· 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
· 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.
· 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
Eg. Paraquat, Fluchoralin (especially in pulse crops)
· Pre emergence – Atrazine, Diuron.
Post emergence – Paraquat and Diquat.
Adjuvant:- are also called as additives.
- Surfactants / wetting agents/surface active agents.
Eg. Soap, Uphar, Tea.
Stabilizing agents include emulsifier and dispersants· An emulsifier causes an emulsion concentrate to disperse spontaneously into small stable droplets.
· Dispersants stabilize suspension.
- 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.
- Humicants/Hygroscopic agents: Humicants prevent rapid drying of herbicides. Eg: Glycerol.
Stickers/filming agents/deposit builders.
Compatibility agents – like “Compex” are used to mix fertilizers and pesticides.
Activators/synergists: “Ammonium thiocyanate” is a widely used activator of amitrole
and the combination is designated as Dmitrole – T.
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
· 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”.
- Triazines Atrazine, atratap
- Triazoles Amitrole, Amitrole – T
- Phenoxy acid 2,4-D, 2,4,5-T, MCPA
- Diphenyl ether Nitrofen
- Nitrites Bromoxynil
- Thiocarbamate Benthiocarb
- Bipyridilium Paraquat, diquat.
- Dinitroanilines Fluchloralin
- Substituted urea Diuron
- Amides & Acetamides Alachlor
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.