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Damages caused by Weeds

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Damages caused by weeds

Weeds are harmful in many ways. The damages caused by them are as under: 1. Reduction in crop yield:- Weeds compete with crops for water, nutrients and light. Being hardy and vigorous in growth habit, they grow faster then crops and consume large amount of water and nutrients, thus causing heavy losses in yields.

2. Increase in the cost of cultivation:- Tillage operations are done to control weeds and it is generally estimated that on an average about 30 percent of the total expenditure for crop production is on tillage operations and more labour is employed for weeding. This results in increasing cost of cultivation and reducing the margin of net profit. 3. The quality of field produce is reduced:- When the crop is harvested from a weedy field the seeds of weeds get mixed with the main crop which results in lowering the quality of the produce .e.g. seeds of weeds in wheat, gram etc. Similarly, bundles of many leafy vegetables like methi or palak contain green plants of weeds. They fetch lower prices in the market. 4. The quality of the livestock products is reduced. Certain weeds eg. Hulhul when eaten by milch cattle impart an undesirable flavour to milk. Similarly weeds like gokhru get attached to the body of the sheep and impair the quality of wool. Certain poisonous weeds like Datura may cause death of cattle if they are unknowingly eaten by cattle. 5. Weeds harbour insect, pests and diseases:- Weeds either give shelter to various insects, pests and diseases or serve as alternate hosts. 6. Weeds check the flow of water:- Weeds block drainage and check the flow of water in irrigation channels and field channels thereby increasing the seepage losses as well as losses through overflowing. The irrigation efficiency is also reduced. 7. Weed secretions are harmful:- Heavy growth of certain weeds like quack grass or motha lower the germination and reduce the growth of many crop plants. This is said to be due to the presence of certain phytotoxins in these weeds. 8. Weeds are harmful to human beings:- certain weeds cause irritation of skin, allergy and poisoning in human beings. 9. Weeds cause quicker wear and tear of farm implements; they get worn out early and cannot work efficiently unless they are properly sharpened or mended. 10. Weeds reduce the value of the land:- Agricultural lands which are heavily infested with perennial weeds like kans always fetch less price, because such lands can not be brought under cultivation without incurring heavy expenditure on labour and machinery.

Weed control Methods

Before adopting an appropriate method for effective control of weeds, it is essential to know about the weed seeds dispersal, mode of propagation, crop-weed competition. Nature has provided weeds either a number of devices that help them to be disseminated widely. The agents which help the dispersal of weed seeds far and wide are water, wind and animals including man. Other means are impure seeds, sewage, sludge, organic manures, agricultural implements, birds, drainage water. The weed control methods can be broadly classified as preventive and control methods. Preventive measures include all such measures through which the introduction of weeds into the crop fields could be avoided. Control methods include ways of their control and eradication after they have grown in the crop field.

Preventive Measures

Since weeds multiply at a much faster rate and are hardier than crop plants, they always have an upper hand if they are allowed to establish in the field. Therefore, it is difficult and costly to eradicate them. The preventive measures are: i) Use clean seeds which free from weed seeds. ii) Use-well decomposed cow-dung or compost. iii) Cut weeds before seeding. iv) Remove weed growth.

Control Methods

They are classified as: 1. Mechanical methods 2. Cropping or cultural methods 3. Biological methods 4. Chemicals methods

1 Mechanical Method

The most common ones are hand pulling, hand weeding, burning, flooding, ploughing, and harrowing etc. Pulling the weeds by hand or with the help of sickle is the oldest and most efficient method. Weeds can be easily uprooted after a good soaking irrigation or rain. This method is costly and time consuming.

2 Cropping or cultural methods

These include proper crop rotations so that neither annual nor perennial weeds have a free growth. Continuous cropping with the same cropping system leads to greater infestation of fallow helps in controlling the weeds effectively. Intertilled crops like cotton and crops like potato / groundnut which necessarily require digging of soil help in reducing the infestation of weeds. Growing crops like sannhemp which have a very vigorous and leafy growth help in smothering the weeds. A suitable combination of mechanical methods and crop rotation is very effective method of weed control.

3.Biological Methods of weed control

Plant or animal enemies of the weeds maybe used for their destruction. The most notable example is control of prickly pear by using cochineal insects. The Kans grass is controlled by growing basket grass, the roots of which are supposed to excrete substances inhibitory to Kans. Presently, the biological control of weed is including the use of plant pathogens, nematodes, parasitic plants and other organisms. Biological methods have been found to be very efficient and economical provided right type of predators, which even under starvation conditions will not feed upon cultivated crop are found out and introduced.

4. Chemical method of weed control

Any chemical that kills the plants or inhibits their growth is known as herbicide and the method of its application is called chemical method of weed control. Such chemicals may be classified as:) selective herbicides, and ii) Non-Selective herbicides. Each group is further divided into two groups viz. foliage applied herbicides are again grouped into two categories viz. contact & Tran located herbicides. Non-selective herbicides are those chemicals which kill the entire vegetation of the treated area and hence they are applied only on waste lands like canal banks, roadsides etc. Selective herbicides:- such chemicals kill only those plants for whom they have selectivity. They are as under:

A) Foliage applied herbicides:-

These chemical are applied over the leaves or green foliage. They are: a) Contact herbicides: - They kill the plant parts which come in contact with the applied chemical. Weed killing efficiency depends upon their concentration, uniformity in coverage of spraying, stage of weed, weather conditions eg. Dicryl, potassium cynate, sodium arsenite. b) Translocated herbicides:- Such chemicals are absorbed by the treated foliage. When sprayed, they are translocated to the roots which are later destroyed. These are best for perennial weeds. They are also known as systemic herbicides eg. 2-4 D.

B) Soil applied herbicides:-

They are used to keep the field completely free from plant growth. a) Soil fumigants:- They form vapour or gas and diffuse through soils. When applied, they kill all the plant growth. They have relatively short stability of few weeks after which the crop seed may be sown or planted in the treated field eg. Carbon-disulphide, methyl bromide. b) Soil Sterilants:- Such chemicals sterilize the soil and make it unfit for any vegetation for varying period of time depending on the nature and dose of the chemical used, soil type, organic matter content of the soil, rainfall etc. eg. Simazine and Atrazine.

C) Aquatic Applications:-

Such chemicals are used by dissolving or emulsifying in water bodies viz canals, ditches, ponds, lakes etc. to control submerged aquatic weeds. eg. Aqualin, Endothal. Methods of herbicide application i) Pre-sowing treatment:- Application of herbicide before sowing or planting of the crop. ii) Pre-emergence treatment:- In this method, the herbicide is applied after sowing or planting of the crop but before the emergence of the crop. iii) Post emergence treatment:- In this method herbicide is applied after full emergence of crop plants. iv) Directed application:- In widely spaced row crop, the herbicides are sprayed over the space left between the rows without treating the crop foliage or crop shoots. Non selective herbicides are applied in this method. v) Band application:- In this method the herbicides are sprayed only over the crop rows and the inter row spaces can be given. Mechanical treatment. This method is adopted for expensive herbicides, which may not be used for overall application due to high costs. Normally soil applied herbicides are applied in this case.

Intext questions

Match the followings: A B i) Mechanical Method a) Form vapour or gas in soil ii) Biological b) Systemic Herbicides iii) Contact Herbicide c) hand Pulling iv) Translocated Herbicide d) Kill all the plants part v) Soil Fumigants e) Cochineal Insects

What you have learnt

A weed is a plant growing where it is not desired. They not only compete with crop plants for plant nutrients, moisture, space and sunlight but also interfere with agricultural operations increasing the cost of labour and tillage. And ultimately affect the yields and quality of farm produce adversely. Weed control methods are classified as preventive and control methods. Control methods are further classified as Mechanical, cropping or cultural method, Biological methods and chemical methods. Any chemical that kills the plants or inhibits their growth is called herbicide. These herbicides are classified as selective and non selective herbicides.

Terminal Questions

i) Define weed and write the characteristics of weeds. ii) Write the damages caused by the weeds. ii) Write the control methods of weeds iv) Write the methods of application of herbicides. v) Write about the selective and non-selective herbicides for Weed Control

Answer to intext Questions

7.1 1) One season to complete life cycle 2) Two season to complete life cycle 3) More than two years 4) Doob 7.2 i) Hand pulling ii) Cochineal insects iii) Kill all the plant parts iv) Systemic herbicides v) Form vapour or gas in soil Suggested activity Prepare weed album of different weeds occurring in crop plants.


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