Basics of weed management

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Basics of weed management

· A weed is a plant growing where it is not desired.

· Burmudo grass/star grass – Cynodon dactylon – Forage grass
Quack grass – Agropyron repens – Soil binding grass.

· Eichornia crassipes – Weed for fisher man (Water hyacinth)

· Weeds as green manure – Croton sparsiflorus.

· Source of a protein – Chlorella pyrenoids.

· Weeds used in crop breeding – Saccharum spontaneum

· Weeds used as pollution indicators.
NO2 gas – Wild mustard
SO2 – Chick weed.

· Medicinal plant – Leucas aspera – snake bite/malaria.

· Reclain alkali soils – Argemone mexicana (Mexican poppy)

· Prickly weed – wild safflower.

· Canada thristle – Cirsium arvense; Bund weed – Convalulus arvensis.

· In tea the presence of “Loranthus” leaves impair its quality.

· Congress/Municipal/Carrot grass – Parthenium hysterophorus.

· Water lettuce – Pistia lanceolata.

· Nut grass – Cyperus rotandus.

· Fail to regrow when they are cut close to the ground level – simple annuals.

· The woody weeds include shrubs and undershrubs are collectively called “Brush” weeds.

· Summer perennial weed – Sorghum halopens (Jhusow grass).

· Winter perennial weed – Circium arvense.

· Migrated weeds (or) introduced weeds are called as Anthrophytes.

· Poisonous weed – Datura fastuosa.

· Facultative weeds are also called apophytes – but often escape from cultivated fields.

· An objectionable weed is a noxious weed whose seed is difficult to separate once mixed with crop seeds.

· In quack grass rhizomes sometimes called “Sobolobes”.
Runners – Bermuda grass
Bulb – Wild onion, wild garlic.

· In Hydrilla verticillata the bulbils originate in leaf axils and called “Turions”.
1. Pappus It is a parachute like modification of persistent calys into hairs.
Eg. Compositae family
2. Comole Weed seeds are covered with hairs partially (or)
3. Baloon It is a modified pappery calyx.
Eg: Physalis minima.

· Wind swings the intact plant Censor mechanism Eg: Mexican poppy.

· Dispersal of weeds through digestion mechanism (animals) is a “endozoochori”.

· Those with the same size and shape as the grain are extreme difficult to separation. Such weeds are called “Satellite”.

· “Neutrophiles” serve as indicator plants.

· Thatch grass – Imperata cylindrica.

· Wild ant (Avena jatva) seeds exhibit 3 types of dormancy.
i.e. Enforced, Innate, Induced dormancy.
a. Enforced dormancy: It is due to the placement of seed in the lower layer of soil, It is
caused by the absence of “red light”.
b. Innate dormancy : Genetically controlled character. It is due to the presence of hard seed
coat and immature embryos.
c. Induced dormancy: due to some sudden physiological changes.

· Agril. ecotype – Continued adoption of an agril. Practice can lead to the development of adoptive ecotypes.

· In summer, furrow planting of crops is a very useful method for reducing weed problem.

· Flooding is a common crop husbandry method of controlling weeds in rice field.

· Bio – agent is a living organism employed to control of pest.

· “Lantana camera” was controlled by Crocidosema lanotana – moth borer.

· Carp fish used to control aquatic weeds.

· Spider mite was found to useful in controlling prickly pear (Opuntia stricta).

· Starvation test is a good safeguard against an introduced bio agent becoming a pest of some
economic crop plant.

· Bio herbicides: In Phytophthora palmivora – De-vine.
Colletotrichum gleoispories – Collego.

· When hand pulling is aided by a forked sharp blade (or) tip of a sickle, it is called
“Spudding”.

· Mowing is cutting of a uniform growth of weeds from entire area at ground level.

· Dredging and chaining is used to control aquatic weed.

· Repeated light application of flame to plant shoots can destroy even roots of deep perennial
weeds. This process is called “searing”.

· When selective herbicides are applied at higher rate they act as non – selective herbicides.
Eg. Diuron.

· Atrazine is a trans locate herbicide when absorbed from soil but a contact herbicide when
sprayed on plant shoots.

· For perennial weeds translocated herberbicides are used.

· Pre plant dessication is applied to destroy existing vegetation.

· Pre plant incorporation: herbiciedes are mixed with weed free seed beds to obtain
residual control.
Eg. Paraquat, Fluchoralin (especially in pulse crops)

· Pre emergence – Atrazine, Diuron.
Post emergence – Paraquat and Diquat.
Adjuvant:- are also called as additives.

  1. Surfactants / wetting agents/surface active agents.
    Eg. Soap, Uphar, Tea.

  2. Stabilizing agents include emulsifier and dispersants· An emulsifier causes an emulsion concentrate to disperse spontaneously into small stable droplets.

· Dispersants stabilize suspension.

  1. Solvents/coupling agents / co-solvents.

· 2, 4 – D is almost insoluble in water, but it can be dissolved in “Poly ethylene glycol (PEG)” to make it water soluble.

  1. Humicants/Hygroscopic agents: Humicants prevent rapid drying of herbicides. Eg: Glycerol.

  2. Stickers/filming agents/deposit builders.

  3. Compatibility agents – like “Compex” are used to mix fertilizers and pesticides.

  4. Activators/synergists: “Ammonium thiocyanate” is a widely used activator of amitrole
    and the combination is designated as Dmitrole – T.

  5. Drift control agents
    · Triazines:- The inhibition of photosynthesis is a characteristic effect of triazines. Their specific action is on the photolysis on the hill reaction.

· Uracil: These are strong inhibitors of photosynthesis.

· Ureas: Produce abnormal meiotic cells in root tips.

· Nitriles – Bromoxynil
Most sensitive site of action is uncoupling of oxidative and P.S. Phosphorylation.

· Bipyridilium – inhibit the reduction of NADP to NADPH.

· Eichlobenil induces dormancy in weed seeds.

· In organic copper or serical herbicides inhibit “Pyruvate dehydrogenase” activity during “glycolysis”.

· Carbamates – These are mitotic poisons.

· Thiocarbamates – Inhibits shoot and root growth of grass weeds.

· Dinitroanilines – Nucleic acid inhibition is the primary mechanism.

· Phenoxy alkanoic acid – 2,4-D, 2,4,5-T cause epinasty, swelling twisting.

· Troazpes – Amibole – development of albino leaves and shoots.

· Selectivity of molinate between Oryza sativa and Echinochloa colonum was chiefly due to differences in the “crown root initiation” levels.

· Activated charcoal has been found a strong absorbant of herbicides 2,4-D.

· In intermediate metabolism the intermediate chemical structure prove more phytotoxic
than the parent compound.

· The reverse metabolism of 2, 4-DB and MCPB was by an enzyme of “b – oxidation
process”.

· Selectivity of propanil between resistant rice and susceptible Echinochloa colonum was found to be due to differences in the “aryl acylamine amidohydrolase” (AAAH) enzyme content of their leaves.

· 2, 4-D for killing broad leaved weeds in monocot crops.

· 2, 4-DB and MCPB are employed for controlling broad leaved weeds MCPA –Methaxone, 2, 4-D (Na salt) – Fernoxone, Nitrofen – TOK – E- 25, Paraquat –Gramaxone; Fluchloralin – Basalin pendimethalin – stomp; Glyphosate – Round – up; Butachlor – Machete Benthiocarb – Saturn.

· Additive effect: It is defined as the total effect of combination is equal to the sum of the effects of the components.

· Synergistic effect: The total effect of a combination is greater (or) more prolonged than the sum of the effects of the two taken independent.

· Antagonistic effect: The total effect of a combination is smaller/lower than the sum of the effect.

· Independent effect: The total effect a combination is equal to the effect of the most active compound.

· Enhancement effect: Response is greater in herbicides when added with adjuvant.

· Amitrole was phytotoxic to groundnut in the presence of high ‘P’ levels.

· Application of herbicides in suspension fertilizers is becoming popular.

· Boron reduces rapid microbial detoxification of 2,4-D.

· Barnyard grass – Echinochloa crusgalli is most common in rice.
Jungle rice – E. colonum – Upland rice.
Water jern – Salvinia molesta.

· Alachlor and metalochlor effective against grasses but weak against broad leaves.

· Acrolein – used for destroying submerged weeds (aquatic).

· Cyperus rotendus – Purple nut sedge; C.esculentus – yellow notes.

· Canary grass – Phalaris minor.

· Witch weed – Striga asiatica.

· Discovery of natural stimulant of striga germination from cotton roots gives the trivial name “Strigol”.

· For controlling striga growing resistant varieties like N – 13 is recommended.

· Parthenin is the main toxicant present in the parthenium.

· Johnson’s grass – Sorghum halopens.

· Atrazine for maize, Sugar cane.

· Trap crop for striga – cowpea.

· Opuntia delloni was controlled by Dactylopius tomentosus.

· Granular formation pre requisite – moisture pressure.

· In Sugarcane, weeding starts with “Blind hoeing”.

· Best herbicides for rice – “Anilophos”.

  1. Triazines Atrazine, atratap
  2. Triazoles Amitrole, Amitrole – T
  3. Phenoxy acid 2,4-D, 2,4,5-T, MCPA
  4. Diphenyl ether Nitrofen
  5. Nitrites Bromoxynil
  6. Thiocarbamate Benthiocarb
  7. Bipyridilium Paraquat, diquat.
  8. Dinitroanilines Fluchloralin
  9. Substituted urea Diuron
  10. Amides & Acetamides Alachlor

C.P. 202:
1. Reasons for low yields of rice in India is cloudyness.
2. Wheat variety brought from mexico and formed the basis for green revolution is Sonara – 64, Lermaroja.
3. The crop most sensitive to excess moisture at early stages of its growth – Maize.
4. Number of ground nut plants/m2 in kharif are 33 and in Rabi are 44.
5. In wheat : single dwarf gene variety is Sonalika
Double is Kalyanasona
Triple is Hira, Malavika.


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