Spraying is the commonest method of applying herbicides. Sprayers are classified as low-volume and high-volume sprayers. Low-volume sprayers usually apply less than 400 litres of the total spray per hectare. With aircraft it is possible to apply as little as 80 litres of spray per hectare. Our small agricultural holdings, coupled with the practice of growing mixed crops present difficulties in the spraying of herbicides with aircraft. However, the delivering of about 800 to 1,000 litres of spray per hectare is good because the drift hazard to sensitive crops is reduced to the minimum. Granular formulations of herbicides appear to hold out much promise in eliminating drift hazard.

A special type of small portable sprayer is the logarithmic portable sprayer being used in experiments in the U.S.A. The advantages of field-testing of herbicides with this sprayer are many. It delivers the chemical at a logarithmically decreasing rate, i.e. if a start is made with a 1% solution, after a certain distance, it will 0.5%, again after the same distance it will be 0.25% and so on, until a dilution is reached that is no longer effective. On a single species one can readily ascertain the minimum lethal dose of a chemical, the maximum tolerated dose, and the maximum dose that produces no symptoms. On mixed vegetation, as in a pasture one can read off the minimum lethal dose for each species and hence determine the selectivity of the chemicals.

Many of the herbicides are corrosive. Wettable powders are frequently abrasive, shorten the life of all parts of the equipment, especially the gear pumps, valves, booms, nozzles, etc. Thorough cleaning and rinsing are necessary. It is a rule to clean any sprayer before and immediately after use.

Multi-purpose sprayers (the same sprayer used for herbicides, insecticides, fungicides, etc.) must be subjected to very rigid sanitation measures. The equipment should be thoroughly cleaned with water or preferably with 1% ammonia water before it is used for other purposes. When an ester is used, the equipment should be rinsed with kerosene before it is washed with water. After use, fill the sprayer tank and hoes with water overnight and the next day give thorough rinsing with water.

2, 4-D FOR KILLING BROAD-LEAVED WEEDS IN MONOCOT crops. Grasses have narrow and erect leaves; the leaf surface is rough and waxy; and the growing-points are located in the crown of the plant below the soil level and are protected by the surrounding leaves. Broad-leaved plants have leaves that are wider and grow horizontally; the leaf surfaces are smooth and even ; and the growing-points are exposed at the tips of the shoots and in the leaf axils. The droplets of the 2,4-D spray bounce off the leaves of grass and wet them only in spots. On the other hand, the droplets stick to the broad leaves and completely wet them, on account of which 2,4-D becomes toxic to broad-leaved plants.

2,4, 5-T is lethal to wild cucumber, whereas 2,4-D is not. Tracer research directed towards the study of penetration, translocation and metabolism of these two chemicals has indicated that 2,4 5-T remains unaltered in the toxic form inside the plant, thus proving harmful to wild cucumber.   2, 4-DB IN LEGUMES. 2, 4-DB and MCPB are employed for controlling broad-leaved weeds in legumes. These chemicals become toxic only after they are converted into the corresponding 2, 4-D and MCPA. This conversion is brought about by certain enzyme systems and as certain legumes lack them, the weeds and not the legume succumb to these herbicides.

SIMAZINE IN MAIZE. This chemical, as such, is harmful to all plants, but it is used as a herbicide in the case of maize, as this crop possesses a mechanism which converts the chemical into non-toxic hydroxy-simazine.

LINURON IN CARROT. A primary reason for the selectivity and lethality of Linuron to carrot and weeds, respectively is due to the differences in absorption, metabolism and phytotoxicity of several of the metabolic derivatives that are formed inside the tissues.

BARBAN IN WHEAT AND BARLEY. The selective control of wild oats in wheat and barley with Barban marks a landmark in the development of selective herbicides. What is remarkable is that although wheat, oats and wild oats belong to the category of grasses, Barban is harmful only to wild oats.

DALAPON FOR KILLING GRASSES. Dalapon, as a systematic killer of annual and perennial grasses, is a systematic killer of annual and perennial grasses, is a new tool in the hands of the farmers. This herbicide has proved highly resistent to breakdown in plants. Studies have shown that it is most effective on young plants, in active growth, and on plants receiving an adequate water-supply. These charecteristics of Dalapon, added to the nature of its formative effects (tubular leaf formation, excessive tillering, stunting) and the fact that it is more effective in tn the acid form than in the form of a sodium salt, indicate that this herbicide is a plant harmone which acts on grasses in much the same way as 2, 4-D does on broad-leaved plants.

The inability of grass roots to produce buds to regenerate themselves, coupled with their strong tendency to retranslocate this lethal harmone to meristematic regions, including shoot tips and buds, may furnish a clue to the selectivity of this compound. Its tendency not to accumulate in the roots may explain the survival of broad-leaved plants whose roots can produce buds; its ability to continue to move into new buds as soon as they formed may explain its strong phytocidal action on perennial grasses that regenerate themselves solely by buds.

Herbicide residues. Herbicides insecticides and fungicides are grouped under the term pesticides. The question of pesticide residue in the final product, that is consumed by man and animals, has been receiving much attention in America where the pest-control measures are employed are employed on an extensive scale. Chemicals are screened through rigorous procedures and the determinations of LD 50 values (lethal dose) are made. If the instructions given on the containers of the products are faithfully followed, there is no possibility of the pesticides leaving toxic residues in food products.

The question of toxic accumulation in food products may be viewed with less alarm in the case of herbicides than in the case of insecticides and fungicides because of the following basic differences in their use. Insecticides and fungicides are dusted or sprayed on the crop plant and not on the weeds; herbicides are sprayed mainly on the weeds and not on the crop plants. Insecticidal, fungicidal sprays have to be repeated many times during the growing season of the crop, whereas only one spray with the herbicide is sufficient to control weeds. In the case of pre-emergence, or pre-planting application of the herbicides, the time-lag between the treatment and the harvesting of the final product renders the toxic chemical innocuous. In the soil, the chemical is detoxified by micro-organisms, cultural conditions and some other natural agencies. Inside the crop plant also, the chemical does not remain in the toxic form in which it is applied; it gets broken down to non-toxic forms.

It is necessary to be unduly alarmed while using imported herbicides in our country, because all of them have been tested and found safe for use on crops.

There are ways of minimising the problem of the toxic build-up of herbicide residues in plants and soil. The use of the proper herbicides at the correct dosage level and in the appropriate manner and at the appropriate time essential for avoiding any possible build-up of toxic residues in the soil. The use of low doses of herbicide is one of the surest ways of minimising herbicide residues, as low doses are adequate enough to suppress the weeds during the first 4-8 weeks after sowing the crops. During that period, herbicide would lose all its toxicity, thus leaving no residues to affect the crops raised in the herbicide-treated field, and further the chances of accumulation of toxic quantities of herbicides in the plant tissues or contaminating the economic products prepared from the crops grown in the treated fields, are reduced when low doses of herbicides are employed. Also, fertilizing the crops adequately and taking timely measures against insects and pests would go a long way in making the crop plants resist the toxic action of herbicides. Other steps that are useful in minimizing the residue problems consists in not disturbing the soil for 4-6 weeks after herbicide treatment; giving a deep ploughing after harvesting a herbicide-treated crop; adding large quantities of organic matter, such as farmyard manure to the soil and its incorporation thereinto; and giving a flood irrigation, followed by proper tillage before sowing another crop in the herbicide-treated land.

If there has been a toxic build-up of residues because of the faulty application of soil-applied herbicides for 2 to 3 years, a wise step to enable such land to sustain all types of crops consists in growing certain crops which can remove large quantities of the herbicide residues from the soil. For example, maize can be grown both for fodder and grain, and another long-duration crop such as sugarcane, would also considerably reduce the amount of residues.

In irrigated farming, unchecked weed growth will deprive the crops of the valuable inputs, such as nutrients and water, leading to the inefficient exploitation of their high-yielding potential. In dryland agriculture, which covers over two-thirds of the cultivable area of the country, the already low and erratic production due to adverse soil and weather conditions goes down still further in the presence of weeds which can grow even under adverse conditions of soil and climate. Chemical weed control in irrigation farming is the only way of making efficient and rational use of the costly inputs used in this type of farming. In dryland agriculture, chemical weed control ensures the stabilization of production in addition to a more efficient use of water by the crop plants.

Minimal cultivation. The concept of minimal cultivation, which is a valuable principle in both irrigated and dryland agriculture, ensures that weeds do not establish themselves securely in arableland. The raising of crops in multiple or relay=cropping sequence does not leave much time for carrying out cultivation. The control of weeds in this situation is possible through herbicide use. In the use of herbicides in such cropping patterns or sequences, it is very important to understand all the possible effects of these chemicals on the long-term effects of herbicides on soil, especially on the microbes which govern soil fertility to a great extent.

Biological methods. In biological weed control, a "natural enemy" of the weed plants is used. The requirement for the success of this method are: (i) the weed species must have been introduced and in the process of introduction, must have been freed from its natural parasites or predators, (ii) the natural predators or parasites must be introduced to prey upon or parasitize the weeds but they, in turn, must have been freed from parasitesin order to carry out their work of destruction unhampered, and (iii) the destructive agent must be highly specialized, so that it is unable to thrive even under starvation conditions on agricultural plants of the new habitat. Root-borders, stem-borders and internal seed or fruit feeders are more highly specialized than the foliage-feeders.

An outstanding example of biological control of plants is that of prickly pear(Opuntid spp.) in Australia. Other noteworthy examples are the destruction of Lantana in Hawaiian islands, and St. Johnswort(Hypericum perforatum) in America. Attempts are under, way for the biological control of nutgrass in Hawaii, the common blackberry(Rubus fruticosus) in New Zealand. Xanthium strumarium (a weed poisonous to hogs) in Austalia, Halogeton glomeratus (a poisonous weed because of a high oxalate content) in south-west America. A lot of thinking is now being done for the control of water hyacinth(Eichhornia crassipes) through biological methods.

Somewhat different, but an interesting example of biological control is the destruction of aquatic vegetation in ditches by using herbivorous fish. The Hawaiian Sugar-Planters' Association reports that Tilapia mosambica of the sunfish family, by eating the vegetation and burrowing in the bottom of ditches to form spawing nests,destroys the roots of aquatic weeds and kills them. This method appears to have an additional merit, i.e. besides controlling submerged aquatic weeds, it can be used as human food. In Russia, this method is being exploited on a large scale. Biological control through "grass carp" which feed on aquatic weeds may be the safest, cheapest and the most effective of the methods of dealing with water vegetation.

The use of geese to control weeds in cotton is popular in some areas in south-west America. The young geese selectively feed on Johnson grass( Sorghum halepenese) without harming cotton.

The eradication of prickly pear by using thr cochineal insect (Dactylopius tomentosus L.) in Maharashtra and Tamil Nadu is the best example of biological control in India. At the Indian Fisheries Laboratory, Bhopal, it was observed that grass carp ( Ctenopharyngodon idella) controls effectively the submerged water vegetation comprising Najasminor, Hydrilla, Verticillata, Ceratophylum demersum, Lemna minor etc. Claims are also made in India of the control of kans grass(Saccharum spontaneum) by growing basket grass, the roots of which are supposed to excrete some substances inhibitory to kans.

In nature, a balance has been struck among all the components, both living(biotic) and non-living (abiotic) of an environment (habitat, system or region). This ecosystem concept has much relevance in that all the components coexist in perfect union, each component satisfying its own niche. The introduction of a foreign element,for e.g. a herbicide, may create some serious upheavals in the ecosystem. Further, the continued use of a herbicide may eliminate all the susceptible weed species and its place may be taken over by some resistant ones,or the existing ones may develop resistance. All these eventualities have to be borne in mind when recourse is taken to chemical weed control. One of the ways to avert such a situation consists in using low doses of herbicides, in rotating herbicides (using one particular herbicide for one year, missing it the next year or using some other herbicide) just like a crop-rotation and alternating intelligently chemical and non-chemical methods of weed control. An integrated weed-control approach is a sure way of minimizing the residue problems in plant, soil, air and water. An integrated weed-control involves the utilization of a combination of physical, chemical and cropping methods of weed control in a well-planned sequence so designed as not to affect the ecosystem. The nature and intensity of the species to be controlled, the sequence of crops that are raised in the rotation, the standard of crop husbandry, the ready and timely availability of any method and the economics of the different weed-control methods are some of thr potent considerations that determine the successful exploitation of the integrated weed-control approach.