The appearance of weeds in the cropped areas can be prevented by adopting the following measures for adoption wherever possible and practicable. It encompasses all measures taken to prevent the introduction and/or establishment and spread of weeds. Such areas may be local, regional or national in size. No weed control programme is successful if adequate preventive measures are not taken to reduce weed infestation. It is a long term planning so that the weeds could be controlled or managed more effectively and economically than is possible where these are allowed to disperse freely.
• Avoid using crop that are infested with weed seeds i.e., use of clean seeds for sowing. Weed free crop seeds may be produced by following the pre-cautionary measures.
• Separating crop seeds from admixture of crop and weed seeds using physical differences like size, shape, colour, weight/texture and electrical properties.
• Using air-screen cleaners and specific gravity separators, which differentiate seeds based on seed size, shape, surface area and specific gravity.
• Through means of seed certification we can get certified seeds and can be used safely because the certified seeds contain no contaminant weed seeds.
• Weed laws are helpful in reducing the spread of weed species and in the use of well adapted high quality seeds. They help in protecting the farmers from using mislabeled or contaminated seed and legally prohibiting seeds of noxious weeds from entering the country. (Weed Control)
• Quarantine laws enforce isolation of an area in which a severe weed has become established and prevent the movement of the weed into an uninfected area.
• Use of pre-emergence herbicides also helpful in prevention because herbicides will not allow the germination of weeds.
• Avoid feeding screenings and other material containing weed seeds to the farm animals.
• Avoid adding weeds to the manure pits.
• Avoid the use of raw dung as manure.
• Pull out seedlings in nurseries carefully without weed seedlings.
• Clean the farm machinery thoroughly before moving it from one field to another. This is particularly important for seed drills.
• Avoid the use of gravel sand and soil from weed-infested.
• Inspect nursery stock for the presence of weed seedlings, tubers, rhizomes, etc.
• Keep irrigation channels, fence-lines, bunds un-cropped areas and roads clean.
• Use vigilance. Inspect your farm frequently for any strange looking weed seedlings. Destroy such patches of a new weed by digging deep and burning the weed along with its roots. Sterilize the spot with suitable chemical.
• Quarantine regulations are available in almost all countries to deny the entry of weed seeds and other propagules into a country through airports and shipyards.
These methods include eradication and control of weeds in the field. (Weed Control)
1. Eradication Measures – It is an ideal weed control method rarely achieved. It infers that a given weed species, its seed and vegetative part has been killed or completely removed from a given area and that weed will not reappear unless reintroduced to the area. Because of its difficulty and high cost, eradication is usually attempted only in smaller areas such as few ha., a few thousand m2 or less. Eradication is often used in high value areas such as green houses, ornamental plant beds and containers. This may be desirable and economical when the weed species is extremely noxious and persistent as to make cropping difficult and economical. Weeds are destroyed immediately before its multiplication, dispersion and acclimatization as and when a new weed species is found. It can be done by,
• destroying the species at the initial stage of introduction and before it produces any propagule (at an early growth stage), and
• degenerating the buried dormant viable seeds by fumigation, flooding, heating and other methods.
2. Control Measures – In these method weeds are not eradicated but their growth is checked and the number of weeds (weed intensity) is minimized so that they do not affect crop yield or it encompasses those processes where by weed infestations are reduced but not necessarily eliminated. It is a matter of degree ranging from poor to excellent. In control methods, the weeds are seldom killed but their growth is severely restricted, the crop makes a normal yield. In general, the degree of weed control obtained is dependent on the characters of weeds involved and the effectiveness of the control method used.
Methods of weed control –
Weed control aims at only putting down the weeds present by some kind of physical or chemical means while weed management is a system approach whereby whole land use planning is done in advance to minimize the very invasion of weeds in aggressive forms and give crop plants a strongly competitive advantage over the weeds. Weed control methods are grouped into cultural, physical, chemical and biological. Every method of weed control has its own advantages and disadvantages. No single method is successful under all weed situations. Many a time, a combination of these methods gives effective and economic control than a single method.
(a) Mechanical methods – This method aims to destroy weeds by cutting and removing or by desiccation and exhaustion of weeds by adopting several methods like hand hoeing, hand pulling, tillage, flooding, burning, mulching by non-living materials etc. However the choice of a method depends on location, extent and habitat of weeds. (Weed Control)
1. Tillage – Tillage removes weeds from the soil resulting in their death. It may weaken plants through injury of root and stem pruning, reducing their competitiveness or regenerative capacity. Tillage also buries weeds. Tillage operation includes ploughing, disking, harrowing and leveling which is used to promote the germination of weeds through soil turnover and exposure of seeds to sunlight, which can be destroyed effectively later. In case of perennials, both top and underground growth is injured and destroyed by tillage.
2. Hoeing – Hoe has been the most appropriate and widely used weeding tool for centuries. It is however, still a very useful implement to obtain results effectively and cheaply. It supplements the cultivator in row crops. Hoeing is particularly more effective on annuals and biennials as weed growth can be completely destroyed. In case of perennials, it destroyed the top growth with little effect on underground plant parts resulting in re-growth.
3. Hand pulling/ hand weeding – It is done by physical removal or pulling out of weeds by hand or removal by implements called khurpi, which resembles sickle. It is probably the oldest method of controlling weeds and it is still a practical and efficient method of eliminating weeds in cropped and non-cropped lands. It is very effective against annuals, biennials and controls only upper portions of perennials. Hand pulling/hoeing is effective only when the weed infested area is small. Repeated hoeing and tillage is essential to control nut grass.
4. Digging – Digging is very useful in the case of perennial weeds to remove the underground propagating parts of weeds from the deeper layer of the soil.
5. Chiseling- It is done by hand using a chisel hoe, similar to a spade with a long handle. It cuts and shapes the above ground weed growth.
6. Sickling and mowing – Sickling is also done by hand with the help of sickle to remove the top growth of weeds to prevent seed production and to starve the underground parts. It is popular in sloppy areas where only the tall weed growth is sickled leaving the root system to hold the soil in place to prevent soil erosion. Mowing is a machine-operated practice mostly done on roadsides and in lawns.
7. Burning – Burning or fire is often an economical and practical means of controlling weeds. Burning the weeds will control the weed problem in sugarcane widely spaced field crops and orchards. It is used to
(a) dispose of vegetation,
(b) destroy dry tops of weeds that have matured,
(c) kill green weed growth in situations where cultivations and other common methods are impracticable.
8. Flooding – Flooding is successful against weed species sensitive to longer periods of submergence in water. Flooding kills plants by reducing oxygen availability for plant growth. The success of flooding depends upon complete submergence of weeds for longer periods. Flooding is done in rice fields to remove the regenerative parts of sedges.
9. Deep ploughing – Perennial weeds like Cynodon dactylon, Cyperus sp. Convonvulus arvensis can be controlled by deep ploughing and flooding with 15-30 cm of water for 4-8 weeks.
10. Mulching can be done in cash crops like sugarcane, cotton and flowers to control weeds.
• Oldest, effective and economical method
• Large area can be covered in shorter time
• Safe method for environment
• Does not involve any skill
• Weeding is possible in between plants
• Deep rooted weeds can be controlled effectively
• Labour consuming
• Possibility of damaging crop
• Requires ideal and optimum specific condition
(b) Cultural methods or cropping methods and competitive methods – Several cultural practices like tillage, planting, fertilizer application, irrigation etc., are employed for creating favourable condition for the crop. These practices if used properly, help in controlling weeds. Cultural methods, alone cannot control weeds, but help in reducing weed population. They should, therefore, be used in combination with other methods. In cultural methods, tillage, fertilizer application, and irrigation are important. In addition, aspects like selection of variety, time of sowing, cropping system, cleanliness of the farm etc., is also useful in controlling weeds.
1. Field preparation – The field has to be kept weed free. Flowering of weeds should not be allowed. This helps in prevention of build up of weed seed population. (Weed Control)
2. Summer tillage – The practice of summer tillage or off-season tillage is one of the effective cultural methods to check the growth of perennial weed population in crop cultivation. Initial tillage before cropping should encourage clod formation. These clods, which have the weed propagules, upon drying desiccate the same. Subsequent tillage operations should break the clods into small units to further expose the shriveled weeds to the hot sun.
3. Optimum plant population – Lack of adequate plant population is prone to heavy weed infestation, which becomes, difficult to control later. Therefore, practices like selection of proper seed, right method of sowing, adequate seed rate protection of seed from soil borne pests and diseases etc., are very important to obtain proper and uniform crop stand capable of offering competition to the weeds.
4. Crop rotation – The possibilities of a certain weed species or group of species occurring is greater if the same crop is grown year after year. In many instances, crop rotation can eliminate at least reduce difficult weed problems. The obnoxious weeds like Cyperus rotundus can be controlled effectively by including low land rice in crop rotation. Inclusion of smothering crop or competitive crops like sunflower, sorghum, sweet, potato, fodder grasses in rotation will effectively control the weeds. The selected crops should grow thickly and develop dense canopy and shade to suppress the weeds. Competitive plants- Parthenium hysterophorus can be effectively controlled by growing Cassia sericea Parthenium through allelopathic effect. Brachiaria mutica (Para grass) is highly competitive to the growth of Typha sp. in ditches.
5. Growing of intercrops – Inter cropping suppresses weeds better than sole cropping and thus provides an opportunity to utilize crops themselves as tools of weed management. Many short duration pulses viz., green gram and soybean effectively smother weeds without causing reduction in the yield of main crop.
6. Mulching – Mulch is a protective covering of material maintained on soil surface. Mulching has smothering effect on weed control by excluding light from the photosynthetic portions of a plant and thus inhibiting the top growth. It is very effective against annual weeds and some perennial weeds like Cynodon dactylon. Mulching is done by dry or green crop residues, plastic sheet or polythene film. To be effective the mulch should be thick enough to prevent light transmission and eliminate photosynthesis.
7. Solarisation – This is another method of utilization of solar energy for the desiccation of weeds. In this method, the soil temperature is further raised by 5–10ºC by covering a presoaked fallow field with thin transparent plastic sheet. The plastic sheet checks the long wave back radiation from the soil and prevents loss of energy by hindering moisture evaporation.
8. Stale seedbed – A stale seedbed is one where initial one or two flushes of weeds are destroyed before planting of a crop. This is achieved by soaking a well-prepared field with either irrigation or rain and allowing the weeds to germinate. At this stage a shallow tillage or non-residual herbicide like paraquat may be used to destroy the dense flush of young weed seedlings. This may be followed immediately by sowing. This technique allows the crop to germinate in almost weed-free environment.
9. Blind tillage – The tillage of the soil after sowing a crop before the crop plants emerge is known as blind tillage. It is extensively employed to minimize weed intensity in drill sowing crops where emergence of crop seedling is hindered by soil crust formed on receipt of rain or irrigation immediately after sowing.
10. Crop management practices – Good crop management practices that play an important role in weed control are:
• Vigorous and fast growing crop varieties are better competitors with weeds.
• Proper placement of fertilizers ensures greater availability of nutrients to crop plants, thus keeping the weeds at a disadvantage.
• Better irrigation practices to have a good head start over the weeds.
• Proper crop rotation programme.
• Higher plant population per unit area results in smothering effect on weed growth.
• Low cost for weed control
• Easy to adopt
• No residual problem
• Technical skill is not involved
• No damage to crops
• Effective weed control
• Crop-weed ecosystem is maintained
• Immediate and quick weed control is not possible
• Weeds are kept under suppressed condition
• Perennial and problematic weeds can not be controlled
• Practical difficulty in adoption
(c) Biological methods – Bio control is defined as the use of living organisms to suppress a pest population, making it less abundant and thus less damaging than that it would otherwise be or Use of living organisms i.e., bioagents viz., insects, disease organisms, herbivorous fish, snails or even competitive plants for the control of weeds is called biological control. In biological control method, it is not possible to eradicate weeds but weed population can be reduced. This method is not useful to control all types of weeds. Introduced weeds are best targets for biological control. (Weed Control)
(i) Qualities of bio-agent- The bio-agent must feed or affect only one host and not other useful plants. It must be free of predators or parasites. It must readily adapt to environment conditions. The bio-agent must be capable of seeking out itself to the host. It must be able to kill the weed or at least prevent its reproduction in some direct or indirect way. It must possess reproductive capacity sufficient to overtake the increase of its host species, without too much delay.
• Least harm to the environment
• No residual effect
• Relatively cheaper and comparatively long lasting effect
• Will not affect non-targeted plants and safer in usage
• Multiplication is costlier
• Control is very slow
• Success of control is very limited
• Very few host specific bio-agents are available at present
(iv) Mode of action
• Differential growth habits, competitive ability of crops and varieties prevent weed establishment e.g., Groundnut, cowpea fast growing and so good weed suppresser.
• Insects kill the plants by exhausting plant food reserves, defoliation, boring and weakening structure of the plant.
• Pathogenic organisms damage the host plants through enzymatic degradation of cell constituents, production of toxins, disturbance of hormone systems, obstruction in the translocation of food materials and minerals and malfunctioning of physiological processes. Specific bio-agent will attack only one or two specific weeds. Non specific bioagent will feed upon variety of vegetation.
• Eichhornia crassipes in controlled by using hyacinth moth (Neconchetina eichhorniae).
• Water fern (Salvinia molesta) is controlled by using curculionid weevil (Cyrtobagous sp.).
• Zygogramma bicolorata, beetle feed on leaves of Parthenium during monsoon.
• Larvae of Cactoblastis cactorum, a moth borer, control prickly pear Opuntia sp. The larvae tunnel through the plants and destroy it. In India it is controlled by cochinial insects Dactylopius indicus and D. tomentosus.
• Lantana camara is controlled by larvae of Crocidosema lantana, a moth bores into the flower, stems, eat flowers and fruits.
• Cuscuta spp. is controlled by Melanagromyza cuscutae.
• Cyperus rotundus – Bactra verutana a moth borer.
• Ludwigia parviflora is completely denuded by Altica cyanea (steel blue beetle).
• Herbivorous fish Tilapia controls algae. Common carp, a non-herbivorous fish controls submerged aquatic weeds. It is apparently due to uprooting of plants while in search of food. Snails prefer submersed weeds.
(v) Bioherbicides The use of plant pathogen, which are expected to kill the targeted weeds. Bioherbicides having pathogenic organisms like fungi, bacteria and virus are used as biocontrol agents. They are applied as chemicals. These are native pathogen, cultured artificially and sprayed just like post-emergence herbicides each season on target weed, particularly in crop areas. Fungal pathogens of weed have been used to a larger extent than bacterial, viral or nematode pathogens, because, bacteria and virus are unable to actively penetrate the host and require natural opening or vectors to initiate disease in plants. Here the specific fungal spores or their fermentation product is sprayed against the target weed. (Weed Control)
(d) Chemical methods – Chemicals, which can kill the weeds or control weed growth are known as herbicides. Using herbicides for the control of weeds is called chemical weed control.
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