WEEDS are the plants, which grow where they are not wanted. They grow in the fields where they compete with crops for water, soil nutrients, light and space and thus reduce crop yields. They also harbor insects pests and micro-organism. Certain weeds release into the soil the inhibitors or poisonous substances which are harmful to the plants, human beings and live-stocks. They increase the expenditure on labour and equipment, render harvesting difficult, and reduce the quality and marketability of agricultural produce. They block the drainage and impede the flow of water in canals and water-transport channels and their growth in the rivers renders navigation very difficult. The dense growth of weeds in water pollutes the water because they deoxygenate the water and kill the fish.

Weed seeds germinate earlier; their seedlings grow faster; they flower earlier; and form seeds in profusion; and mature ahead of the crop they infest. Nature has bestowed these qualities on weeds so that there seeds are collected unwarily along with the produce of the crop at harvest and get distributed to other places where the produce may be taken. They have the remarkable capacity to germinate under varied conditions, but very characteristically they are season-bound and the peak period of germination always takes place in certain seasons in regular succession year after year. Another charecteristic of the weed seeds is the possession by them of the phenomenon of dormancy which is an intrensic physiological power of the seed to resist germination even under favourable conditions, and also the seeds do not lose their viability for years even under adverse conditions.

Weeds belong to the class Angiospermae (flowering plants) which have two subclasses: Monocotyledoneae (monocots) and Dicotyledoneae (dicots). On the basis of the habitats, they are divided into terrestrial and aquatic categories, and on the basis of the duration of life they are divided into annuals, biennials and perenials. Annuals live and produce their seeds in a single growing season. Biennials need two growing seasons; in one season they pass through their vegetative or rosette stage, followed by reproductive stage in the next season. The multiplication of both the annuals and the biennials is through seed. Perenials live indefinitely and are propagated not only through seeds but often vegetatively through underground structures, such as rhizomes, stolons, bulbs and tubers.

Perenials are of two types: the simple and the creeping. The former multiply only through seeds. They have no normal means of spreading vegetatively. However if they are injured or cut, the severed portion produce new plants. Creeping perenials are spread by creeping roots, creeping above-ground stems (stolons), and creeping underground stems (rhizomes). Some weeds propogate themselves by means of tubers which are modified rhizomes adapted for the storage of food.

Aquatic weeds (hydrophytes or water inhabitants) are classified into three types, viz. submerged, emersed and floating. Submerged aquatics are anchored to the bottom of the habitats, e.g. a ditch, and grow entirely beneath the surface of the water. Emersed once have their roots beneath the surface of the water, but the leaves and stems are above the water-line. Floating weeds or surfaced aquatics either float freely on the water or float only in a limited area.

There is still another group of weed known as the parasitic weeds. These are of two kinds: the total parasite and the partial parasites. These weeds parasitize certain host plants, which they directly attack and deprive them of water, nutrients and assimilates.

Crop-weed competition.   In non-irrigated areas, the competition between weeds and crops is largely for water. The transpiration coeffiicients for kunda (Ischaemumpilosum), hariali(Cynodon dactylon) and Tephrosia pupurea are 556, 813 and 1,108 respectively, whereas this coefficient is only 430 for jawar (Sorghum vulgare). A saving of 750 to 1,250 tonnes of water per hectare of soil, forming a one-metre deep column, is possible by keeping the soil free from weeds.

In irrigated tracts, the competition is severe for nutrients. The unchecked growth of weeds, in a wheat fields measuring one hectare, removes about 20 kg of nitrogen, reducing the grain yield by about twelve quintals. The mineral requirements of weeds are high. Wild mustard (Brassicasinensis) requires twice as much nitrogen, twice as much phosphorus, four times as much potash and four times as much water as a cultivated oat plant. Weeds in a fallow land deplete the soils of both moisture and nutrients. Pohli (Carthamus oxyacantha) removes about 60 kg of nitrogen from one hectare.

Weeds generate a formidable competition with crop plants for light. One of the secrets behind the high-yield potential of the dwarf varieties of crop plants is that they can capture solar energy more efficiently. Unchecked weed growth in the fields cropped with dwarf varieties will intercept solar radiation to the detriment of the crop plants. Likewise, the competition for space that the weed offer to the valuable crop plants may assume, at times, so alarming proportion as to smother the latter.

The identification of weeds. Both the nature of the weeds and that of the crops which they infest, influence the action of the weedicides. For example, the same types of weeds occur in maize and cotton which are kharif crops. 2,4-D can be used for killing the weeds in maize, but this weedicide should never be used on cotton which is highly sensitive to it. The successful application of chemicals to smother the weeds in growing crops is determined by the knowledge one has regarding the biology of both the crops and the weeds. The degree of success from the adoption of physical methods of weed removal is determined largely by the knowledge one has of the peak periods when the seeds of weeds are formed and of the nature or duration of their dormancy.

A description of some of the important weeds occuring in our country and the methods of their identification and control in different crops is given at the end of the chapter.

Preventive measures.  ; Nature has provided weeds with a number of devices that help them to be disseminated widely.The agencies that facilitate the dispersal of weed seeds far and wide are water, wind and animals, including man. The troubles that weeds create in crops, soil and water are summed up in the adage "one year of seedling is seven years of weeding". To avoid such a situation, a wise step is to follow the principle "prevention is better than cure".

As weed seeds are so readily dispersed by natural agencies and by the farmer himself, it is important to prevent weeds, whether in crops, on borders or bunds, in fences or in irrigation-channels, from flowering and setting seed. Preventive methods consist in sowing crop seeds not contaminated with weed seeds,using manure and irrigation water not laden with them and the enforcement of weed control laws and seed-certification measures.

Control methods. Measures against weeds comprise mechanical(cultivation and moving), cultural or cropping, biological and chemical means. Each of these methods has certain merits and a prudent farmer can make use of one means or a combination of them to control weeds efficiently and economically.

Mechanical methods. Hand-pulling or hand-weeding, hoeing, tilling, mowing, burning, flooding, smothering, etc. are examples of physical methods of weed control, involving the use of physical energy through implements,either manual, bullock-drawn or power-operated. Hand-weeding is the most efficient method,but it is back-breaking,time-consuming and costly. With the gradual industrialization of our country, coupled with the raising of standards of living and literacy, manual labour is becoming scarce. Further, the high wages paid to the hired labour narrow down the profits of the cultivator.

Tillage is a practical and economical method of controlling annual weeds. The plough (disc,mouldboard,etc.), the harrow (disc,spike and spring-toothed) and the cultivator (duck-foot, blade,bakhar, etc.) are implements in use to eradicate weeds.

Cultural or cropping methods. Weeds under many conditions are better compititors than crop plants for light, water, nutrients and soil space. However, farming practices are capable of changing the condition in such a way as to enable the crop plants to compete with weeds successfully or to reduce their interference to the minimum and thus preventing them from acting as impediments to increased crop production. Seeds with good germination will give the crop a vigorous and close stand and thus enable it to steal a march on the weeds. Varieties which are well adapted to a region will obviously compete better with the weeds than varieties poorly adapted to it. The plant breeder has to evolve quick-growing and short-duration varieties of crop plants with a large leaf area and good branching and the agronomist has to work out the proper seed-rate,depth,time and method of sowing, applying the most effective methods of irrigation and fertilizers and adopt a proper system of rotation.

Some crops can compete better with weeds than others. For instance crops like Sudan grass, sorghum and cowpea are good competitors while crops,such as linseed,groundnut and lentil are poor competitors. The raising of highly competitive crops (also known as smother crops) is useful in reducing weed infestations. One disadvantage from which many smother crops suffer is that their seedlings grow slowly. If these crops are grown for seed, some measures have necessarily to be taken to control weeds in the early growth of these crops. However, if they are meant for fodder, the thick sowing resulting in dense growth smothers the weeds.

Chemical methods  The controlling of weeds in the growing crops with weedicides increases their yields and ensures the efficient use of irrigation,fertilizers and plant-protection measures, such as the spraying of insecticides and fungicides. The removal of weeds from the growing crops facilitates easy harvesting and gives a high-quality produce without admixture with weed seeds. Chemical weed control can be adopted quite in time and in situations and under conditions which make manual or mechanical weeding difficult. A great advantage of this method lies in killing weeds in the crop row or in the immediate vicinity of crop plants. The chemical method is easier, less time-consuming and less costly than weeding by hired labourers.

A herbicide is any chemical that kills the plants or inhibits their growth. Selective herbicides remove certain weeds from certain crops. The selectivity is not absolute, but is governed by the amount of the chemical applied, the way it is applied, the degree of wetting of the foliage, the amount of rainfall following the application, the tolerance of different plants to a specific chemical and the differences in the growth habits of the crops and the weeds. Non-selective herbicides remove a wide range of vegetarian, although plants differ in their susceptibility to any particular chemical. These two major groups of herbicides are further classified as given below.

Selective herbicides
A. FOLIAGE APPLICATIONS. These applications are made to the leaves of growing plants, uaually as sprays,but in a few cases as dust applications.
   (i) Contact herbicides. Contact herbicides kill only the plant or the portions of the plant that actually come into contact with the chemical. The herbicide to be effective must cover the foliage. Selectivity is dependent upon the differential wetting,differences in forms of the plants, and also upon the placement of the spray. Some examples are dicryl,potassium cyanate,selective weed-oil (carrot oil), sodium arsenite,Solan, Propanil and sulphuric acid.

(i) (ii) Translocated herbicides. Translocated herbicides move within the plant,a property that makes them effective in destroying the roots of perennial weeds. A low-volume application is possible in their case. The physiological differences among plants determine the selectivity. Systemic herbicide is also a term used to name a herbicide which is translocated. Examples of this group of herbicides are Caebyne, 2, 4-DB, MCPB, 2, 4-5-T, 2, 4, 5-TP and MSMA.

B. SOIL APPLICATIONS. Soil fumigants and soil sterilants fall into this group. They are used where it is desired to remove all plant growth or to keep areas free from plant growth.

(i) Soil fumigants These materials are employed for killing all plant growth before sowing more desirable species. They function as a vapour or as a gas that diffuses through the soil and have a relatively short life in the soil as a result of which re-planting in the treated area is possible within four weeks or less. Carbon disulphide, Chloropicrin, Vapam, methylbromide and cyanamide are some soil fumigants in use.

(ii) Soil sterilants. These chemicals make the soil sterile (incapacitate it to support vegetation) for varying length of time,depending on the chemical used, the amount applied,rainfall,soil type,organic matter, etc. The inorganic chemicals,borates and chlorates, and the organic compounds,Trizines (Simazine and Atrazine), urea herbicides (Diuron, Monuron, Fenuron, Linuron, Neburon), Uracils (Bromacil, Terbacil) are some soil sterilants.

C. AQUATIC APPLICATIONS. A number of chemicals are used for controlling some submerged aquatic weeds by dissolving or emulsifying them in water in canals,ditches,ponds and lakes. Some chemicals in use are aqualin, aromatic solvents, chlorinated benzenes, copper sulphate, Endothal, Fenac, Sodium arsenite and 2,4-D.

Formulation of herbicides. Formulation refers to the way in which in which the basic weed-killing chemicals are prepared for practical use. Herbicides are formulated to be applied as solutions in water or oil,emulsions,wettable powders,granules and dusts.

Solutions of water or oils. The salts of most herbicides are soluble in water. They are dissolved in convenient amounts of water and then sprayed. A few examples are sodium and amine salts of 2, 4-d, 2, 4,5-T, MCPA, amine salts of DNBP, sodium salts of Pentachlorophenol, TCA and Dalapon.

The "parent acid" formulations of some of these are soluble in oil (DNBP,PCP). They are often used to increase the toxicity of oil sprays or to fortify the oil.

Emulsions. An emulsion is one liquid dispersed in another liquid, each maintaining its original identity. The two liquids are prevented from reacting with each other by the addition of an emulsifying agent. Ester formulations of 2,4-D are oil-like and form emulsions. Emulsions are milky.

Wettable powder. It is a type of formulation in which a herbicide is absorbed generally on an inert carrier, together with an added surface-acting agent, and finely ground so that it will form a suspension when agitated with water. Simazine, Atrazine, Monuron, Diuron and Neburon are wettable powders.

Granular herbicides. In these formulations the herbicide is absorbed on, mixed with, or impregnated into, a generally inert carrier in such a way that the final product consists of granular particles. Many carriers are used, e.g. clay,sand vermiculite and finely ground plant parts (ground corn cobs, tobacco leaf trash,etc.). Granular materials can be spread by hand or with mechanical spreaders. These materials have advantages over sprays,because water is not needed for application, costlier spraying equipment is dispensed with, and the granules fall off the leaves of valuable plants without causing injury. Another advantage is that as the active weed-killing principle in these formulations is gradually released, they suppress the growth of weeds for long.

Dusts. Insecticides and fungicides are very often made in the form of dusts. However, only a few herbicides are applied as dusts because of the drift hazard.

Calculation of the dosage levels. The quantity of the toxic ingredients, to which the weed-killing property of a chemical is due, is different in different formulations of herbicides. The active ingredient is that part of a chemical formulation which is directly responsible for the herbicidal effects. In some herbicides, the entire molecule may be the active unit. If the chemical is 99 per cent pure, it is considered to have 99 per cent active ingredient. The strength of the wettable powders is expressed in terms of the active ingredient (a.i.). In others, the herbicidal activity is calculated on an acid-equivalent basis. The acid equivalent (a.e.) refers to that part of a formulation which can be theoretically attributed to the acid. In these cases, the acid equivalent is given as the active ingredient. Examples are 2, 4-D, MCPA, MCPB, 2,4-DB, CMPP and Picloram.