophenolmercury in Germany by Bayer tunder the trade name Uspulun (Bayer SeedGrowth2014.
The organomercury compounds gave a broader spectrum of control of seed-borne disease in cereals, compared to copper seed treatments which controlled only buat and could be phytotoxic.

6. 1929, ethyl mercuric chloride was introduced, under the trade name Ceresan , which gave more effective protection against a broad spectrum of fungal diseases compared to previous mercury compounds.
Liquid treatments, using alkyl mercury active ingredients, were reintroduced about the time of World War II. This was because of the reduced hazards and inconvenience to operators dressing the grain in specially designed machines, although alkyl mercury compounds are more toxic than arylmercurials.
By the early 1940s, seed treatments of a wide range of vegetable crops, with various mercury compounds, were recommended to protect the seeds and young, developing seedlings against excessive damping of.

7. Copper compounds: Bordeaux Mixture continued to be a widely used copper based fungicide and remained the standard treatment. The mixture could be “home-made by dissolving gramlated copper sulphate in water in the spray tank and adding much hydrated lime, although commercially prepared Bordeaux mixture Was physically much better and contained a n Bordeaux mixture controlled shot hole Stigmina carnophila ) in almonds increasing yields 200% four to five applications of Bordeaux mixture reduced cranberry fruit rots by 50%, sugar beet yields were increased by 20% with control of cercospora and in tomatoes, phoma rot was reduced from 35 to 6%.
In the case of the Irish potato famine, potato crops were significantly affected by late blight ( Phytophthora infestans ) and a summary of twenty years of experimental data in Vermont (1890-1910) showed ed an an average yield increase of 64% in potatoesas a result of controlling it with Bordeaux mixture.
Bordeaux mixture, like the older copper-based sprays such as Burgundy mixture and ammoniacal copper carbonate, tended to fall out of favour due to their phytotoxic

8. In the 1930s, there was a significant amount of experimental work to develop copper-based fungicides which were less phytotoxic. The work was based on the idea that “insoluble” copper compounds would cause less injury to40 J. Unworthy et al.plants compared to “soluble” copper compounds but would still act on the spores of the fungi. Ordeaux mixture was being replaced by basic copper sulphate , cuprous oxide, copper oxychloride and copperphosphat . Synthetic Organosulphur Compounds : Following the “inorganic” sulphur fungicides were the early group of the so- calledorganosulphur compounds which were discovered somewhat serendipitously when,as reported in the Michigan Technic Magazine, “a rubber chemist suggested to aplant pathologist that derivatives of dithiocarbamic acid, parent substance of a wellknown group of rubber accelerators, be tested as insecticides.

9. Insecticide: Losses due to insect pests could be quite significant with, for example, in the early1900s losses due to the grape berry moth of 25–50% in New York, whilst in Wisconsin, it was common for 35–40% of cabbage heads to be unfit due to caterpillar damage. Losses due to borers in sugarcane in Louisiana were estimated at 8–30%. During this period, particularly in the USA, there was an increasing trend towards monoculture agriculture on farms which resulted in potentially greater crop losses from an insect pest complex that was less diverse but more abundant than previously. This provided ideal feeding conditions for certain insect pests, and, beginning in the 1930s, there was a need for more effective insecticides. However, up until the 1940s, insecticides still tended to be based on natural products, petroleum-based products, sulphur and arsenic.

10. Natural products: Nicotine used either as a smoke, or as extracts from tobacco leaves, had been used as an insecticide since the late seventeenth century. In the period 1900–1934, various nicotine salts or compounds were prepared some of which entered worldwide markets as insecticides notably nicotine sulphate , one of the most stable derivatives of nicotine, which was introduced into the United States in 1912 as Black Leaf 40. Nicotine derivatives were mainly used to give quick action as contact insecticides. However, the fixed nicotine derivatives were active over a much longer period and could be used against leaf chewing insects . Pyrethrum: Pyrethrum has a long history of use as an insecticide, and one aspect of its development was to develop “pyrethrum extenders” which could be added to pyrethrumformulations to increase their efficacy. Today, such compounds, with little or no intrinsic action on their own, are known as synergists. Piperonyl butoxide , now widelyused as an insecticide synergist, was synthesized nd its efficacy demonstrated.

11. Sulphur compounds: sulphur and lime sulphur also have insecticidal and miticidal properties. Flowers of sulphur were widely used against mites such as the red spider, the six-spotted spider mite and the rust mite of citrus. Sulphur was particularly important in controlling false spider mites ( Brevipalpus phoenicis , B. obovatus and B. californicus ) which vector several virus and virus-like diseases of citrus and other crops. Leprosis leprosis which nearly destroyed the Florida citrus industry in the early1920s but which began to disappear in the late 1920s which coincided with the increased use of sulphur . Crop losses due to soil pests,such as grubs, wireworms, cutworms, root maggots and root aphids, were also significant with an estimated loss of upwards of 12 million dollars in the USA in 1912.. Arsenic compounds The most widely used insecticides in the early part of the twentieth century werethe arsenic-containing compounds first introduced in the nineteenth century, such asParis Green, Scheele’s Green and London Purple. Lead arsenate , first introduced in1892, was also popular and replaced Paris Green for certain applications, since it was less phytotoxic and adhered better to the treated foliage, giving a longer insecticidal effect.lead arsenate was rapidly adopted as an insecticide throughout theworld , particularly where codling moth was a pest of apple.Lead arsenateremained the preferred treatment against codling moth on apples which, if not treated,would lead to damage of between 20 and 95% to apples in every orchard.

12. Herbicide Desipite different cultivation practices and hand weeding significant losses can beattributed to the presence of weeds reducing crop yields, with, for example, a 10%reduction in corn, 12–15% in spring cereals and 5–9% in winter cereals . Weeds reduce crop yields by competing for nutrients and space and in addition, they can harbour insect and disease organisms that attack crop plants. For example, black stem rust can use the common barberry, quack grass or wild oats as a host before attacking cereal crops . Although some chemicals were said to be useful for total weed control for tennis courts, driveways and railways, it was suggested that in general that the use of chemicals was a complex, expensive and uncertain process.

13. Inorganic compounds Crop losses due to weeds in growing crops were still significant, and a large number of inorganic compounds were investigated as potential herbicides. Sodium chlorate . The use of sodium chlorate was particularly prevalent in New Zealand for the control of ragwort ( Seneciojacobaea ) which was a particular problem for dairy farmer.ley’s Exploding Trousers”. When sodium chlorate is mixed with organic matter such as wool or cotton fibres , an exothermic reaction can take place leading to clothing exploding and catching fire; this was a particular problem when applicators without2 The Battle for a Sustainable Food Supply 45protective clothing sprayed the chemical for weed control. Sulphuric Acid Sulphuric Acid were also being investigated for the selective control of annual broadleaf weeds infield crops. In a Canadian study, several inorganic compounds, sulphuric acid, coppers nitrate and sulphate , iron sulphate , sodium chloride, sodium dichromate, sodiumchlorate , ammonium thiocyanate and bisulphate , were investigated for the control of broadleaf weed.

14. Organic compounds DNOC ( dinitro -o-cresol ) was first used as an insecticide in 1892, it wasnot until 1932 that it was patented as a selective herbicide in cereals by Truffautand Pastac in France . DNOC the most effective dyestuff to control weeds in cereal. Underthe trade name of SINOX , it was used throughout Europe as a selective spray incereals and flax. Cereals and onions, the growing point is protected by leaves, which is not the case for common broad-leaved weed. In the early 1940s, the discovery by four groups of researchers in the USAand the UK of the first “hormone” herbicides, 2,4-D ((2,4-dichlorophenoxy) aceticacid ) , 2,4,5-T (( 2,4,5-trichlorophenoxy )acetic acid) and MCPA ((4-chloro-2-methylphenoxy)acetic acid), initiated an agricultural revolution and modern weedscience.

15. Phenoxy acetic acid which are selectively active against broad-leaved weeds. Unlike DNOC and its derivatives, the selectivity of the growth regulating compounds does not only depend on differences in external plant form but also in a fundamental difference in the biochemistry of mono- and dicotyledonous plants. The Golden age of pesticides Hence, for example, the introduction of the dithiocarbamate family of fungicides, the methylcarbamate and organophosphate families, aswell as linden and DDT for insecticides, and the dinitro and phenoxy acid families of herbicides. With the discovery of these compounds which had excellent properties as pesticides, there was increasing interest in the chemical control of pests and diseases leading to the so-called Golden Age of Pesticides in the 1940s and 1960s.The many war uses for inorganic compounds and the necessity for organic insecticides—especially nicotine, rotenone and pyrethrum for the control of human disease-bearing insects have created serious shortages for the control of insects injuring agricultural crops and domestic animals. These scarcitieshave resulted in a great production of new and promising insecticides”. Amongstthese new insecticides was DDT which at the time was referred to as “a substance theproperties of which are so remarkable that it has been accorded the doubtful honourof becoming ‘news’. Very little of the vast amount of experimental work done hasyet been published, and the door has thus been opened wide to the wildest speculation and exaggeration”

16. Fungicides Dithiocarbamate Phthalimde Quinones Dithiocarbamate The use of dithiocarbamates as accelerators in the vulcanisation of rubber was described in an extensive review published in 1923 (Bedford and Winkelman 1923);however, it was not until the late 1930s that derivatives of dithiocarbamic acid, thiram disulphides and zinc diethyl dithiocarbamate were tested against fungi growing on plant. . In the early 1940s, trials were carried out with some metal dialkyldithiocarbamates, including sodium, iron, lead, zinc, copper, silver and mercury dimethyl, diethyl and dibutyl dithiocarbamates and selenium diethyl dithiocarbamate , against various fungal diseases. The first commercial dithiocarbamate was tetramethylthiuram disulphide , or thiram ,which was an effective seed dressing and could also be used to protect fruit and vegetable crops from a variety of fungal diseases. . Nabam and zineb were found to have significant fungicidal properties and were particularly useful against late blight in tomatoes and potatoes.

17. 1953, nabam and zineb were used on approximately 75% of the areaused for potatoes in the USA. Whilst potatoes were important in the initial development of the dithiocarbamates , tests showed that these fungicides were effectiveagainst a wide range of disease organisms, on a wide variety of crops Molecular structure of zineb :

18. Phthalimde : Phthalimides which, like the dithiocarbamates , were useful as accelerators in the vulcanizations of rubber were investigated as fungicides.salts of amides and imides. Each of the compounds was found to have fungicidal properties' with one, N-( trichloromethylthio )- tetrahydrophthalimide , commonly known as captan , showing promising results as an agricultural fungicide . Captan Captan came to be a widely used fungicide to protect against a variety of fungal problems on a very wide range of fruit and vegetable crops. For example, in 1993 in the USA, it was estimated that 50% of all apple acreage,60% of almond acreage and 100% of Florida strawberry production were treated with captan .

19. Quinones Just as the dithiocarbamates and phthalimide fungicides arose from research withcompounds originating in the rubber industry, another family, the quinones , camevia a similar route. ”. Trials confirmed thattetrachloroquinone, known as chloranil or commercially as Spergon , worked as a seed protectant against, for example, powdery and downy mildew on vegetables, including cabbage, cauliflower and broccoli. quinone , 2,3-dichloro-1,4-naphthoquinone , known as dichlone or commercially as Phygon , was found to bemore stable and was particularly effective against brown rot of stone fruit and scabon apples and pears, as well as blossom blights. Another non-systemic, protectant fungicide, the organochlorine chlorothalonil (2,4,5,6-tetrachloro-1,3-benzenedicarbonitrile ), was registered for use on crops It found widespread use as a broad-spectrum product against Oomycetes, Ascomycetes, Basidiomycetes and Fungi imperfect on a wide varietyof crop.

20. Insecticide: Chlorinated hydrocarbon organophosphate Methylcarbamate

21. Chlorinated Hydrocarbon Both lindane and DDT were synthesised many years before their potentialas insecticides was realised . After the discovery of the broad-spectrum insecticidalproperties of lindane , gamma-1,2,3,4,5,6-hexachlorocyclohexane, in 1943, at Imperials Chemical Industries in the UK, it became widely used for control of both soil. Lindane Lindane found particular use as a seed treatment against wireworms, which can causesignificant stand losses and ultimately crop yield reductions, and other soil pests ina range of vegetable and field crops . DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) was developed as the firstof the modern synthetic insecticides in the early 1940s. It was initially used withgreat effect to combat malaria, typhus and the other insect-borne human diseasesamong both military and civilian populations.

22. Later, DDT was introduced into agriculture and was hailed as a miracle product(Muir 2012) due to its properties of: Broad spectrum of activity Persistence—did not need to be reapplied often Practically insoluble in water—was not washed off by rain • Relatively inexpensive Easy to apply. After the discovery of the significant insecticidal properties of lindane and DDT,other chlorinated hydrocarbons were synthesised and, on the basis that every newchlorinated hydrocarbon might be a potential DDT, screened for insecticidal activity. One of these, toxaphene , was produced by the chlorination of camphene and wasclaimed to be the most complex mixture of active ingredients.

23. Molecular structure of organochlorine Insecticide from DDTFrom left to right DDT , Methoxychlor and Dicofol .

24. In a research programme looking for new uses for cyclopentadiene , a byproduct of synthetic rubber production, it was unexpectedly found that itreacted with hexachlorocyclopentadiene to give mono- and bis -adducts via theDiels –Alder reaction. These adducts were tested for insecticidal activity, and it wasfound that the mono-adduct, chlordene (4,5,6,7,8-hexachloro-3a,4,7,7a-tetrahydro4,7-methano-1h-indene), did indeed have some activity. However, it was too volatile to compete with DDT as a persistent insecticide, but this could be overcomes by chlorinating the reactive double bond to give the less volatile chlordane(1,3,4,7,8,9,10,10-octachlorotricyclo[5.2.1.02,6]dec-8-ene), (Brooks 1977). Discovered in 1945, chlordane was widely used on agricultural food crops such asvegetables , small grains, maize, oilseeds, potatoes, sugar cane, sugars beet, fruits.

25. Aldrin and dieldrin were used for soil treatment against various soil pests, as a seedtreatment on grains, sugar beet, beans, leeks and onions and as a foliar applicationon various agricultural crops and fruits . ). The main use of endrin was tocontrol insects on cotton, but there was some use in controlling insect pests on rice,sugar cane, grain crops and sugar beet (FAO 1971). Another important insecticidein the cyclodiene family of insecticides is endosulfan (1,9,10,11,12,12-hexachloro4,6-dioxa-5-thiatricyclo[7.2.1.02,8]dodec-10-ene 5-oxide). The introduction of the chlorinated hydrocarbon insecticides in the late 1940s andearly 1950s brought significant benefits to agriculture, particularly in tropical anddeveloping countries, where their general effectiveness, and being relatively inexpensive , allowed farmers to protect their crops in an efficient manner.

26. Rice —in southern Brazil, irrigated rice attacked by weevils lost up to 20% of theyield if not treated with aldrin , whilst upland rice lost up to 25% of the yield ifattacked by termites and other soil pests. Maize —in Guatemala in land infested with wireworms and other soil pests, lossesin the order of 40% occurred if the land was not treated with aldrin . In Mexico,up to 60% of the crop yield was lost if areas with a severe infestation of soil pestswere not treated with either aldrin or heptachlor. Sugarcane —in most countries where sugarcane is grown, attack by indigenoussoil pests could lead to a loss of up to 50% of potential yield unless treated withaldrin , benzene hexachloride or lindane .

27. Sugar beet —in Chile and other South American countries, aldrin was essential toprotect sugar beet over a sufficiently long period • Bananas —in countries where bananas are grown, aldrin , dieldrin or kelevan ( acyclodiene chlordecone derivative) was essential to protect the crop against thebanana weevil borer. Potatoes and sweet potatoes —in some countries, particularly in the tropics, theseimportant staple food crops cannot be grown economically without treatment withaldrin , dieldrin or chlordane against wireworms and other soil pests.

28. Organophosphate Organophosphate insecticides have two distinctive features;they are generally more toxic to vertebrates than other classes of insecticides, andmost are chemically unstable or non-persistent. This last characteristic led to them being used in agriculture as substitutes for the more persistent organochlorides Organophosphates can be divided into three separate groups: aliphatic,phenyl and heterocyclic derivatives. ). Somecommonly used aliphatic organophosphates are malathion, (diethyl 2-[( dimethoxyphosphorothioyl ) sulfanyl ]succinate), dimethoate (O,O-dimethyl S-[2-( methylamino )-2-oxoethyl] phosphorodithioate ), dichlorvos (2,2-dichlorovinyldimethyl phosphate) and mevinphos (2-methoxycarbonyl-1-methylvinyl dimethylphosphate ). common phenyl derivatives include parathion ( O,Odiethyl O-(4-nitrophenyl) phosphorothioate ) and methyl-parathion ( O,Odimethyl O-(4-nitrophenyl) phosphorothioate ), Fig. 2.27, whilst examples ofheterocyclic derivatives are diazinon (O,O-diethyl O-(2-isopropyl-6-methyl-4-pyrimidinyl) phosphorothioate ), azinophos -methyl (O,O-dimethyl S-[(4-oxo1,2,3-benzotriazin-3(4H)- yl )methyl] phosphorodithioate ), chlorpyrifos ( O,Odiethyl O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate and phosmet (S-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl] O,O-dimethyl phosphorodithioate ).

29. Apples —if codling moths were not controlled, crop losses of up to 50% couldoccur in one or two years, and this pest can be controlled with azinphos-methyl,phosmet methyl-parathion and chlorpyrifos . Avocados —the major pest of California avocados is greenhouse thrips which leavescars on the fruit resulting in losses of up to 50%; the thrips can be controlled withmalathion Blueberries —the blueberry maggot, from the blueberry fruit fly, is the major insectpest of blueberries in Maine. The larva feeds from inside the fruit leading toshrunken berries; the presence of infested fruit at harvest can result in the condemnation of whole fields of harvested fruit. The most widely used insecticidesare azinphos -methyl and phosmet . Cherries —the most important insect pests in Michigan tart cherry orchards arecherry fruit fly and plum curculio, both of which directly damage the fruit.Azinphos -methyl, phosmet and chlorpyrifos are widely used against these pestsbecause they give effective control of all the major insect specie Corn —corn rootworm larvae are the most damaging pest in corn production, resulting in yield losses of up to 55% due to root pruning and lodging. Chlorpyrifos andterbufos are widely used against both cutworms and rootworms.

30. Dates —four species of beetles are pests of dates in California, and these have beenknown to cause crop losses of up to 75%; malathion will give control of these pests. Sugarbeet —the sugarbeet root maggot is the most destructive insect pest of sugarbeets in the USA, terbufos and chlorpyrifos can control this pest which, if leftunchecked , can reduce yields by up to 42%. Sugarcane —the sugarcane borer reduces sugar yield by causing retarded growthand stunted stalks, with a resulting loss in plant weight. Before azinphos-methylwas introduced, about 13% of yield was lost in Louisiana despite the use of inorganic insecticides.• Walnuts —codling moth can be a serious pest in walnuts; chlorpyrifos has beenwidely used to control this pest since it is effective and is less toxic to beneficialorganisms in the orchard.

31. The mode of action of the organophosphate insecticides is to inhibit the normalbreakdown of acetylcholine by blocking the acetylcholinesterase enzyme. The resultis a build up of acetylcholine and continuous stimulation of the receptors of targetcells . Research into alternative chemicals with an anticholinesterase activity led in the early 1950s to the development of the N-methyl carbamate family of insecticides . Thus, although the organophosphates and carbamates are distinct families ofchemicals , they both have a similar mode of action .

32. Methyl carbamate the N-methyl carbamates share a similar mode of action with the organophosphates, there is one important difference in that theN -methyl carbamyl -acetylcholinesterase combination dissociates more readily than the phosphoryl-acetylcholinesterase complex. This difference limits the duration ofN -methyl carbamate poisonings and gives a greater difference between the onset of symptoms and the lethal dose compared to most organophosphate compound. ,

33. The importance of the N-methyl carbamate , and organophosphate, insecticides in maintaining crop yield was highlighted in a report from the Agricultural and Food Policy Center at Texas A&M University. This report calculated the loss in yieldand increase in costs, if both families of insecticides were withdrawn from the Usmarket . Even if alternative insecticides or other methods of pest control were used, there would still be significant yield losses, and increased costs, for some crops..

34. Herbicides Crop yield losses would be very significant without the use of herbicides overall, it has been estimated that globally the potential crop yield loss due to weeds is 34%, with wide variations for different crops and regions. For example, on average in North America, 52.1% of soybean yield would be lost with no herbicide weed control; even if best management practiceswere follow the corresponding value for the loss of corn yieldwas 50 . However, crop yield losses could be even greaterunder low moisture conditions with losses of 84% for soybean and 96% for corn(MAFF Canada 2017). In a study, under field conditions in the UK, looking at theeffect of weed species, cleavers , wild oat fieldspeedwell (Veronica persica Poir .), common chickweed and scented mayweed ( Matricaria recutita ) on beans, wheat, barley and oilseed rape,it was found that the highest grain yield loss was caused by cleavers at 33.9%, withsmaller losses due to wild oats at 26.5%, chickweed at 26.3%, speedwell at 21.6% andmayweed at 8.8% .

35. As an example, fieldbindweed was recognised as a serious weed pest in parts of the USAearly in the twentieth century, and studies using various inorganic chemicals, sodiumchlorate , sodium arsenite , sodium hypochlorite, zinc chloride and copper sulphate tocontrol it were carried out. Of the chemicals used, sodium arsenite and zinc chloridekilled the tops of plants but regrowth soon appeared after each treatment and onlysodium chlorate gave promising results . Bindweed cancause significant yield losses if uncontrolled, see Table 2.2, and studies continued into its eradication, focussing on sodium chlorate, but also using sodium chlorideand carbon bisulphide , the latter being used as a fumigant .

36. 1940 to the 1960s, several important new families of herbicides were introducedon to the market, such as the phenylurea , triazine , dinitroaniline , bipyridinium and chloroacetamide herbicides.

37. Phenoxy acid compound Phenoxy acid herbicides are selective for broadleaf weeds, notably against those in cereal crops. They work by mimicking indole acetic acid, an auxin plant growth regulator, thereby producing rapid uncontrolled growth lethally damaging the weeds. The phenoxy acid group of herbicides is comprised of three separate sub-groups; phenoxyacetic, e.g. 2,4-D ((2,4-dichlorophenoxy)acetic acid)and MCPA ((4-chloro-2-methylphenoxy)acetic acid), phenoxybutyric,e.g . 2,4-DB (4-(2,4-dichlorophenoxy) butanoic acid) and MCPB (4-(4-chloro-2-methylphenoxy) butanoic acid), and the phenoxypropionic chemicalssuch as 2,4-DP (2-(2,4-dichlorophenoxy) propanoic acid) and MCPP (2-(4-chloro2-methylphenoxy) propanoic acid).

38. The phenoxy acid herbicides were first marketed from the mid-1940s to the early1960s. As previously mentioned, the first phenoxy acid herbicide was 2,4-D, firstintroduced in 1945, and widely used on a variety of small grain, fruit, nut and vegetable crops for the control of broadleaf weeds and woody plants. Overall, the phenoxy acid herbicides had a significant impact on agricultureand on securing an economically sustainable food supply.

39. In a 2006 study in Canada, it was estimated that if the phenoxyacid herbicides, 2,4-D, MCPA and MCPP, were withdrawn completely for use onwheat and barley, there would be an increase in costs of 338 million dollars. Wheat and barley, there would be an increase in costs of 338 million dollars. This increase being due to increased cost of alternative herbicides and lower yields, as the herbicides used would be less effective. This increase in costs can be considered tobe conservative as the loss in yield was estimated as 0.5–3.5%, whereas other studies have estimated yield losses of 15–37% .

40. Phenyl urea compound Phenylurea derivatives were used extensively as herbicides for the selective controlof germinating grasses and broad-leaved weeds in many crops, including fruit trees,berries , asparagus, sugarcane and especially cereals. Berries, asparagus, sugarcane and especially cereals. They are typically photosystem inhibitors and concentrate in the leaves at the site of photosynthesis. They then block the electron transport system causing a build up of destructive high energy products which destroy chlorophyll and ultimately the leaf tissue .The first phenylurea herbicide, monuron (3-(4-chlorophenyl)-1,1-dimethylurea), was reported in 1951 .

41. For example, 25 black-grass plants per square metre can cause losses, on average, of between less than 5–25%, whilst 100 plants can cause losses of between 5 and 50%,depending on conditions. The phenylurea herbicides isoproturon andchlortoluron were found to give excellent control of black-grass, thus mitigating thepotential yield losses . In Pakistan, where wheat is the staplecrop , a study using isoproturon was reported to give an increase in grain yield of76% when compared to a “weedy” check plot.

42. Phenylurea herbicides rapidly had an important role in weed control and were used globally on a wide range of crops, as can be seen,for example, from the large number of residue tolerances granted in the USA. Triazine compound The first triazine was prepared in 1952, and triazine herbicides were subsequently registered for use in more than 100 countries on a great number of crops . Like the phenylurea herbicides, they are photosystem II inhibitors. The triazine family of herbicides can be divided into three separate groups, chlorotriazines , methoxytriazines and methylthiotriazines , each of which vary in terms of selectivityand use.

43. The most important triazines were atrazine (6-chloro-N-ethyl-N-isopropyl1,3,5-triazine-2,4-diamine) and simazine (6-chloro-N,N-diethyl-1,3,5-triazin2,4-diamine). Atrazine and simazine can be used on corn and sweet corn in theUSA , and crop yield losses in this crop, after replacing these two herbicides with alternatives, have been estimated at about 20%.

44. Atrazine is also used in sugarcane , and cropyield losses have been estimated to be up to 25% if it was not available. The use of simazine for weed control in strawberries has also been investigated, and it was shown that crop yields could be increased more than threefold,compared to untreated control plots, with applications 6 and 12 weeks after planting. In grapes , simazine treatment gave a 33 and 95% increase in yield, compared to untreated controls, in two successive years, this discrepancybeing explained by differences in rainfall between the two years .

45. Dinitroaniline Compound Dinitroaniline compounds had been researched as dye intermediates many yearsbefore some members of this chemical family were discovered to be useful asselective herbicides. it was found that 2,6-dinitroanilineswere more effective than either the 2,3- or 2,4-dinitroanilines, with the first member of the substituted 2,6-dinitroanilines being reported in 1960. Unlike the phenylurea and triazine herbicides, the dinitroanilines are not photosystem II inhibitors, but act by inhibiting mitosis (cell division), thereby limitingnormal root growth . The first successful dinitroanilinecommercialised was trifluralin (2,6-dinitro-N,N-dipropyl-4-( trifluoromethyl )aniline)which was registered in the USA in 1963 as a selective pre-emergent herbicideand was used on a large variety of food crops, most notably broccoli, cabbage,onions , a number of leafy green vegetables, beans, tomatoes, potatoes, wheat, soybeans , sugar beet and sugar cane.

46. Bipridinium compound The discovery of the bipyridinium herbicides is a good example of the benefit ofscreening readily available chemicals to test for possible pesticidal effects. Theirmode of action, which requires both light and oxygen, involves disrupting PhotoSystem I by diverting electrons from the iron– sulphur centres to form, for example,reduced parquet species which react with oxygen to ultimately give highly reactive radicals which attack membranes, thereby inducing breakdown of cells in the sprayed green tissue. For example, diquat has been successfully usedfor preharvest desiccation to allow easier harvesting in peas, beans, oilseed rape and potatoes. Paraquat has been widely used as a herbicide in almost100 countries and by an estimated 25 million farmers, mainly on small farms.

47. On paraquat in Vietnam, the Philippines and China shows that its use can result insignificant increases in crop yields, for example, 18% for maize and 13% for tea in Vietnam, whilst in China, increases of 8, 4 and 3.8–7.1% were obtained for wheat,rice and oilseed rape, respectively.

48. Chloroacetamide compound The chloroacetamide family of herbicides originated from a research programmeinitiated by Monsanto in 1952 to look for compounds that would be activeagainst grasses. The first of the chloroacetamide herbicides to be commercialised , in 1956,was allidochlor (N,N-diallyl-2-chloroacetamide), a pre-emergence herbicide whichwas recommended for the control of grasses. This was followed by propachlor (2-chloro-N-isopropyl-N-phenylacetamide) in 1964, butachlor (N-( butoxymethyl )-2-chloro-N-(2,6-diethylphenyl) acetamide in 1968 and alachlor (2-chloro-N-(2,6-diethylphenyl)-N-( methoxymethyl ) acetamide ) in 1969, Fig. 2.40; later additions to the family were metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(1-methoxy2-propanyl) acetamide ) and acetochlor (2-chloro-N-( ethoxymethyl )-N-(2-ethyl-6-methylphenyl) acetamide ).

49. The chloracetamide family of herbicides contains residual herbicides that share acommon mode of action for the control of more than 30 grasses and broadleaf weedsparticularly in corn, soya and rice. Particularly in corn, soya and rice. They inhibit and distort early seedling growth,which appears to be due to inhibition of cell elongation and cell division ( Fuerst 1987).The utility of alachlor in reducing crop yield losses has been demonstrated with the control of yellow nutsedge ( Cyperus esculentus L.) in corn.

50. Red rice is major problem wherever rice is grown; it is verycompetitive with rice and difficult to control as both belong to the same speciesand share many morphological and physiological characteristics. Treatment of rice with the chloroacetamide herbicides, alachlor , dimethenamid 2-chloro-N-(2,4-dimethyl-3-thienyl)-N-(1-methoxy-2-propanyl) acetamide ), Fig. 2.42,and metolachlor or acetochlor , 15 days before sowing, it was demonstrated thatsignificant yield increases were obtained compared to control plots; for example, in1998 the rice yield was 8440 kg ha−1 for acetochlor treated plots compared to just200 kg ha−1 for control plots .

51. The battle continues It was not until the nineteenth century that great strides were made in crop protection,particularly for the control of fungal diseases and insect pests with chemicals. The large number of inorganic chemicals used came from the growth in mining and the earlier advances in inorganic chemistry, whilst the use of dyestuffs and aromatic chemicals followed the development of organic chemistry in the latter half of the nineteenth century. Prior to 1940s, fruit and vegetable crops were routinely treated against fungalproblems with inorganic chemicals, for example, sulphur , lime sulphur , copper andBordeaux mixture. Bordeaux mixture, for example, proved effective against fungalattacks in controlling grape downy mildew, potato late blight and numerous otherdiseases . By the early 1900s, spraying Bordeaux mixture on potatoes was commonpractice in North America and Europe. In Vermont, for example, a yield increase of64% was obtained as a result of late blight control in potatoes. However, chemical control of weeds was much less successful. Several inorganicchemicals , working as contact herbicides, including copper salts, sodium nitrate andferrous sulphate were available, but the use of herbicides was not widespread.

52. For herbicides, 2,4-D led the revolution in selective weed control, which before1945 relied on cultivation techniques, including hand weeding, or on inorganic saltswhich had been used since the late 1800s with varying degrees of success. Time of the introduction of new herbicide classes, which coincided with a reductionin available labour and increased mechanisation , farmers were ready for improved methods of selective, chemical weed control .

53. Conclusion Ever since mankind changed from a hunter/gatherer way of life to that of a farmer,it has been necessary to combat the pests that threaten the food supply. Early techniques for pest control included religion or folk magic, but hand removal of weedsand insects would have proved to be more reliable. During theso -called Golden Age of Pesticides in the 1940s to the 1960s, there was increasinginterest in organic chemistry and thousands of compounds were routinely screenedfor their pesticidal These new synthetic pesticides, such as the phenoxyacid herbicides, the dithiocarbamate fungicides and the N-methyl carbamate insecticide.

54. As part of the ongoing battle against pests to the increased resistance of pests and increasing resistance to pesticides, various crops were introduced which in themselves were resistant to insects, for example, Bt corn, and cropsthat are tolerant to non-selective herbicides such as glyphosate with soybeans and canola.

55. Thank you