In this article we will discuss about:- 1. Definitions of Weed 2. Classification of Weeds 3. Benefits 4. Losses 5. Ecology 6. Crop-Weed Association and Competition 7. Control.
Definitions of Weed:
Weed is a plant growing at a place and time where it is not desired. Thus, any plant other than groundnut in a sole crop of groundnut is a weed. While all weeds are unwanted plants, all unwanted plants at a particular situation may not be weeds in other situations. As long as a plant is growing at a place and time without interfering with man’s activities, all unwanted plants may not be weeds.
Common definitions for weed found in literature are:
1. A plant out of place.
2. A plant interfering with the intended use of land.
3. A plant with negative value.
4. A plant growing with desired plant.
5. An uninvited guest.
6. A plant growing where it is not desired.
Such definitions indicate that a plant appropriately is a weed only in reference to humans at a particular point of time. Thus, Cynodon dactylon (perennial grass) could be an undesirable plant (weed) in groundnut crop but a valuable plant in a pasture.
Sorghum and pearl-millet plants in cotton fields are weeds while they are food crops when cultivated as sole or intercrops. Basically, it is the situation involving space and time, as well as the farmer’s interest on it, which class a component of vegetation as weedy. There is no part of the flora classified as weeds by nature or scientists.
From the above it is clear that weeds are plants growing in places and at times when the farmer wanted some other plants to grow in that place. As long as a plant is growing at a place and time without interfering with human’s interest, it is not a weed. As such, while all weeds are unwanted plants, all unwanted plants may not be weeds.
In agriculture, weeds may be defined as that part of the non-crop vegetation that reduce quality and quantity of existing crops to such an extent that any benefits accrued from weeds cannot compensate for the unfavourable effects left on crops. To a farmer, weed is an unwanted plant interfering with his crop growth.
Weediness is a situation in which there is abundance of weeds. Weeding is a farm operation where mechanical methods of weed control are followed to provide best opportunity for the crop to establish and grow vigorously up to the harvest. Depending on their migration into crop lands, association with specific crop, problems created, survival etc., weeds are named differently.
Parasitic Weed:
A weed that depends partially or fully for its growth on its host plant.
Obligate Weed:
A crop-land weed incapable of surviving in a wild community.
Objectionable Weed:
A problem weed, whose seed once mixed with crop seed is extremely difficult to separate.
Noxious Weed:
An undesirable troublesome weed difficult to control.
Facultative Weed:
Weed of wild community origin, escaping sometimes to crop-land.
Alien Weed:
Weed not native of a country.
Associated Weed:
Non-parasitic weed association with a specific crop.
Satellite Weed:
A weed that has become an integral part of a crop ecosystem.
Weeds have been part of agriculture scene since man started cultivating crop plants and weed management is an integral part of crop production with the introduction of improved cultivars. Weed management is an approach in which weed prevention and weed control have companion roles.
It implies a systems approach in which all available tools are used to reduce the propagule seed bank, prevent weed emergence and minimise competition from weeds growing with desired plants. Thus, weed management has both immediate and long term objectives. This approach also implies a consideration in a broader context of their interactions with production practices.
Classification of Weeds:
There are over 30,000 species of weeds around the world of which about 18,000 has been identified as causing serious losses. Several weed species often respond alike to a common control measure because of similarity in some aspect of their ontogeny, physiology or morphology. This enables generalisation of effectiveness or failure of control measures against weeds as a group instead of individual species. Weeds are classified into several groups for planning effective control measures.
Morphology:
Weeds in crop fields, based on morphology, can be classified into five groups.
i. Broad-Leaf Weeds:
These are mostly dicots having broad leaves with netted venation and exposed apical meristem. Cleome viscose, Eclipta alba, Amaranthus spinosus and Portuloca oleracea are examples of this group.
ii. Grasses:
These are monocots with 2-rank leaves that are usually long, narrow and upright with parallel veins. Propagation is with seeds, rhizomes, stolons and stem cuttings. Examples are Echinochloa crusgalli and Cynodon doctylon.
iii. Sedges:
These are monocots like grasses but have leaves occurring in two rows. They have modified rhizomes. Cyperus rotundas and Cyperus diffofrmis belongs to this group.
iv. Algae:
This is a large and diverse group of simple photosynthetic plants. Examples of this group are Chlorella, Spirogyra and Cladophora.
v. Ferns:
These are seedless vascular plants which produce spores. Ceratopteris siliquosa, Marsilea crenata and Salvinia molesta belong to this group.
Association:
Based on the association weeds with crops and seasons, they are of three groups.
i. Season-Bound Weeds:
They grow in specific season of the year irrespective of the crop species cultivated. Sorghum halepense is a summer perennial weed whereas Cirsium arvense is a winter perennial weed.
ii. Crop-Bound Weeds:
These are usually parasite on the host crop. Cuscuta (dodder), Orahanche (broomrape) are weeds of the tropics.
iii. Crop-Associate Weeds:
Weeds are associated with certain crops. They grow with the crop due to their requirement of microclimate and habitat and survive along with the crop in the form of mimicry. Seeds of crop are contaminated with seeds of weeds. Arena fatua, Phalaris minor and Chinopodium album are associated with wheat crop. Amarantlius viridis and Echinochloa crusgalli are always associated with rice crop.
Origin of Weeds:
Many weeds of India originated in other parts of the world. Based on their origin, they are classified into three groups.
i. Alien Weeds (Foreign Origin):
Argemone mexicana, Parthenium hysterophorus, Eichhornia crassipes and Eupatorium odoratum.
Many alien weeds become so aggressive as to fast displace the indigenous flora of the new ecosystem. These are called invasive alien weeds (IAW). There are thirteen alien weeds notified as IAW including Parthenium hysterophorus, Salvinia molesta, Ipomea carnea and Phalaris minor.
ii. Apophytes (Indigenous):
Cynodon dactylon, Cyperus rotundus, Echnocldoa colonum and Saccharum spontaneum.
iii. Antrophytes Introduced by Man:
Phalaris minor, Corchorus acutangulus and Avena hidoviciana.
Ontogeny:
Based on ontogeny (life cycle), weeds are classified into three broad groups:
i. Annuals:
These weeds grow and mature within one year, but usually they complete their life cycle in one season. They are sub-divided according to the season of prevalence.
ii. Biennials:
These weeds live for two seasons with a duration upto two years. In the first year, they usually remain vegetative and in the second year they produce seeds. Biennials, generally, do not come up in annual field crops, but infest perennial crop fields, pastures, lawns and orchards.
Common weeds of this group are:
a. Alternanthera echinata.
b. Daucus carota.
c. Cirsium vulgare.
d. Zingiber casumunar.
iii. Perennials:
They live for three or more years and produce seed more then once in their life cycle. They may propagate by seed or vegetative parts or by both. They are further subdivided into simple, bulbous and creeping perennials. Simple perennials are propagated only by seeds. They do not have a quick regenerative capacity from vegetative organs. Ipomoea carnea and Lantana camara belong to this group.
Bulbous perennials possess modified stem with scales and propagate by bulbs, bulbils as well as by seeds such as Allium vinea’e. Creeping perennials reproduce by seed as well as rhizomes (Sorghum halepense), stolons (Cynodon doctylon), roots (Convolvulus arvensis) and tubers (Cyperus sp).
Nature of Stem:
According to aerial stem characters, weeds may be:
Many weeds have subaerial stems with storage organs which help them to propagate and perennate besides other modes of reproduction. Such organs may be nuts as in Cyperus rotundus or rhizomes as in Inula indica. Subaerial stems without storage organs may be runners (Oxalis repens), stolons (Colocasia antiquorum) or offsets (Pistia stratiotes).
Forms of Stem:
Allelopathy:
Any direct or indirect harmful effect by one plant on another through production of chemical compounds (or) antagonistic interaction of phytochemicals on the growth of other plants is called allelopathy. Simply, the phenomenon of one plant having detrimental effect on another through production of chemicals is called allelopathy.
The inhibitory substances or allelochemicals may be exudated from roots, lechate of plant organs or seeds, decomposition of dead plant parts as end products or byproducts or metabolites liberated from the plants. Allelochemicals are released by washing, leaching, exudation and volatilization.
Allelopathy differs from competition in that the latter is involved in the removal or reduction of some factor or factors (water, nutrients and light) from the environment that is required by some other plant sharing the habitat.
Allelopathy involves any direct or indirect harmful effect of one plant through release of chemical compounds on the other. Allelopathic chemicals inhibit seed germination or reduce the growth of other plant species. They include phenolic acids, coumarins, terpenoids and flavonoids.
Effect of Weeds on Crops:
There are several reports on the allelopathic effect of weeds on crops:
1. Seed exudates of Avena fatua (wild oat) affect germination and early growth of wheat.
2. Leaf and inflorescence of Amarantlius spinosus affect the vegetative growth of fingermillet and maize.
3. Leaves and inflorescence extract of Triantliima portulacastrum and Parthenium hysterophorus affect the germination and seedling growth of sorghum.
4. Tubers of Cyprus esculentus affect the growth of maize and groundnut.
5. Leaf extracts of Camelina alyssum (flax weed) greatly reduce the yield of flax.
6. Argemone mexicana affects germination and seedling growth of wheat and finger-millet.
7. Celosia argentina affects germination and growth of pearl-millet.
8. Datura affects germination and growth of sunflower.
Effect of Weeds on Weeds:
Certain allelopathic effect of weeds on other weed plants has been reported. This phenomenon helps the weed to dominate in that environment. Extract of leaves, leachates and decaying leaves of Polygonum orientate contains flavonoids which are toxic to germination and growth of Ameranthus spinosus.
Decaying rhizomes of Sorghum halepense affect the growth of Setaria viridis, Digitaric sanguinalis and Amaranthus sp. Whole plant extracts of Amarnathus viridis, Trianthima portulacastrum and Flavaria australasica affect the germination of Echinochloa colonum.
Effects of Crops on Weeds:
There is evidence in support of crop plants showing allelopathy on weeds. Root exudation of maize inhibits the growth of Chenopodium album and Amaranthus retroflexus. Cold water extracts of wheat straw reduced the germination and growth of Ipomea sp and Abutilon indicum.
Benefits from Weeds:
Weeds in the history of agriculture have been a mixed blessing. Weed infested fields have served as direct recruiting grounds for new crops. Several cultigens stared as weeds. Rye (Secale cereale) and oats (Avena sp) developed from weeds in barley and wheat fields. In Central America, two cultivated amaranths, Amaranthus hypochondriacus and A. cruentus and chenopodium (Chenopodium nuttaliae) probably originated as weeds in cultivated fields.
Multi-cropped fields invaded by weeds have served as primitive laboratories in the development of crop plants. The common potato (Solanum tuberosum) is tetraploid and resulted from a cross between a diploid cultigen (S. stenotonnim) and a weedy diploid (S. sparsipilutn). The latter species imparted resistance to root knot nematodes and the common potato adapts to a wider range of conditions than its parents.
1. Source of Resistance to Pests and Diseases:
Weeds have been a constant source of new genes for resistance to pests and diseases. Bread wheat (Triticum aestivum) is a cross between emmer wheat (T. dicoccum) and Aegilops squcirrose (syn. Triticum tauschii); the later, a wild grass, has conferred cold tolerance to bread wheat. Resistance of potato to cyst nematode (Heterodera rostochiensis) spread from a wild tetraploid (S. oplocense) to weedy S. sucrense and then to S. tuberosum.
2. Fodder Value:
Several weeds of grass-lands serve as food for animals. Weeds like hariyali (Cynodon doctylon) and bind weed (Convolvulus arvensis) are good fodders for milch animals. Digitaris sanguinalis, Dichanthium annulatum, Eclipta alba, Phalaris minor and Melilotus indica are valued as succulent feed. Some weeds such as Amaranthus viridis are used as green vegetables for human consumption.
3. Weeds as Green Manures:
Several species are used as green manures. Weeds belonging to legume family are collected as green manures, especially for low land rice, Tephrosia purpuria belonging to legume group is a common green manure weed in south India. Aquatic weeds such as Eichhornia crassipes and Pistia stratiotes are used for composting.
4. Medicinal Value of Weeds:
A number of weeds have medicinal value of great importance. Leucas aspera is used in snake bite. Calotropis procera is a good medicine for gastric troubles, Argemone mexicana is used for curing skin disorders, Phyllanthus niruri is useful in treating jaundice and Striga orobanchoides is used for the treatment of diabetes.
5. Other Benefits:
Weeds are valued for several other economic benefits. Cyperus rotundus is useful for making agarbathis, Andropogon sp is useful for manufacturing aromatic oil, Cymbopogon citralus, C. nardus and C. martini are valued for essential oils.
Imperata cylindrica and Typha elephantina are useful for thatching roofs of huts. Weeds like Cynodon doctylon, Dicanthium annulatum and Saccharum squarrosus are used for protecting different types of bunds, especially in soil and water conservation.
Losses due to Weeds:
Growth requirements of crops and weeds are identical. Therefore, when weeds are allowed to grow with crops, they compete with crop plants for all the growth factors, natural or costly inputs. Unfortunately, weeds absorb nutrients and moisture faster and smother the crop plants leading to reduced yield. It is estimated that losses due to weeds exceed losses from any other pest or disease.
Weeds compete with crop plants, largely, for soil moisture, nutrients and sunlight.
The intensity of weed competition depends on the:
a. Type of weed species.
b. Severity of infestation.
c. Duration of infestation.
d. Competing ability of the crop.
e. Climate which affects weed and crop growth.
1. Yield Losses:
Among the annual agricultural losses in India, weeds account for 33 per cent, insects 20 per cent, diseases 26 per cent and others 21 per cent. The annual economic losses account for Rs. 2,000 crores in the country. Recent estimates show that average crop yield reductions due to weeds vary from 12 to 72 per cent depending upon the crop (Table 8.3).
Reduction in yield due to weeds is highest in carrot (72.5%) followed by sugarbeet (70.3%), onion (68%), cotton (47.5%) and rice (41.6%).
2. Crop Pests and Disease Incidence:
Weeds act as alternate hosts to pests and pathogens in off-season which infect the crop later and cause severe damage.
3. Poor Quality of Produce:
Weeds often reduce the quality of farm produce in several ways. Parasitic weeds, Aeginitia indica and Striga litura reduce the quality of sugarcane juice. Wild onion or wild garlic mixed with forage impart off-flavours to milk.
Weed seeds like wild mustard, mexican poppy mixed with wheat grains or edible mustard cause objectionable odour to the flour and even prove poison. Cacklebur weed seriously impairs sheep wool quality. Tea quality is impaired by the presence of parasitic weed, Aeginitia indica.
4. Human Health Problems:
Some of the weeds cause health problems and allergic reactions. Hay fever and asthma (pollinosis) are aggravated by Franseria sp. Direct contact with weeds such as Parthenium lvysterophorus (congress weed), Amrosia and Helenium sp leads to dermatitis (allergy).
The hair of Urtica sp (string neetle) causes severe itching and inflammation. Tsetse fly causing sleeping sickness multiply with brush weeds. Pistia lanceolata (water lettuce) and Salvinia auriculata (salvina) are breeding sites of mosquitoes causing malaria, yellow fever, encephalitis and filarisis.
5. Animal Health Problems:
Many weeds are poisonous to animals when ingested. Lantana camara induces hypersensitivity to light in animals. Diarrhoea in milch animals and blood strains in their milk has been attributed to the weed Rhododendron sp Sorghum lialepense at its tillering stage is poisonous to gracing animals because of its high prussic acid content. Sweet clover (Melilotus alba) when fed in large quantities acts as antiblood coagulant.
6. Problems of Water Contamination:
Aquatic weeds render water unfit for drinking purpose. They markedly reduce the flow of water in irrigation and drainage channels. Typha sp (cattails) and Carex aquatilis (water sedge) are menace to fisheries. Aquatic weeds upon decomposition emit offensive odours and pollute the atmosphere.
7. Reduction in Land Value:
Lands infested by perennial weeds such as Cyperus rotundas, Cynodon doctylon, etc makes the land unsuitable for economic crop production. Large extent of rice growing lands have been rendered uneconomical for crop production due to severe infestation of perennial weeds in the absence of regular cultivation for want of adequate irrigation water. The value of such lands has gone down considerably.
Ecology of Weeds:
Ecology is the interrelationship between organisms and their environment. Weed ecology is concerned with growth characteristics and adaptations that enable them to survive the changes in environment. Environment includes climatic, edaphic and biotic factors which influence the distribution, prevalence, competing ability, behaviour and survival of weeds.
Persistence of Weeds:
The adaptive capability of weeds to any environment indicates their persistence Management practices for a crop with which the weed is associated determine the persistence of the weed species.
1. Climatic Factors:
Solar radiation, rainfall, humidity, temperature and wind largely affect the weeds persistence. Since weeds grow along with crops, tolerance to shade is a major adaptation that enables them to survive. Photoperiod influence flowering, seed setting and final seed production. Temperature influence weed distribution.
Soil temperature influence weed seed germination and dormancy, the major survival mechanism of weeds. Rainfall and humidity influence persistence and distribution. Nutsedge is common in humid tropics. Pigweed (Amaranthus sp) prefers mesophytic conditions, while Opuntia sp (prickly pear) can grow well under xerophytic conditions. Velocity and direction of wind influence the occurrence and persistence of weeds.
2. Soil Factors:
All the soil factors influencing crop plants also influence growth and development of weeds. Weeds such as Puccinallia sp (alkaligrass) and Agropyron repens (quackgrass) known as basophiles can grow well in alkali soils. Similarly, weeds like Digitaria sanguinalis and Borreria sp called as acidophiles comes up well in acid soils. Several weeds growing well in neutral soils are called as neutrophiles.
Weeds can thus serve as indicator plants. Quackgrass (Agropyron sp), Canada thistle (Cirsium arvense) and wild carrot (Daucus carota) are abundant in depleted and eroded soils. Commelina benghalensis thrive well in moist soil, Typha sp lives only in waterlogged soils and lmperata cylindrica can persist drought.
3. Biotic Factors:
Plants and animals are the biotic factors that affect weed persistence. Competitive ability of weeds with crop and also with other weeds determines their survival under a given set of crop management practices. Crops like tobacco serve as hosts to parasitic weeds like orobanche.
Survival Mechanism:
Annual and biennial weeds are principally reproduced by seed, while perennials survive both vegetatively and by seed. Weed seed spread rapidly with the help of certain morphological appendages and through agencies like cattle, human, machinery, wind, water, etc. Seed dormancy enables them to survive for long periods under adverse conditions.
1. Seed Production:
Survival of weeds depends on their capacity to produce sufficient seed or other propagules even under adverse conditions. An annual weed can produce enough seed to cover an area of about one ha in the next season. One plant of hedge mustard (Sisymbrium altissinum) produces about 5 lakh seeds and that of Amaranthus retroflexus, Portulaca oleraceae and Solanum nigrum produce around 2 lakh seeds. Bulbs, tubers, rhizomes, stolons, etc offer an additional survival mechanism for perennial weeds.
2. Dispersal of Weeds:
Weed seeds and fruits are dispersed very fast in various ways. Wind, water, animals, man and farm machinery are the principal agencies of weed seed dissemination.
i. Wind:
Weed seeds have structural modifications like wings, parachute, chaffy, etc. which aid in their wind distribution to a longer distance. Winged fruits are seeds of umbelliferae, parachute fruits with tuft of hair in compositae like Canada thistle (Cirsium arvense), comate seeds with silky hairs like milkweed (Calotripis sp) and prickly poppy (Argemone mexicana), chaffy seeds of Solanaciae like ground-cherry (Physalis minima), etc., disperse their seed by special adaptations.
ii. Water:
Some weed seeds have special structures with which they can float in water are carried away on flowing water. Such seeds washed into streams by runoff during rainy season are carried to crop lands. Irrigation water is also important in the spread of weed seeds. Pigweed, lambsquarters, bind weed, mustards, cocklebur, etc., are the best examples of weed dissemination by water. Most aquatic weeds disperse through water.
iii. Man and Animals:
Weed seeds with special structures like hooks, spines, barbs, awns, etc., tend to cling to man clothing or animal skin and carried long distances. Fleshy fruits eaten by birds or other animals are often incompletely digested and the viable seed dispersed through the animal excreta. Lantana seeds are dispersed through Indian myna.
Tribulus terrestris (puncture vine) and Xanthium strumarium (cocklebar) are dispersed through animal hooves and hairs. Commercial seed contaminated with weed seed are dispersed by humans.
iv. Farm Implements and Manure:
Farm implements carry weed seed from field to field and from one area to another. FYM is an important source of weed spread, since viable seeds are present in the dung of farm animals. If mature weeds are composted, the seeds retain their viability and infest the fields to which it is applied.
3. Dormancy:
Dormancy is the resting stage for seed. It is a state in which viable seeds fail to germinate under the environment favourable for germination. Many annual weeds produce dormant seed which can grow in flushes over years. Seed dormancy is an efficient survival mechanism of weeds.
One year seeding is seven years weeding is due to the seed dormancy. Seeds of Dactyloctenium sp, Indigofera sp, Lathyrus sp and Citrulus sp remain viable for periods varying from 2 to 15 years. The longevity of Striga seed in soil could be upto 20 years. The field bind weed (Convolvulus arvensis) seed is viable upto 22 years.
4. Vegetative Reproduction:
Vegetative propagules of perennial weeds located deep in the soil enable them to persist for years even if their aerial growth is destroyed. Perennial grasses such as Imperata cylinclrica and Agropyron repens which propagate through rhizomes have the ability to regenerate even from a small piece of rhizome.
Purple nutsedge (Cyperus rotundus) and yellow nutsedge (C. esculentus) propagate through seed as well as tubers that have varying degrees of dormancy. Thus, perennial weeds, through their ability to store food materials in underground parts and propagule dormancy, can adapt to most cropping systems and their eradication is a serious problem.
Crop-Weed Association and Competition:
Crop-Weed Association:
Climatic, edaphic and biotic factors of environment determine the distribution of species, their competitive ability, prevalence and intricate relationships. Crops grown under specific agro-ecological situations have their characteristic weeds.
Major factors contributing to crop-weed association are similarity in climatic needs, seed size, seed germination, tillage needs, cropping and harvest practices. As weeds do not belong to a particular plant species, atleast some weed species enjoy the favour of a crop field habitat and flourish along with crop plants.
Some of the weeds always associated with specific crops are:
Crop-Weed Competition:
Struggle for survival and existence is called competition. Competition occurs when each of the two or more organisms seeks the measure it wants of any particular growth factor and when the immediate supply of that factor is below the combined demand of the organisms.
Water, nutrients, light and space are the major factors for which competition usually occurs. Competition between crop plants and weeds is most severe when they have similar vegetative habits and common demands for available growth factors.
Dominance of a habitat by the crop or weed depends on the rapidity of seed germination, seedling establishment and subsequent growth and development. Differences in photosynthetic area, root development and its growth largely determine the competitiveness of plants. The degree of weed competition is determined by the weed species infecting the crop, density of infestation and duration of infestation.
Weed species differ among themselves in competitive ability. Annual monochoria (Monochoria vaginalis) is more severe competitor than another grass Echinochloa crusgalli in rice crop. If weeds are controlled within two or three weeks of emergence, their influence on final crop yield is minimal. Weeds are capable of absorbing just as much as or at times even bigger amounts of nutrients and water than crop plants.
It has been reported that for every 1.0 per cent increase in yield, weed infestation should be eliminated to the extent of 3 per cent in barley, 2 per cent in wheat, 0.5 per cent in maize, 3 percent in tomato and 8 per cent in apple.
Soybean has lower competitive ability with Cyperus rotundus followed by groundnut, blackgram, greengram, sorghum and maize. It clearly shows that crops differ appreciably in competitive ability with weeds.
Critical Period of Crop-Weed Competition:
The critical period for crop-weed competition is the period from sowing upto which the crop has to be maintained in a weed free environment for remunerative crop production. Differences in final crop yield due to weeding at this period and that due to weed free environment throughout the crop period will be marginal.
The crop canopy in the early period of growth is inadequate to smother the weed growth. By early reproductive phase, the crop develops leaf area adequate to smother the weed growth.
Hence, weed competition in crop field is invariably severe in early stages of crop growth than at later stages. Generally, in a crop of 100 days duration, the first 35 days after sowing should be maintained weed-free for optimum yield. Critical periods of weed competition for major crops are described in Table 8.4.
Controlling of Weeds:
A. Mechanical Methods:
Considering the problem of weeds in crop fields and need for weed research in India, a Coordinated Weed Control Scheme on wheat, rice and sugarcane was initiated as early as 1952 in 11 states of the country by the ICAR to monitor the weed flora and also to find out the relative feasibility of economical weed control. Later, a number of crop research institutes of ICAR and SAUs were involved in weed control research.
It was in 1978 the weed research programme was strengthened through All India Coordinated Research Programme on Weed Control by the ICAR in collaboration with the United States Department of Agriculture.
Initially, six centers were started at different SAUs for a period of six years. Later, seven more centers in II phase and nine more centers in III phase were added during 1982-83 and 1985-86 respectively for a period of five years each. The programme was continued with plan funds of ICAR (Fig. 8.1).
This project assisted farming community through the scientific technologies developed, which are effectively utilised for alleviating the yield losses due to weeds in field crops. In VII Plan, it was decided to establish a National Research Centre for basic as well as applied research in weed science in India. Accordingly, the National Research Centre for Weed Science (NRCWS) was approved in 1986 and came into existence on 22.04.1989.
The NRCWS was further upgraded as Directorate of Weed Science Research (DWSR) in 2009 with the following mandate:
1. To undertake basic and strategic research for developing strategies for efficient weed management in different agroecological zones.
2. To provide leadership role and coordinate the network research with state agricultural universities for generating location specific technologies for weed management in different crops, cropping and farming system.
3. To act as repository of information in weed science.
4. To act as a centre for training in research methodologies in area of weed science and management.
5. To collaborate with National and International agencies in achieving the above objectives.
6. To provide consultancy.
Weed control methods are, broadly, classified into four groups:
(i) Mechanical methods.
(ii) Agronomic (cultural) practices.
(iii) Biological methods.
(iv) Chemical methods.
Mechanical and/or physical methods are being employed ever since man began to grow crops. They include hand hoeing, hand pulling, tillage, digging, sickling, burning, flooding and mulching, Choice of each method depends on the location, extent and habitat of weeds.
a. Hand Hoeing:
Hand hoe has been the simplest weeding tool for centuries. Hoeing is particularly effective on annuals and biennials as weed growth can be completely destroyed. In case of perennials, it can destroy only top growth. It is effective on creeping perennials, which have shallow root system.
b. Hand Pulling:
It is physical removal or pulling out of weeds by hand. It is very effective against annual and biennial weeds, as they do not regenerate from pieces of roots left in the ground. It is very economical where weeds are scattered around that makes herbicide use uneconomical or herbicides are not effective on a particular weed species.
c. Tillage:
Weed control by tillage is achieved primarily by:
i. Burial of small annual weeds under soil with tillage implements or tools.
ii. Disruption of intimate contact between weed plant and soil for water and nutrients.
iii. Injury of root and stern of weeds leading to their reduced competitiveness or regenerative capacity.
Tillage operations such as ploughing, disking, harrowing and levelling for seedbed preparation promote weed seed germination which can be destroyed by subsequent tillage operations. Annual and biennial weeds without well-developed tap root system can be effectively controlled by tillage. Perennial weeds can also be controlled considerably by continuously deep ploughing for a period of three to five years.
d. Digging:
It is very useful to remove underground propagating parts of perennial weeds from deeper layers of soil. Digging is followed by hand pulling of the weed. Digging, a labour intensive method, is restricted to perennial weed control in small areas where other methods are not effective.
e. Sickling:
Sickling by hand is used to remove top weed growth, to prevent weed seed production and to starve the underground parts. It is specially ideal on sloppy lands, where root system has to be left in the field to minimise soil erosion.
If the soil condition does not permit use of implements or tools for controlling weeds, especially at critical stages, sickling is usually followed. Herbicide needs can be considerably brought down by sickling before applying the herbicide.
f. Burning:
It is an economical and simple means of weed control. It is used to:
i. Dispose of accumulated vegetation.
ii. Destroy dry tops of matured weeds.
iii. Kill green weed growth when other methods are impracticable.
iv. Destroy buried weed seeds and other propagating plant parts.
Fire is used to burn crop residues and weeds after the harvest of crops like sugarcane, cotton, potato, maize etc. When burning is used for selective control of annual broadleaf weeds and grasses in widely spaced crop rows, it is known as flame cultivation.
g. Flooding:
Weed infested field is ploughed deep and flooded with 20-30 cm standing water for 5 to 10 weeks. It is successful against weed species sensitive to longer periods of submergence in water. Flooding is an efficient method of weed control for some perennial weeds like Cyperus sp, Cynodon doctylon and Convolvulus arvensis. Lowland rice is usually rotated with other irrigated crops for controlling such problem weeds.
h. Mulching:
It has smothering effect on weeds by restricting the photosynthesis. Mulching is effective against annual weeds and some perennial weeds like Cynodon doctylon, Sorghum halepense, etc. Mulching with straw, hay, paper and films is successful in cash crops. Availability and cost of the materials limit its large scale use.
i. Soil Solarisation:
High soil temperature can suppress weed seed germination and kill weed seedlings. During the process of heating soil surface, termed solarisation, soil temperature is raised above thermal depth point for most weed seeds and seedlings.
Placing transparent and opaque polyethylene sheets on moist soil and trapping solar radiation can achieve solarisation. Its effectiveness for weed suppression depends on warm, moist climate and interise radiation to raise soil temperature high enough to kill weed seeds and seedlings.
During solarisation soil temperature is increased around 50°C. Solarisation for a period of 5 to 6 weeks may be adequate for controlling annual weeds, while over 5 months may be necessary for controlling perennial weeds. It is an expensive technique for large scale application.
j. Chemical Stimulants:
Chemicals that stimulate germination of both viable and dormant seed include growth regulators respiratory inhibitors, nitrogen compounds etc. Ethylene can be used to control striga by stimulating its seed germination.
Nitrates such as ammonium nitrate are known to cause increased emergence of wild oats, Elusine indica, Echinochloa colonum etc. Sodium azide is known to stimulate oat seed germination. Striga is a chemical stimulant of parasitic weeds, striga, orobanche etc.
B. Agronomic (Cultural) Practices:
Agronomic practices for weed control are mostly non-monetary or involve relatively less expenditure. It may not be possible to solely depend on agronomic practices for complete weed control, as desired level of weed control may not be possible with these practices.
However, weed intensity can be reduced to some extent to improve crop yield. The main objective of cultural practices is to provide a short term relief to crop during initial growth periods so that the crop may take a lead in its growth and development.
Crop management practices are directed in favour of crops and suppressing weed growth.
Such management practices are:
1. Good seedbed preparation to minimise early weed infestation such that the crop dominate the weeds.
2. Timely sowing, optimum stand establishment and adequate fertiliser use leading to vigorous crop for better competition with weeds (selective crop stimulation).
3. Weeding before the weeds produce seeds for minimising weed infestation in the succeeding season.
4. Crop rotation for controlling crop associated weeds.
5. Including smother crops such as legumes in intercropping systems.
6. Line sowing for using inter-cultivation implements.
7. Summer ploughing to desiccate the weed seed and vegetative propagules by exposing them to hot sun.
8. Band placement of fertilisers nearer to crop rows.
9. Fertiliser application to the crop at a time when the nutrients uptake is at its peak.
a. Planting or Sowing Time:
It is an established fact that weed seeds are thermosensitive and sensitivity varies with the species. By adjusting the sowing time in such a way either to sow the crop before the germination of weeds of a locality/field or after the emergence of weeds which can be uprooted by repeated ploughing. In advance/normal sowing, the crop, generally, gain in its growth over the weeds and exert smothering effect on weeds.
Moreover, due to unfavourable growing conditions, the weeds though germinate along with the crop but sometimes fail to attain its proper growth and hence do not gain any advantage in the competitiveness over the crop. In early/normal sown wheat, wild oats prove to be more aggressive whereas under late sown conditions with the fall of temperature Phalaris minor attain the advantage of aggressiveness over wild oats.
b. Use of Clean Seed:
To prevent primary infestation, crop seed must be made free from weed seeds. Phalaris minor weed was imported along with Mexican wheat seed and then spread to many parts of the country through movement of wheat seed from place to place.
In general, small sized crop seed become the major source to carry weed seed to new areas than the crops with bigger seed size. It is thus imperative to clean the crop seed thoroughly to check the future spread to the weeds to new areas.
c. Tillage:
All the annual weeds present at the land preparation are uprooted or destroyed with tillage. Clean cultivation helps in removal of under and above ground parts of perennial deep-rooted weeds and gradually due to repeated tillage operations these weeds are eliminated from the crop areas. Pre-sowing tillage operations must be thorough and at shorter intervals to prevent reestablishment of weeds.
Inter-cultivation at appropriate time with hand toots or animal drawn implements is the common methods of weed control in different parts of the country. At times of labour scarcity during the crop season, intercultural operations are the only economical methods of weed control, especially in dryland crop production.
d. Stale Seedbed:
The objective of this technique is to induce germination of weed seed with irrigations before sowing the crop so that 2-3 flushes of germinated weeds are destroyed. This method is ideal for crops in which germination of crop and weed seed is synchronised.
Majority of weed infestation can be reduced by this method. Pre-sowing irrigation followed by seeding is ideal method of crop establishment compared to dry seeding followed by irrigation for relatively weed free environment.
e. Method of Sowing:
Closer row spacing always gives the crop an upper hand over the weeds. Thick crop stand limit the weed establishment and subsequent weed growth. Later flushes of weeds are also completely smothered as the crop canopy cover does not permit light to penetrate to weeds.
Bidirectional seeding also helps in reducing weed growth as the distribution of plants over the space becomes adequate and healthy crop canopy structure can be generated which can cover the weeds effectively.
f. Seed Rate:
Higher seed rate for establishing thick crop stand enjoys competitive advantage over weeds. The objective of higher seed rated is to provide good stand to crop so that it may share more of the growth resources over the weeds.
With low crop stand, weeds will have an edge over the crop and use much of available space, light and other growth factors. Under this situation, weeds smother the crop and their removal becomes difficult. As such adequate stand establishment is essential for minimising the weed infestation.
g. Crop Rotation:
Certain types of weeds get adapted to a particular crop sequence due to their akin nature of growth or preference to the cultural practices given to crop. Phalaris minor is a typical weed of rice-wheat sequence as the ecological conditions required for growth and development of Phalaris minor are met with this rotation.
Phalaris infestation can be reduced by replacing wheat with other rabi crops like berseem, raya, winter maize etc. These crops not only change the growing conditions but also exert smothering effect on Phalaris minor. Wild oats can be completely eliminated by growing berseem for 3-4 years.
Cyperus rotundus and Trianthima portulacastrum are serious weeds in many kharif crops. Transplanted lowland rice in such fields can eliminate problem of these weeds as these weeds are unable to grow under anaerobic conditions of rice crop. Similar such weeds can be eliminated by growing lowland rice for 3-4 years.
h. Intercropping:
Quick growing crops in between crop rows at wider spacing can control weeds through smothering on them. Intercropping can increase the competitive ability of crops to reduce weed growth. The magnitude of reduction in weed growth depends on nature of intercrops, their relative proportion in the mixture and spatial arrangement of the crops. Efficient intercrops are cowpea, greengram, blackgram, soybean and other pulse crops.
i. Mulching and Crop Residue Management:
Mulches are effective in minimising weed growth in wide row crops like sugarcane, cotton and maize. Besides suppressing the weed growth, mulches aids in conserving soil moisture. Smothering effect of mulches on weeds prevent penetration of light to the weeds leading to their death. Any crop residue can act as mulch. Straw, plastic and paper mulches can also be used as mulches. Close, growing pulse crops are effective live mulches.
j. Water Management:
The role of land submergence for weed suppression in lowland rice has been well documented worldwide. Under normal irrigations to wheat, wild oats make luxuriant growth and suppress wheat growth.
With limited irrigation, growth of wild oats is suppressed (reduction in dry matter production) and remains below wheat crop canopy. Deep and profuse root system of wheat crop, relative to wild oats, can use moisture from deep soil layers and suppress the growth of wild oats.
k. Nutrient Management:
Creating unfavourable conditions for weed growth, relative to crops, leads to good crop canopy development quickly to provide smothering effect on weeds. Placement of fertilisers closer to root zone of crop can restrict fertiliser use by weeds. As such, plants can use the placed fertiliser efficiently for quick initial growth to smother the weed growth.
Broadcast fertiliser is equally shared by both crop and weeds and the crop suffers badly due to better growth of weeds. Basal application of fertilisers by placement aids in vigorous crop growth early in the season to suppress weed growth.
C. Biological Methods:
These methods involve utilisation of natural living organisms (bio-agents) such as insects, pathogens and competitive plants to limit the weed infestation. The objectives of biological control are not eradication, but reduction and regulation of the weed population below the level of economic injury.
A bio-agent may be either specific or nonspecific. Specific bio-agent attacks only one or two specific weeds, while the non-specifics feed upon a variety of vegetation. The specific bio-agents include, primarily insects and plant pathogens. The choice of pests and pathogens should meet certain criteria before they can be declared successful as bio-agents.
Important criteria are:
1. Host specificity.
2. Fast damaging ability.
3. Adaptability to new environment.
4. Fasting ability for long period during periods of declining weed population.
5. Efficient reproduction.
6. Freedom from natural enemies.
The merits of biological control are relative cheapness, comparatively long lasting effects and least harm on the environment and the non-target organisms.
Some outstanding examples of successful biological control of weeds are:
a. Specific Bio-Control:
1. Control of Eichhornia crassipes (water hyacinth) using Necochetina eichhorniae, N. bruchi (hyacinth weevils) and Sameodes albiguttalis (hyacinth moth).
2. Salvinia molesta (water fern) is controlled by Crytobagous sp (curculinoid weevil). Nyphula responsalis defoliate water fern besides pistia, typha, azolla and Mcirsilea sp.
3. In Florida, Alternanthera philoxaroides (alligator weed) is controlled by flea beetle Agasiees hygrophila.
4. Two weevils, Microlarinus lypriformis and M. lareynii can control Tribulus terrestris (puncture vine).
5. Release of Dactylopius tomentosus (scale insect) controlled Opuntia dillenii (prickly pear) in many parts of India.
6. Lantana camara (lantana weed) in Andhra Pradesh of India has been effectively controlled by a moth Crocidosema lantana and a bug Teleonemia scrupulosa.
7. Zygogramma bicolorata beetle feeds on parthenium plants during rainy season.
8. Bioherbicides using pathogens such as fungi, bacteria and virus are also used as biocontrol agents.
b. Bio-Herbicides:
A bio-herbicide is a plant pathogen used for weed control through application of its inoculum. A list of bio-herbicides of Sustainable Agriculture is given in Table 10.5.
c. Nonspecific Bio-Agents:
Grass carp, also called white amur is voracious feeder on several weeds including lemna, hydrilla, chera and eloode. Besides silver carp, common carp and tilapia also feed on algae.
d. Competitive Plants:
The weed, Parthenium hysterophorus is suppressed by Cassia sericea. Plant leachates of C. sericea have kaolines which accumulate in the soil and interfere with the germination and growth of parthenium only. Brachiaria mutica (paragrass) is highly competitive to the growth of Typha sp in ditches.
Some success has been achieved in biological control of weeds in crop fields. Ludwigia parviflora weed in rice crop can be controlled by Haltica cycinea (steel blue beetle). Larvae of moth Bactra verutana have been found to bore into shoots of Cyperus rotundas in India, Pakistan and USA.
D. Chemical Methods:
Use of chemicals for weed control offer great potential in crop production. Chemical weed control functions on the basis that certain chemicals are capable of killing some plants (weeds) without significantly affecting other plants (crops). .Such chemicals are called herbicides.
Chemical weed control has been largely a post-World War II development. Apart from introduction of dinoseb in 1933, MCPA and 2, 4-D in 1945, it is now around 60 years since the first effective organic herbicide was marketed.
At global level, the usage of herbicides is higher than any other agrochemical. The percentage of herbicide consumption is 43 as against 31 per cent with insecticides, 21 per cent with fungicides and 5 per cent with other agrochemicals.
In India, although 2, 4-D was first tested as early as in 1946, chemical concept of weed control struck deeper roots only when tea growers began using herbicides on a commercial scale in late 1970s. In India, consumption of insecticides is more (75%) as compared to herbicides (13%), fungicides (8%) and other agrochemicals (4%). Herbicide market in India has grown about eight folds in the past 15 years.
Chemical weed control has some definite advantages over other methods:
1. Most effective compared with other methods of weed control.
2. Highly suitable for closely spaced crops.
3. Suitable for adverse soil and climatic conditions.
4. Provides early season or zero day weed control.
5. Controls the weeds without any mechanical injury to the associated standing crop.
6. Reduces the need for repeated cultural operations during the crop season.
7. Controls even many perennial weeds which cannot be effectively controlled by other methods.
Some of the drawbacks with chemical control of weeds are that they must be applied at proper time, they have harmful residues which may affect succeeding crops and require minimum technical knowledge.
Chemical Weed Control in Different Crops:
A large number herbicides have been tested for their effectiveness in controlling weeds in different crops. Recommended herbicides for different crops are given in kg ha-1 on active chemical basis unless stated otherwise.
1. Cereals:
2. Pulses:
3. Oilseeds:
4. Commercial Crops:
For calculating the weight of commercial product and formulated product required for application, Herbicide Spraying may be referred.
Weed Control in Intercropping Systems:
Control of weeds may be a greater problem in intercropping than when the component crops are grown as sole crops. Usually, intercrops are better than component crops as sole crops in weed suppression. Most herbicides are crop specific.
A herbicide that does not harm both the component crops, usually, does not control a large number of weed species. The more complex the intercropping system, the less likelihood of finding herbicides.
Control of Problem Weeds:
Cyperus rotundus : Deep cultivations and exposing soil to hot sun for 3-7 days or MSMA/ DSMA (4.5) twice at an interval of 10 days on standing nutsedge or 2, 4-D (2.0) repeated twice, glyphosate (0.8 to 1.5) on standing nutsedge/directed spray on plantation and orchard crops, with repeated spot sprayings.
Ipomea Cornea:
2, 4-DEE (1.0 to 2.0) as post-emergence.
Water Hyacinth:
Paraquat (1.0) + 2, 4-D (1.0), paraquat (1.0) + fluroxypyr (4.0) provide satisfactory control.
Imperata Cylindrica:
Glyphosate (0.8) with or without ammonium sulphate 2% or MSMA (4.5) or dalapon, (3.0) repeated twice at 15 days interval.
Saccharum Spontaneum:
Glyphosate (1.2) at 6-7 leaf stage.
Eupatorium Odoratum and Chromolaena Dadorata:
2, 4-DEE (1.0 to 1.5), paraquat (1.0) as post-emergence on 2 m tall weed.
Pennisetum sp (P. Polystachyon and P. Pedicellatum):
Post-emergence application of glyphosate (1.2), paraquat (0.8) + diuron (1.0) gives complete kill of 2-3 months old grass.
Trianthema Monogyna in Cotton:
Pendimethalin (0.75 to 1.5) as pre-emergence.
Control of Parasitic Weed:
Important parasitic weeds in India are loranthus, dodder, orabanche and striga.
i. Loranthus (Bird Vine):
It is a semi-stem parasite on trees like citrus, mango, teak, casuarina etc. Copper sulphate and 2, 4-D combination applied in the holes made on the stem of trees are useful. Copper sulphate 7 g and 1.0 g fernoxone (2, 4-D) per tree is effective.
ii. Doddor (Cuscuta):
It is a total stem parasitic annual plant with twining thread like yellow or orange stems and leaves. The plant twines the host like lucerne very closely.
Control measures include:
a. Planting crops other than legumes and flax for several years to reduce weed seed in the soil.
b. Application of CIPC at 6 to 8 kg ha-1 in lucerne crop infested with dodder.
c. Application of paraquat at 1.0 kg ha-1 over infested lucerne.
d. Pronamide application at 1.5 kg ha-1 15 days after cuscuta germination.
iii. Orabanche (Broom Repe):
It is a total parasite infesting tobacco and a number of solanaceous plants. A long rotation with tobacco crop once in three years preceded by a chilli crop can reduce the loss due to the parasite.
Pre-emergence application of sodium 2, 4-D ethyl sulphate (sesone), post emergence allyl alcohol (0.1 to 0.2%) and directed post emergence DNDC at 0.40 to 0.75 per cent can give good control. Lowland rice cultivation is also an effective method of controlling the orabanche.
iv. Striga (Witch Weed):
Striga weeds are semiroot parasitic plants. Crops such as sorghum, maize, tobacco, sugarcane and sunflower are infested by this parasitic weed. Raising trap and catch crops is the best way to control striga. Trap crops like cotton, field peas, cowpea, flax and soybean stimulate the parasite seeds to germinate but are not themselves parasitised.
Catch crops stimulate the parasite seeds to germinate and are parasitised. Hence, catch crops should be destroyed before striga sets seeds. Sudan grass is very effective catch crop. Herbicides like 2, 4-D and MCPA at 1.0 kg ha-1 are effective after emergence of the weed.
Control of Aquatic Weeds:
Aquatic weeds grow in water and complete life cycle, atleast a part, in water. Waterhyacinth, typha, water fern and hydrilla are the common aquatic weeds which need control measures.
i. Waterhyacinth:
Waterhyacinth (Eichhornia crassipes) is a perennial, free floating aquatic weed. The seeds germinate on the banks of water bodies and the seedlings then move into the main water along with water waves. The mats are so strong as to even stop ships.
Manual removal of weed is the successful way of its control. Most commonly used herbicide is 2, 4-D at 1.2 to 8.0 kg ha-1 depending on weed mat density. To be effective, the spray liquid concentration of 2, 4-D should not be less than 0.3 per cent.
Best time of application is summer, when the weed is growing actively. Combination of paraquat 1.0 kg and amitrole 3.0 kg ha-1 or amitrole and fenac 0.1 kg ha-1 are very effective. Amitrole alone at 1.0 per cent is also effective.
ii. Typha:
Three species of typha (T. latifolia, T. aungustifolia and T. elephantina) are common in India. It has quick regeneration capacity from stout rhizomes.
Manual weeding is difficult because of its deep underground rhizomes. In shallow water areas, combination of dalapon 15 kg ha-1 and amitrole 3 kg ha-1 mixed with 0.5 per cent surfactant can give good control. Diuron 15 kg ha-1 mixed with MSMA 2 kg ha-1 can give conspicuous control. A combined spray of 2, 4-D amine 1.5 kg ha-1 with aminotriazole 1.5 kg ha-1 is very effective.
iii. Water Fern:
Water fern (Salvinia molesta) is a free floating plant reproduced by offsets. Physical control by dredging with a rope is successful if care is taken not to miss plants. Granular C-288 herbicide 30 kg ha-1 formulation (0.75 kg ai ha-1) can completely control salvinia at low water level in rice field or lowlying areas without any injury to rice crop.
iv. Hydrilla:
Hydrilla verticillata is an annual submerged troublesome aquatic weed in calcareous waterlogged soils. Excellent control of hydrilla can be achieved with single application of tank mixed diquat and CSP (1.0 and 4.0 ppm) on the water surface. It is also highly susceptible to anhydrous ammonia and endothall. Common snail (Marisa cornuarieties) feed on hydrilla and destroy them.