Weed Control

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-organisms. 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 labor and equipment, rendering 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.

Natural Powers of Weeds

Weed seeds germinate earlier, their seedlings grow faster; they flower earlier; and from seeds in profusion; and mature ahead of the crop they infest. They have remarkable capacity to germinate under varied conditions, but are season-bound and the peak period of germination always takes place in certain seasons. Another characteristic of weed seeds is the possession of phenomenon of dormancy which is an intrinsic physiological power of the seed to resist germination even under favorable conditions and the seeds don not loose their viability for years even under adverse conditions.

Types of weeds

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

Perennials are of two types: the simple and the creeping. The former multiply only through seeds. They have no normal means of spreading vegetatively. Creeping perennials are spread by creeping roots, creeping above-ground stems (stolons), and creeping underground stems (rhizomes). Some weeds propagate themselves by means of tubers which are modified rhizomes adopted for the storage of food.

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

Parasitic weeds are of two kinds: the total parasites and the partial parasites. These weeds parasitize certain host plants, which they directly attack and deprive them of water, nutrients and assimilates.

 

Preventive measures:

Weeds have been provided with a number of devices by nature to help them to disseminate widely. Water, wind and animals facilitate the dispersal of weed seeds.

Weed seeds are so readily spread by natural agencies and farmer that 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 measures consist of 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 mowing), cultural or cropping, biological and chemical means.

Mechanical methods: Hand-weeding, hoeing, tilling, mowing, burning, flooding, smothering etc are examples of physical methods of weed removal. This process is the most efficient method but is time consuming, back-breaking and costly.

Tillage is practical and economical method of controlling annual weeds. The plough (disc, mouldboard, etc), the harrow 9disc, spike, and spring-toothed) and the cultivator (duck-foot, blade) are some implements.

Cultural or cropping methods:

Weeds under many conditions are better competitors than crop plants for light, water, nutrients and soil space. Farming practices should be capable enough 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. The plant breader 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.

Chemical methods:

The weeds are controlled in the growing crops with weedicides which further increases the yield and ensure 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. This method is easier, less time consuming and less costly than weeding by hired laborers.

 

Classification of Herbicides

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 vegetation, although plants differ in their susceptibility to any particular chemical.

 

Selective herbicides

A) Foliage Applications:

These applications are made to the leaves of growing plants, usually 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. Examples are dicryl, potassium cyanate, selective weed-oil (carrot oil), sodium arsenite, Solan, Poranil and sulfuric acid

ii) Translocated herbicides: These move within the plants, 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. Examples are Carbyne, 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 vapor 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 cynamide 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, Triazines (Simazine and Atrazine), urea herbicides ( Diuron, Monuron, Fenuron, Linuron, Neburon), Uracils (Bromacil, Terbacil) are some soil sterilants.

C) Aquatic Application:

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