L.K. Baishya, I. Walling and D.J. Rajkhowa
ICAR RC for NEH Region, Nagaland Centre
Weeds have been existing on the earth ever since man started domesticating/cultivating plants and animals around 10,000 B.C. Weeds have been recognized as a problem since then and the battle against weeds is a never ending one. Between humans and continuing food supply, there stand four natural hazards, e.g. weather, weeds, insect pests and plant diseases. Sometimes these hazards work independently and many times they work hand in hand. Weeds are the most underestimated crop pests in tropical agriculture although they cause maximum reduction/loss in the yields of crops than other pests and diseases. Of the total annual loss of agricultural produce from various pests in India, weeds roughly account for 37%, insects for 29%, diseases for 22% and other pests for 12% (Yaduraju, 2006). They decrease quantity and quality of produce/food, fibre, oil, forage/fodder, animal products (meat and milk) and cause health hazards for humans and animals. Thus weed control is indispensable in every crop production system.
- Prevention of weed
First prevention means stopping weeds from infesting an area. It advocates not permitting a weed alien to an area to enter into and infest that area. The area could be of any dimension, a block, a district, a state or a country. However, the success of weed prevention decreases vis-à-vis difficulty in achieving it increases if the size of area increases. In true sense, it was earlier intended towards prevention of exotic weeds. Secondly, in crop field situation, prevention highlights on the aspects how best the seed reservoir/bank could be exhausted, although complete exhaustion is a rare possibility. In this situation, preventing weeds to emerge from soil and grow vigorously and/or preventing existing weeds to come to seeds, tubers, rhizomes or other propagules for dissemination are considered to be preventive approaches. The weed control/management measure adopted the year before, in fact, may be a preventive approach for the coming year. Even certain cultural practices/methods such as stale seedbed; competitive crop and cultivar; time, method & rate of sowing; spacing; irrigation; time, method and rate of fertilization; intercropping; crop rotation; cover crops; etc have bearing towards reducing growth and seed production of weeds. Thus prevention requires a set of practices rather than a single practice to tackle the introduction and spread of weeds. Their individual effect is small, but the impact is enormous when the very/same practices followed together for a long period under certain situation. Impact assessment/quantification of prevention approach should focus on the combined effects of all practices adopted together rather than that of a single practice. In certain agro-ecosystems/crop field situation, a single option of prevention not attended may mar the effect of all other practices carefully attended towards prevention. This makes impact analysis on quantitative term difficult and prevention a more qualitative aspect. The measures/options normally suggested towards prevention of weeds seem to be simple and easy, but difficult to execute/follow and achieve the result compared with control.
i) Pure and clean crop seeds and seed certification
Always use pure, clean crop seeds, which do not add seeds of the existing or new weed species to the soil seed bank. It acts as an insurance/check against increasing weed (both existing and new weeds) problem in the long run.
ii) Stale seed bed
Stale seedbed technique is a cultural-cum-preventive measure. It is cultural from the point of view of pre-sowing 2-3 tillage invariably adopted by farmers since long time to prepare a good seed bed and to control early flushes of weeds.
iii) Well-decomposed farm yard manure (FYM)/compost:
Fresh or undecomposed farm yard manure/compost is a source through which weed seeds are added to soil. Therefore, it should be always well-decomposed/well-rotten and free from weed seeds.
iv) Clean farm machineries and farm animals
Farm machineries like tillage and harvest implements are another source from which weed seeds get into crop field. They should be cleaned properly and the soil sticking/adhered to the implements particularly ploughs, tractors must be removed before it is carried to another field. Accordingly similar treatment may be given to the farm animals since several weed seeds adhere/entangle with the furs and skins of animals by means of some hook like structures and disperse. These are very small efforts towards weed prevention and have hardly been ever evaluated or quantified.
v) Clean irrigation channels and water or alternative irrigation method/system
Irrigation water carries soil and weed seeds to a crop field. Some sub-surface (drip) and over-head (sprinkler) irrigation systems may be installed as per feasibility and applicability in crops, soil and climatic conditions
vi) Clean farm bunds, roadsides, fences and other non-crop areas
Weeds on farm bunds, paths/roads and fences should be controlled occasionally always before they go for flowering to avoid weed perpetuation in the field. Similarly weeds not controlled by measures adopted should be removed from the field before they flowered
vii) Sand, soil from an infested area should not be transported and used to a clean or cultivated area.
viii) While up-rooting crop seedlings, weed seedlings up-rooted may be removed before crop seedlings are taken to main field. In transplanted crops like vegetables, rice, this may be an important mechanism of spread of weeds. Otherwise, weed control has to be exercised in the nursery.
ix) Weed/plant quarantine laws
An enactment is always required to check movement of noxious and pernicious weeds such as Striga sp, Orobanche sp, Parthenium hysterophorus, Eichhornia crassipes, Chromolaena (=Eupatorium) odoratum, Salvinia molesta, Lantana camara etc. It could be both inter-state and inter-country movement. Weed law prevents dissemination by manual, physical or mechanical ways of weed species in general and noxious weeds in particular across regions, states or countries. It also prevents farmers from using mislabeled or contaminated crop seeds. Weed law exists only in Karnataka in India, which declares Parthenium hysterophorus as a noxious weed. It should be enacted across states and countries and should include most of the noxious weeds. Again noxious weeds can be prohibited ones (having seed as well as vegetative propagation and difficult to control) and restricted ones (objectionable in cropping areas but controlled). Weed Quarantine Law enforces isolation of an area where a serious weed has established and prevents further movement of the weed into a non-infested area. To achieve this, import of weed seeds separately or as admixture with crop, should be prevented strictly. To enforce the laws, a number of quarantine stations across the length and breadth of a country requires be operational in places from which possible entry of weeds through food grains, feed and other means is apprehended. Airports for prevention of inter-country/inter-continental movement and rail stations, bus stops, for checking the intra-country/inter-state movement of weed seeds should have a strong regulatory authority of which at least a member should be weed scientist, preferably weed biologist. All noxious weeds of the world should be identified and enlisted before declaring weed laws. Similarly Plant Quarantine (Regulation of Import into India) Order was enacted/issued in 2003 and till June 28, 2006 nine Amendments have been incorporated into it. This imposes regulation on the import of plants, plant products, soil, earth, clay, compost, sand, peat, sphagnum moss, germplasm, transgenics or genetically modified organisms, live insects, microbial cultures, wood or timber. No consignment of plants and plant products such as seeds of coarse cereals, pulses, oilseeds and fodder seeds and seeds/stock materials of fruit plants for propagation, if found infested or infected with a quarantine pest or contaminated with noxious weed seeds shall be permitted to be imported. Pest Risk Analysis (PRA) and fumigation towards disinfection/ disinfestation as applicable will be pursued well before shipment of the product or in certain cases before unloading the materials in India. The order/law if function properly may prevent entry of exotic weeds including noxious.
- Eradication of weed
Eradication means complete elimination or removal of all live weeds/wild plants/plants including their seeds and vegetative propagules from certain area. It is taken to the belief that once a weed is eradicated from an area, it will not reappear unless introduced. Eradication is very difficult to achieve or hardly achieved in terms of complete exhaustion of seed bank and vegetative propagules of weeds from soil. On the contrary, very high dose of soil sterilants and fumigants applied for the sake of eradication unnecessarily leads to soil and water pollution. That’s why it is not or rarely desirable and its adoption is discouraged now-a-days on the ground of high cost, difficulty in accomplishment and for urges in maintaining biodiversity for pest management in crop fields. Eradication brings about temporary or long sterilization effect of soil. Better it is advised to apply/try in small areas such as vegetable and rice nurseries, flower beds, in industrial and factory premises, where immediate residue may not likely cause hazard. Eradication may lead to more soil erosion from the treated area. Therefore, management of weeds to certain extent rather than their eradication is advocated and exercised. However, it may be adopted in certain crop fields if the reasons are justified, e.g. for perennial (Cyperus rotundus, Cynodon dactylon, Convolvulus arvensis, Cirsium arvense etc.) and parasitic weed control in some arable lands and noxious and invasive weed control in non-crop situations. Its adoption is justifiable around oil refineries, industrial and factory premises, housing complexes and railway tracts for fear of fire hazards from the existing/growing weeds. Eradication may also be adopted in and around farm yards where harvested farm produces are kept for threshing and further processing. For eradication programme, whatever chemicals opted should be non selective and should act through soil since soil needs to be sterilized. Several soil sterilants, e.g. methyl bromide, metham, EPTC, NaClO3, chloropicrin (tear gas) and dazomet (DMTT) are recommended for the purpose. They are of course out-dated and their rate of application is very high. Therefore, certain triazines (e.g. simazine, atrazine), phenylureas (e.g. linuron, diuron) or even phenoxyalkanoic acids (e.g. 2,4,5-T but banned in India) may be used for the purpose, but the dose should be much higher than recommended for usual weed control in the crop fields. For example, in crop field, simazine (triazines) is used @ 1.0-2.0 kg/ha depending on soil type, soil pH and organic matter content, but the same as a soil sterilant, may be used as high as 40 kg/ha or more on pathways, roadsides, fence lines, industrial and factory premises and other non-crop areas.
- Control of weed
Earlier there used to be spoken of three major concepts, viz., prevention, eradication and control of weeds. People did not think much about management of weeds. They use to use “control” to mean control of weeds and definitely not to mean management of weeds. Unlike weed prevention, control is exercised normally after the weed problem arises/exists (Zimdahl, 1999) or known to users from previous history of weed infestation in a certain area/crop field. Control is easier to practise than prevention and eradication and can be put/made to work well with short-term economic or cultural planning goals. However, prevention and eradication require long-term thinking and planning. That is why weed control options have been adopted widely among people. Methods/tools, severity of weed infestation in terms of population, species diversity and age/stage of growth, production systems and capital invested greatly affect the degree/level of weed control achieved in a farm. Generally when a single method is exercised towards control of pests/weeds, it is more a control than management aspect of pests/weeds. Employing one single method, e.g. herbicide, managing a weed population to the desired density or economic threshold level is hardly/rarely possible unless other options tried or exercised. Sometimes a pesticide/herbicide may even kill pests/weeds to the level of 100 per cent, which assumes to be as good as eradication. Therefore, for management necessarily more than one or several options/methods should be tried to bring down weed population below economic threshold value. Thus control remained as an old concept and in recent years it has got a different connotation, e.g. management. Management has become more popular because it has some significant bearing over control in terms of maintaining bio-diversity and for preventing sudden insurgence of other pests/weeds etc. However, control still bears significance when many farmers use only herbicide to control weeds in crops. For example, to control Phalaris minor or Avena ludoviciana in wheat, farmers resort mainly to herbicides in north-western wheat belt of the country. We talk of management of weeds much, management is hardly executed.
Knowledge of weed seed characteristics, morphology, ontogeny, nature of competition and degree of association with crops are pre-requisite for suggesting some efficient weed control measures. It makes the users/scientists quite acquainted with the nature and spectrum of weeds existing in the crop fields and accordingly guides them to adopt certain measures. Identification and naming of a particular weed based on its genus, species or certain biological characters may not be much useful to users since weed control usually, unless specific weed problem in certain area, aims at composite weed culture and not on individual species of weeds. Therefore, some common characteristics of the species, which are clearly visible and easily understandable by users, are to be exploited for making of their classes/groups and for recommending suitable control measures.
Weed management decisions vary according to plant life cycles, infestation size, environmental parameters and management objectives. Hand-pulling a perennial weed species such as Canada thistle or leafy spurge is a futile effort, but very effective for control of a biennial plant such as diffuse knapweed. Releasing bio-control insects for control of dalmatian toadflax on a 500 acre property is wise, but relying on insects for controlling small patches on a 40 acre pasture is inefficient. Successful weed management requires proper plant identification, selection of effective management methods and monitoring the effects over time. Prevention is the most essential aspect of weed management. Eradication is very difficult to achieve or hardly achieved in terms of complete exhaustion of seed bank and vegetative propagules of weeds from soil. On the contrary, very high dose of soil sterilants and fumigants applied for the sake of eradication unnecessarily leads to soil and water pollution. That’s why it is not or rarely desirable and its adoption is discouraged now-a-days on the ground of high cost, difficulty in accomplishment and for urges in maintaining biodiversity for pest management in crop fields.
The author is grateful to Dr. T.K. Das, Principal Scientist, Division of Agronomy,IARI, New Delhi as many references were undertaken from his literatures.