In this article we will discuss about:- 1. Definition of Weeds 2. Misutilisation of Weeds 3. Distribution 4. Factors Governing the Weed Composition 5. Propagation 6. Dispersal 7. Economic Uses 8. Prevention 9. Control 10. Eradication 11. Integrated Weed Management.
Definition of Weeds:
Jethro Tull, a Great Britain farmer, was the first person to use the term weed as a plant growing where it was not desired, in his famous writing ‘The New Horse Hoeing Husbandry’ published as the first book on agriculture in 1731. Since then several definitions of weeds have been suggested around this basic information. Of these definitions, a comprehensive and widely accepted one is- a weed is a plant growing out of place and out of time.
To elaborate, weeds are plants growing in places and at times where or when we wanted either some other plants to grow or no plant to grow at all. This definition of weed, as one would note, does not identify a particular portion of plant kingdom as weedy. Rather, it tags the weedy nature of a plant to the situation in which it occurs and the attitude of an individual towards it.
To cite a few examples, bermudagrass (Cynodon dactylon) and foxtail (Cenchrus ciliaris) are valuable plants in pastures, but in crop fields these form troublesome weeds. Likewise, quackgrass (Agropyron repens) is a very good plant to bind certain erodable lands, but it forms a serious weed in crop fields and orchards. Pricklypear (Opuntia spp.) forms a valuable biological fence, besides being a useful plant in holding the desert soils.
But in pastures and rangelands of many countries, including India, it has become a very aggressive weed on millions of hectares of otherwise productive land. Waterlily (Nyphaea sp.) and lotus (Nelumbo sp.) are amongst the most troublesome aquatic weeds of the world, yet these are cultivated in specific water bodies as ornamental plants.
Lantana (Lantana camara), with its multicolour flowers, has frequently attracted the gardeners who train it in different fashions. But on thousands of hectares of agricultural lands, grazing grounds, and river beds, it has spread as a noxious weed in the country. Similarly, several plant species growing on roadsides and wastelands may be of no concern to us at certain times when they are not to be considered as weeds.
But when they hinder the efficient use of roads and highways, immediately one considers these plants as weeds. Thus, it is the situation, involving space and time, and the individual interest in that situation, which class a plant as weed. In this context it is well said that while all weeds are unwanted plants, all unwanted plants may not be weeds.
Despite the good intentions of the above accepted definitions of weeds, for all intents and purposes, about 30,000 plant species have been identified as definite weeds in the world infesting crop lands, water bodies, woodlands, gardens, orchards, air fields, utility rights-of-way, etc.
Misutilisation of Weeds:
Certain weeds have been used for purposeful adulteration of food in peace time, and for toxicating mankind in war time. A recent example of adulteration of crop seeds with weeds seeds is that of Mexican poppy seeds mixed with mustard seeds. The resulting oil is very harmful to the humans. Growing of chicory with lucerne and berseem likewise is an unhealthy adulteration of these forage crops.
The history reveals that during the war time, rootstocks of false hellebore (Veratum sp.) were used to poison the drinking waters of the enemy. Another weed henbane, was used to extract ‘truth serum’ from it in Korean War. It forced the prisoners to reveal truth, unknowingly. But such things are now only of historical importance. Presently, allowing the weeds to grow in crop fields and use these later as forage is the biggest misuse of weeds in India, and other developing countries.
Distribution of Weeds:
India, with her great variety of agro-climatic situations spread over her length and breadth, is blessed with favourable conditions for the cultivation of a variety of crops. But, by the same token, we also have m the country numerous, widely differing weed species An examination of the weed flora composition reported in literature from different countries fully supports this.
So much so that even m the same crop the weed composition is found to differ widely in different places. Several known and unknown factors govern the weed composition obtained at a given place and time.
Factors Governing the Weed Composition:
Therefore, these are given here only in brief, as follows:
1. Extent of Dispersal:
The weed composition available at a given site is governed primarily by the kind of weed seeds and other propagules present there in the soil which, in turn, depends upon the extent to which dispersal of different weeds to the new area had taken place. The absence of a weed from a particular place does not necessarily mean that it was not adapted to that farming situation.
A weed could be missing from a place just because its seeds had not yet got chance to reach that site. For example- a few years back Parthemum hysterophorus was limited to waste lands of some southern states of the country. But now it occurs throughout the country, and in a state like Maharashtra, it has become also a serious weed of crop fields.
2. Climatic Elements:
Climatic elements control the distribution of plants, in general on earth Weeds are no exception to it, except that their adaptability is much wider. As a result, the selection pressure of climatic elements on weeds is less intense than on the crops.
3. Soil Moisture:
This is the most dominant factor influencing the weed species composition in any field. The dry lands, irrigated lands, and paddy lands afford completely different kinds of weed flora because of differences in their soil moisture strata. Even the amount and time of rainfall or irrigation affect the weed species composition. Water table status also controls the kind of weed species present m he field due to variable soil moisture reaching the root zones of the weeds.
4. Soil Type:
Moisture retention capacity of soil, pH, salinity, drainage, and nutrient status have their own roles to play in the distribution of dominant weed flora at a place. A very common observation is the prominence of Chenopodium album on normal soils and of C. murale on the salt affected soils.
Likewise, while Tribulus terristris and Celosia argentia are dominant on the coarse texture, low moisture retentive soils, Trianthema portulacastrum and Portulaca sp. grow profusely on the fine texture, moist soils. The high status of available zinc in soils is conducive to a weed like Setaria lutescens while Clienopodium and Portidaca spp. are more comfortable on K2O rich soils and Digitaria spp. on the P2O5 rich soils.
This is a very strong factor controlling the kind and intensity of weed flora present in different fields. In the same crop grown by different agronomic practices, the weed flora is found to vary sharply. Rice cultivation, which is conducted in several fashions in the country, is a very common example of wide variation in weed flora in this crop.
Similarly, potato grown on flat beds usually have different weed problems than the ridge planted crop. So is true of small grains sown after a pre-sowing irrigation as against when these are sown in dry soil and irrigated thereafter. Crop rotations and monocultures also exhibit distinct differences in weed flora and weed intensities.
Weed flora is not static. It changes with change in one or more of the above factors. The new weed problems frequently replace the older ones. When herbicide use is introduced in the systems, the change in weed flora is further hastened by the development of resistant biotypes and species.
Propagation of Weeds:
(i) Propagation by Seeds:
Majority of our weeds reproduce by distinct seed formation. They are largely monoecious plants, though some like Canada thistle (Cirsium arvense) and eelgrass (Vallisneria spiralis) are dioecious.
Weeds, particularly the annuals, have tremendous seed producing capacity which helps them to multiply year after year at exponential rates. Production of a few thousand seeds per plant is common in most weed species. Some of the annual weeds like Ageratum, Amaranthus, and Portulaca spp., may produce even one lakh or more seeds per plant (Table 3.1).
Contrary to it, usually the perennial weeds produce only few seeds, although weeds like Chromolaena odorata, Cuscuta spp., Mikania micrantha, Mimosa spp., Sorghum halepense, and Saccharum spp., are prolific seed producers, despite being perennials. This makes their control all the same more difficult.
Added to the prolific seed production feature found in most weeds is the ability of their seeds to undergo dormancy under unfavourable soil and atmospheric conditions and thus, remain viable in soil for several years.
The weed seed bank in soil ensures their continued progenies year after year, even when man adopted some rigorous weed control programme. The task is made more difficult by the fact that weed seeds have very high dissemination capability in nature. A few weedy plants that may escape farmer’s hoe, may produce seeds sufficient to sow the entire field.
There is yet another interesting feature of seed production in weeds. Under stress conditions they terminate their vegetative growth early and ensure production of at least a few seeds before withering away. Besides, there are a few weed species which are short-lived by nature, irrespective of the environment.
These are called ephemerals, e.g. Phyllanthus niruri and P. fraternus, which complete their life cycles within 2-3 weeks. Then there are weeds which produce seeds, both aerial and underground. Dayflower (Commelina benghalensis) is one such weed with us.
The viability of fresh weed seeds has been determined to vary from 5% (ex. Datura stramonium), to as high as 78% (ex. Bidens pilosa). Cyperus rotundus seeds are non-viable.
Weed seeds and fruits of different species differ widely in their shapes and sizes. Some seeds may be as small as dust particles (ex. Cuscuta arvensis), while others as large as 10 mm long (ex. Xanthium strumarium). They may be light like wool (ex. Calotropis gigentea) or heavy like minute stone pieces (ex. Vicia sativa).
Some are smooth and shiny (ex. Chenopodium album), while others are rough and spiny (ex. Tribulus terrestris).
(ii) Vegetative Propagation:
All perennial weeds possess well developed vegetative propagation organs. These may be in the form of either rhizomes and root-stocks (ex. Sorghum halepense, Agropyron repens, Saccharum spontaneum, Cynodon dactylon, and Typha angustata); runners (Oxalis corniculata); suckers (ex. Hieracium spp.); offsets (ex. Eichhornia crassipes and Pistia lanceloata); tubers (ex. Cyperus rotundus)-, bulbs and bulbils (ex. Wild allium spp. and Oxalis latifolia)-, or roots (ex. Cirsium aroense, Convolvulus arvensis, Alhagi camelorum, and Pluchea lanceolata).
A weed like bermudagrass possesses more than one kind of vegetative propagules i.e. rhizome and runners. In some cases the detached stem pieces can give rise to new weedy plants (ex. Opuntia and Cuscuta spp.). In certain annual weeds vegetative regeneration may occur from the crown region (ex. Parthenium hysterophonis). In an aquatic weed like Hydrilla verticillata, aerial buds, called turions, are present.
The underground vegetative propagation system of weeds may be shallow, say within 20-30 cm soil depth (ex. Cynodon dactylon), or deep, up to 100 cm or more in soils (ex. Pluchea lanceolata). The implication of the vegetative propagation system present in several of our weeds is of direct concern to the farmers. It makes their control more cumbersome since small pieces of their propagules can give rise to new plants which multiply into fresh colonies. Thus, although tillage may destroy the aerial shoots of perennial weeds for some time, yet it actually helps such weeds in dispersing their rhizomes and tubers all around. When a translocated herbicide is used against a perennial weed, it may hold it back for considerable time without fear of its seed dispersal, but none of the present herbicides penetrates in soil or translocates in plants deep enough to kill the entire underground roots and rhizomes of the weed, permanently.
Dispersal of Weeds:
(A) Dispersal of Weed Seeds:
The dispersal of seeds away from the mother plant is a built-in mechanism provided by nature to the weeds. It safeguards against their own inter-plant competition, and aids encroachment of new lands and waterbodies by the weeds.
There are many ways the weed seeds attain their movement to short or long distances, including sometimes cross countries.
The major ways of weed seed dispersal, particularly on farmlands, are as follows:
i. Dispersal with Farm Produce:
Weeds which mature at the same time and height as the grain crop, get harvested and threshed together with the crop. While seeds of some of the weeds species can be separated from the grain by simple sieving operations, those with the same shape and size as the grain are extremely difficult to remove.
Such weeds are called satellite weeds since they become part and parcel for the specific crop, year after year. Avena fatua is one such weed in small grains with us. Similarly, in the seed producing crops of lucerne, Cuscuta and Cichorium spp. can be considered as satellite weeds.
ii. Dispersal with Silage, Dung, and FYM:
Weeds harvested with the soilage and silage crops may disperse their seeds around feeding stalls from where they eventually reach the agricultural fields. Also, animals feeding upon the mature weeds can pass out in their dung considerable number of viable seeds.
Experiments have revealed that cattle can pass out up to 10% of the ingested weed seeds in dung in viable form. One can watch during rainy season numerous weed seedlings emerging from the cattle droppings in undisturbed places, including from the manure pits. As a result, usually a field provided with heavy load of FYM is found more weedy than the fields where chemical fertilizers were used, instead.
iii. Dispersal with Wind:
Many weed seeds and fruits are equipped with special organs that keep them afloat in air for considerable distances. For instance, in some weed species their minute fruits are equipped with persistent calyx which is modified into hair to form a parachute like structure called pappus.
This is commonly found in weeds belonging to asertaceae and asclepiadaceae, families. Then there are weed seeds that are covered with hair to achieve the necessary boyancy, for example, cattails (Typha spp.) and tigergrass (Saccharum spontaneum). Groundcherry (Physalis minima) seeds are enclosed in a papery balloon like structure.
The fanweed (Thlaspi arvense) fruits get airborne due to their feathery wings. Some weed seeds float in air just because they are light enough, like those of Cuscuta arvensis. King (1966) used inverse of density of weed seeds to define lightness of weed seeds and named it as Voilures Coefficient. The term, however, is no more in use.
The wind uproots and rolls several weedy plants to long distances during the summer months in tropical and subtropical regions; threshing out their seeds all along the way. Carthamus oxyacantha disperses its seeds largely in this manner in the country.
Some weeds are of inderminate growth habit. They mature their fruits in lots even when the plant is green. Wind shakes these plants strongly enough to split open their mature fruits and throw out the seeds, some distance away from the mother plant. This mechanism, of seed disperal, called censer mechanism, is found in Mexican poppy (Argemone mexicana).
The dispersal of weed seeds and fruits by wind is often the major cause of our new weed problems. Weed seeds and fruits can travel several kilometers with wind unnoticed before hiting the ground. This is why farmers have to be vigilant about any new looking weed seedings on their farms.
iv. Dispersal with Water:
Weedy irrigation and drainage channels provide ready source of movement of weed seeds and fruits from their banks into the agricultural fields along with the irrigation water. Weeds also disperse frequently with flow of weedy canals and rivers.
v. Dispersal through Birds:
Several birds pick-up weed fruits and seeds on their wings, beaks, and feet, and drop these all over during their flight. Lantana camara has been internationally dispersed in this fashion by birds like Indian Myna and the Chinese turtledove. The seeds of the parasitic weed Loranthus spp. get stuck to the beaks of birds feeding upon its fruits, and are then transferred to the fresh tree branches. Oxalis latifolia bulbs and bulbils are dropped undigested by the birds on the ground while flying.
vi. Dispersal by Man:
Careless acts of man are greatly responsible for the dispersal of weeds. For example, when farm machinery is shifted from one field to another without proper cleaning, it disperses many new weed seeds. Weed seeds and fruits are carried on wheels of bullockcarts, automobiles, and even aeroplanes. Man has also carried weed seeds with him throughout the globe unknowingly during travel. Numerous weed seeds are dispersed by man with raw agricultural produce transported from one country to another.
(B) Dispersal of Weed Vegetative Propagules:
On farmlands, careless cultivation of the land is perhaps the foremost factor in the dispersal of vegetative propagules of weeds. It detaches and drags the underground propagules to the uninfested spots where they grow into new colonies. Also, the transplants of paddy and several vegetables often carry with soil clinging on their roots, several weedy rhizomes, tubers, and seeds.
Aerial bulbs of wild onion (Allium spp.) can disseminate just like weed seeds. Also, some weed seedlings growing in crop nurseries may reach the main fields unnoticed along with the crop seedlings either because of their small size or by mimicry as seen in case of Echinochloa spp. in rice nurseries.
Man often moves some good looking wild plants from one place to another to decorate his surroundings. From here these plants spread as weeds by different agencies to other areas. Two such living examples of luring weeds with us in India are waterhyacinth (Eichhornia crassipes) and lantana (Lantana camara).
Economic Uses of Weeds:
Several well-known weeds have been put to certain economic uses since ages. Of these, their medicinal use is perhaps the most ancient one in India. Typha and Saccharum spp. are used in cottage industry for making ropes and thatch material. Cyperus tegedum and Clinogyne dichotoma dry shoots are used on large scale to make mats. The lantana sticks have been used to make furniture.
Cichorium intybhus roots are used for adding flavour to coffee powder. Weeds like Chenopodium album, Amaranthus viridis, and Portulaca sp., form good leafy vegetables. Certain weeds have been used to donate specific genes to our crop plants, for instance, Saccharum spontaneum has been used widely in developing the present day noble canes for North India.
Fruits and rhizomes of certain weeds are used as vegetable and food material. Attempts are underway to convert weedy vegetation into useful manure, animal feed, paper pulp, biogas, and edible proteins on commercial basis, to make physical control of weeds more attractive.
More recently, certain weeds have exhibited nematicidal properties. Their incorporation into the soil has been found to result in reduced rootknot nematode population. Some promising weeds in this respect are Crotolaria, Parthenium, Calotropis, and Eichhornia spp.
Some entomologists have found that parasites and predators of certain crop pests also survive on insect pests of certain weeds. Thus, such weeds help in maintaining the continuity of life cycle of certain useful parasites. Trichogramma chilonis is one such parasite. It feeds upon the eggs of Catopsilla sp. laid on the weed Cassia occidental, as well as upon the eggs of castor semi-looper which damages our castor plants.
Enlightened with the economic potential of certain weeds, some authors tend to define weeds as plants for which economic uses are yet to be discovered. But this in no way lowers the undesirable nature of weeds in agriculture or reduce our concern towards them as harmful plants.
Prevention of Weeds:
Strictly speaking, prevention of weeds embodies all measures to deny the entry and establishment of new weeds in an area; large or small. However, in practice, weed prevention also includes measures to check the every year spread of the already existing weed species on the farm.
Weed prevention is a long term planning so that later the weeds could be controlled or managed more effectively and economically than is possible when these are allowed to disperse freely. Often this project is more successful when it is practised on cooperative basis, simultaneously by a number of neighbouring farmers, although there are many things which the individual farmer can also usefully include in his farm plans.
The important weed prevention recommendations are as follows:
1. Avoid using crop seeds that are contaminated with weed seeds, for sowing.
2. Avoid feeding screenings and other grain material containing weed seeds to the farm animals.
3. Avoid adding the uprooted weeds to the manure pits.
4. Clean the farm machinery thoroughly before moving it from one field to another. This is particularly important for the seed drills.
5. Avoid the use of gravel, sand, and soil from the weed-infested areas.
6. Inspect nursery stock for the presence of weed seedlings, tubers and rhizomes.
7. Keep irrigation channels, fence-lines, and uncropped areas weedfree.
8. Use vigilance. Inspect your farm frequently for any strange looking weed seedlings. Destroy such patches of any new weed by digging these deep and burning the weed along with its roots. Sterilize the spot with suitable chemicals.
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. But these are not very effective in many countries, including India, for one reason or the other. The enormous increase in traffic is one big reason for it.
Control of Weeds:
Weed control on farm lands is the process of limiting any given weed infestation to the extent that it permits economic crop production. The objective here is to limit the growth of unwanted plants without any attempt to eliminate them from the ecosystem. The extent to which a given weed growth should be limited will depend upon the cost involved in the operation and the benefits anticipated from the operation.
On non-crop lands and in waterbodies one should manage the vegetation in such a way as will encourage the growth of some beneficial plants and suppress the remaining, relatively unwanted, plant species. The indiscriminate destruction of vegetation in non- crop areas is not the objective of good weed control. A land without natural vegetation will not be much worth living on.
Thus, before bringing any individual plant under a weed control measure, we must be able to non-erroneously identify the possible gains that may result from our action. In view of this fundamental scientific concept of weed control, now the weed scientists prefer to use the term Weed Management and consider it as a part of vegetation management in a particular area. We shall be using, therefore, the term weed management most of the time in the subsequent text.
There are at least four approaches to manage the weeds effectively. Each of these methods has its own merits and demerits, which may vary with place and time. It is for the farmer to make a proper choice from these alternatives, and, if needed, combine two or more of these methods to achieve the desired objectives precisely and economically, with minimum environmental pollution.
The important components of weed management are as follows, particularly in relation to agriculture:
1. Good crop husbandry procedures
2. Physical destruction methods
3. Employment of herbicides
4. Use of biological control agents
5. Allelochemical control.
Some useful information on allelochemical control of weeds is provided here as follows-
The weed scientists and agricultural chemists have attempted to exploit the naturally occurring allelochemicals from certain plants to suppress specific, target weeds. This has been termed Allelochemical control of weeds. In this approach specific allelochemicals were identified and extracted from the relevant plants and sprayed on the target weeds.
Although the technique gave very encouraging results in the laboratory, yet under field conditions the approach proved infeasible and uneconomical. Some scientists simultaneously attempted addition of raw powder of plants containing allelochemicals to the waterbodies to kill certain aquatic weeds, for example, powder of Parthenium hysterophorus and Coleus amboinious to control waterhyacinth. Again the field feasibility of the technique could not be realised.
The latest approach to allelochemical control is the application of sprays containing specific bacteria or fungi, capable of releasing the desired allelochemicals on the target weeds to destroy these. This technique has been fully commercialised as an eco-safe method to control certain noxious weeds, at least in some western countries, including the USA. This approach is also termed bioherbicidal control of weeds.
Weed Eradication:
In variance with weed control, weed eradication is the complete elimination of all live plant parts and seeds of a weed from an area. Although very attractive, it is a very expensive adventure. It will often cost more than the land was worth. As such, weed eradication is not a common practice, except against certain noxious weeds like Lantana, Saccharum, and Striga spp., which otherwise take thousands of hectares of land out of crop production each year.
The weed eradication programme against such weeds on farmer fields should begin when their growth is still limited to small patches. Once the weed has invaded large, continuous areas, its eradication usually becomes uneconomical. The initial, intensive efforts to destroy all the weed parts should be followed by many years’ of vigilance to prevent the escapes from establishing into wide patches.
Integrated Weed Management:
Also called systems approach, the Integrated Weed Management (IWM) involves deployment of different methods of weed prevention and control in right proportion and at appropriate time against the target weeds without damaging the environment in any way.
On crop lands, the good crop husbandry approach should form the primary component of IWM, and other efforts should be harmonised with it. In fact, IWM programmes should be strongly based on plant community and ecological behaviour of weeds.
According to the FAO, “the integrated campaign against pests is a method whereby all economically, ecologically, and toxicologically justifiable methods are employed to keep the harmful organisms below the threshold level of economic damage, keeping in the foreground the conscious employment of natural limiting factors”.
Management of Cuscuta (dodder) in lucerne fields is cited here as a very good example of Integrated Weed Management schedule, as follows:
1. In fields with history of Cuscuta, adopt crop rotations with non- susceptible crops. Grow lucerne only once in three years in such fields.
2. Do not move animals and farm machinery from the dodder- infested fields to the dodder-free ones.
3. Treat the densely infested patches of lucerne with a non-residual herbicide like paraquat.
4. Do not feed the cuscuta infested crop to the animals.
5. Do not collect lucerne seeds from the crop infested with dodder.
A good IWM plan should be flexible enough to incorporate innovations and practical experiences of local farmers. Also, the IWM programme should be developed for the whole farm and not just for one or two fields. So to say, it should be extended to irrigation channels, roadsides, and other non-crop surroundings on the farm from where most weeds find their way into the crop fields.
At the end, the intensity of any IWM programme against weeds should commensurate with the present and future benefits from the operation, else we are not really interested.