• Phalaris minor developed resistance to isoproturon due to its longer usage.
• Orobanche is a complete root parasite on tobacco and tomato.
• Striga is a partial root parasite.
• Onion is a poor competitor with weeds because of shallow roots, slender leaves, sparse leaves and slow growth.
• 15-45 Days after transplanting (DAT) is the critical period of crop weed competition in onion.
• For the control of weeds
a) Recommended herbicide should be used
b) Recommended dosage should be used If increased dosage is used herbicide will lose selectivity
c) Herbicide should be applied at correct time
• Of the more than 3 lakh species known in this world 3000 species are of economic value for us and are cultivated ones.
• Weed is a plant growing where it is not desired.
• Jethro Tull a great British farmer was the first person to refer the word weed and its definition in his famous writing “Horse Hoeing Husbandry”.
• Bermuda grass, Cenchrus ciliaris, Eleusine indica, Panicum maximum (Guinea grass) when grown in crops considered as weeds otherwise forages.
• Plants may become weeds only in a particular situation and time so long as any plant growing on time without interfering with man’s interest.
• While all weeds are unwanted plants, all unwanted plants may not be the weeds.
• Weed science society of America defined weed as a plant growing where it is not desired.
• European weed research society defined weed as any plant or vegetation excluding fungi interfering with the objectives or the requirements of the people.
• W.S. Blatchley defined weed as a plant out of place or growing where it is not desired
• W.W.Robbins et al., defined weeds as obnoxious (unpleasant) plants.
• J.L.Harper defined weeds as higher plants which are a nuisance.
• G.C. Klingman defined weed as a plant growing where it is not desired or a plant out of place.
• Weed control as a science was initiated in 1942.
• 2,4-D was first used in a large scale in 1947.
• The development of 2,4-D in 1946 and later triazines, ureas which shows tremendous potential for plantation crops was beginning of herbicide usage in India.
• Weeds account for 45% loss of agriculture produce Insects – 30% Diseases – 20% Others (rodents) – 5%
• Weeds compete with crop plants for moisture, nutrients, space and sunlight and reduce the crop yields.
• The reduction in crop yields depends on crop-weed competition.
• Crop-weed competition depends on types of weed flora associated with crop, weed density, duration of infestation, and competitive nature of crop plants.
• Crop – reduction in yield due to weeds
Wheat = 15-30%
Rice = 30-35%
Maize, sorghum, pulses, oilseeds = 18-85%
Bengal gram = 30%
Redgram = 34%
Onion = 90.7%
Potato = 28%
• Redgram is very deep rooted so sustain from weed competition by obtaining nutrients from deeper layers.
• Weed infestation in sugarcane leads to reduction in sucrose content.
• Weeds growing within and outside the fields provide shelter to insect pest and disease pathogens of crops and act as host to these both during crop season and off season.
• Weevils survive on Solanum weed sps.
• Ergot of Bajra pathogen survives on Cenchrus ciliaris.
• Stem Borer on rice will survive on Echinochloa as well as Panicum.
• Caterpillar which is a pest on Redgram, cotton survives on Chenopodium album, Amaranthus sps, Datura and so on.
• Parthenium causes skin rashes.
• Ambrosia sps. causes hay fever.
• If seeds of Argemone mexicana mixed with mustard seeds cause blindness.
• Examples of thorny and spiny weeds are Acanthospermun hispidum, Argemone Mexicana, Mimosa pudica (Touch me not), Achyranthes aspera.
• Certain weeds cause sickness to farm animals and some may prove fatal due to high level of some alkaloids. Ex: Sorghum halepense (Johnson grass) Tribulus terrestris (produces thorny fruit)
• Tribulus terrestris causes sores in the hooves (foot) of grazing animals.
• Weeds are good source of organic matter to soil and improve soil fertility.
• Weeds comprise of 2.5 times more N, 1.5 times more P, 3.5 times more K than crop plants.
• Parthenium, Eichhornea crassipes, Eupatorium are used as compost and also as green manure plants. They should be used before flowering.
• Many of the weeds which are legumes enrich the soil fertility by fixing atmospheric nitrogen.
• Weeds which are fodder value to livestock are Panicum maximum (guinea grass), Panicum repens, Brachiaria mutica (Para grass), Brachiaria reptans, Digitaria sps, Chloris gayana, Cenchrus ciliaris, Eragrostis sps, Dichanthium annulatum, Setaria sps.
• Weeds which are used as food for human beings are Amaranthus viridis, Celosia argentea (cocks comb weed), Alternanthera sessilis, Portulaca sps, Digera sps, Chromolaena odorata.
• Centella asiatica which is a common weed in wetland rice acts as blood purifier and improves memory power.
• All parts of Centella asiatica have medicinal value.
• Phyllanthus niruri is used for curing jaundice.
• Tylophora indica is used for curing asthma patients.
• Solanum nigrum and Solanum xanthocarpum are used to cure asthma, cough and fever.
• Argemone mexicana seeds resemble to that of mustard. Oil is used for skin treatment.
• Spirulina sps (weed in aquatic bodies) is rich in protein.
• Weeds serve as soil binders. Ex: Dichanthium annulatum, Vetiveria zizanioides
• No soil cover is worse than weedy one.
• Hydrilla verticillata is used in aquariums.
• Lantana camara produces beautiful flowers of different colours.
• Very good lawn is developed from Cynodon dactylon.
• Extracts from parthenium plant can be used against pulse beetle.
• Incorporation of calotropis, parthenium, Crotalaria into the soil reduces nematode population in the soil.
• Weed Jatropha gossypifolia is used as biofuel.
• Weeds are used as live fences. Ex: Lantana camara Opuntia dillenii
• 2,4-D is effective for broadleaved weeds.
• Lantana camara is a wasteland weed.
• Echinochloa and Marsilia are wetland weeds.
• Euphorbiaceae members produce milky latex.
• Tribulus terrestris possess short spines on fruit.
• Solanum xanthocarpum has spines on stem and leaves.
• Acanthospermum hispidum has triangular fruit with spines.
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