Chemical Weed Control Point Wise Notes For Competitive Exam

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• Herbicide is derived from two latin words

Herba = plant

Caedere = to kill

• Literally the term weedicide is correct but technically weedicide is not an appropriate term. It should kill only weeds not others. But at higher doses it loses selectivity and kills all vegetation including crops. Weeds and crops are herbaceous plants. So herbicide is most appropriate term.

• First herbicide registered and used for selective control of weeds in crops was 2,4-D.

• The credit for introduction of 2,4-D goes for Marth Mitchell (USDA) in lawns for control of broad leaved weeds.

• Common salt and inorganic salts of K and Mg are non selective and could hardly be used in crops.

Kerosene oil also has herbicidal action.

Sulphuric acid is used for weed control in small grain cereal crops.

Dilute H2SO4 at a conc. of 6-10% has been used for broad leaved weed control in cereals for a quite long period in Europe.

• In 1900 the herbicidal role of calcium cyanamide was discovered and later in 1908 sodium arsenite and chloropicrin (tear gas) came into use as herbicides.

• In 1919 sodium chlorate was first widely used as soil sterilant for controlling perennial weeds.

• Until recently sodium chlorate used to be recommended as a total weed kiler on road side, factory premises and non cropped areas.

• The first important discovery in the field of selective weed control was the introduction of 4,6- Dinitro – O – Cresol (DNOC) in France in 1933 is a contact herbicide applied to cereal crops to control broad leaved weeds. This was widely used in Netherlands. It is not successable because of phytotoxic effect on crop plants, mammalian toxicity.

Pokorng synthesized 2,4-D in 1940 and described its chemical synthesis in 1941.

Zimmerman and Hichcock too synthesized 2, 4-D in 1942. This event was more highlighted ad taken as first time discovery of 2,4D.

Marthan Mitchell in 1942 & 1944 from USDA reported its herbicidal activity to selectively control the broad leaved weeds in the lawns.

Hammer and Tukey in 1944 observed that 2,4-D successively control weeds in cereals.

2,4-D is first selective organic herbicide in world because of its widespread use in crops mainly cereals all over the world.

• Sister compounds of 2,4-D that is derivates of phenoxy alkanoic acids i.e., MCPA, 2,4,5-T, MCPB, MCPD came into market during late 1940’s.

• Between 1945 & 1960’s several photosynthesis inhibitor herbicides like urea, triazine, uracil group of herbicides and mitotic inhibitors like carbamates, dinitroanilines, lipid synthesis inhibitors like thio carbamates were developed in the world and the concept of preemergence herbicide originated during this period.

• Now atrazine is the principal herbicide used in maize and sorghum all over the world.

• In 1970’s and 1980’s new group of herbicides called sulfonyl ureas and imidazolinones were synthesized.

Sulphonyl ureas and imidazolinones became very popular because of their high potent nature and their usage in very low quantity. They are ecofriendly because of low residual nature.

• In china butachlor is the most popular herbicide used in rice. The other most popular herbicide is pyrazosulfuron ethyl highly marketed in china, japan for weed control in rice.

• Herbicide is a chemical mostly organic which selectively kills the weeds when applied to crop area without phytotoxic effect on crops.

• In market any approved herbicide is identified with 3 names

a) Common name: it is technically accepted short name

b) Chemical name: describes chemistry of molecule or reads out complete molecular structure

c) Trade name: given by the manufacturer

1) Common name: 2,4-D

Chemical name: 2,4-Dichlorophenoxy acetic acid

Trade name: Agromex, weedor

2)Common name: Glyphosate

Chemical name: N-(phosphomethyl) glycine 

Trade name: Glycil (Excel India crop care ltd) Roundup (monsanto)

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