WEED CONTROL METHODS

Weed control and weed management are the two terms used in weed science. Weed control is the process of limiting infestation of the weed plant so that crops can be grown profitably.

Weed management includes prevention, eradication, and control by regulated use, restricting invasion, suppression of growth, prevention of seed production and complete destruction. Thus weed control is one of the aspects of weed management.

A) CULTURAL METHODS

Several cultural practices like tillage, planting, fertilizer application, irrigation, etc., are employed for creating favorable conditions for the crop. These practices if used properly, help in controlling weeds. Cultural methods, alone cannot control weeds, but help in reducing the weed population.

1) Field Preparation

1. The field has to be kept weed-free. The flowering of weeds should not be allowed. This helps in the prevention of the build-up of the weed seed population in the fields.
2. Irrigation channels are important sources of spreading weed seeds. It is essential, therefore, to keep irrigation channels clean.
3. Deep ploughing in summer exposes underground parts like rhizomes and tubers of perennial and obnoxious weeds to the scorching summer sun and kills them.
4. Conventional tillage which includes 2 to 3 ploughing followed by harrowing decreases the weed problem.
5. Running blade harrows cuts weeds and kills them.
6. In lowland rice, the puddling operation incorporates all the weeds in the soil which would decompose in course of time.

 

2) Planting Method

1. Sowing of clean crop seeds without weed seeds should be done. It is a preventive method against the introduction of weeds.
2. Sowings are taken up one to three days after rainfall or irrigation depending on soil type. Weeds already present in the soil start geminating within two or three days.
3. Sowing operation with seed drill removes some of the germinating weeds.
4. Transplanting is another operation that reduces the weed population. Since the crop has an additional advantage due to its age.

 

3) Varieties

1. Short statured, erect leaved varieties permit more light compared to tall and leafy traditional varieties.
2. Weeds continue to germinate for long time in ‘dwarf varieties resulting in high weed growth.

4) Planting Density

1. Plants of one type do not generally allow germination of other plants near their vicinity. Closer planting of crops suppresses germination and the growth of weeds.
2. Wider planting should be avoided

 

5) Fertiliser Application

1. Plants differ in their capacity to respond to fertilizer application.
2. Crops like sorghum, maize, pearl millet, and rice growing at a faster rate when nitrogenous fertilizers are applied and cover the soil earlier.
3. Weeds like Cynodon dactylon, Cyperus rotundus do not respond to nitrogen application and they are suppressed by fast-growing crops.

 

6) Irrigation and Drainage

1. Depending on the method of irrigation, weed infestation may be increased or decreased.
2. Frequent irrigation or rain during the initial stage of crop growth induces several flushes of weeds.
3. In lowland rice, where standing water is present most of the time, the germination of weeds is less.
4. Continuous submergence with 5 cm water results in reducing weed population whereas, under the upland situation, weed population and weed dry matter is very high

 

B) PHYSICAL METHODS

Physical force either manual, animal, or mechanical power is used to pull out or kill weeds. Depending on weed and crop situation one or a combination of these methods is used.

1) Hand-weeding

Pulling out weeds by hand or uprooting weeds by using a small hand tool is known as hand weeding.

2) Hand hoeing

The entire surface soil is dug to a shallow depth with the help of hand hoes, weeds are uprooted and removed. This method is adopted in irrigated upland crops like finger millet, pearl millet, onion etc.

3) Digging

• Weeds are removed by digging up to deeper layers so as to remove underground storage organs.
• It is very useful in the case of perennial weeds and it is done with the help of pick axes or crowbars. Cynodon dactylon can be effectively controlled by this method.

4) Mowing

• Mowing is the cutting of weeds to the ground level.
• Mowing is usually practiced in non-cropped areas, lawns, and gardens wherein the grass are cut to a uniform height to improve the aesthetic value.
• The common mowing tools are sickle, scythe, and lawnmower.

5) Cutting

Weeds are cut above the ground surface leaving stubble. It is the most common practice against brush and trees. Cutting is done with the help of axes and saws.

6) Burning and Flaming

Burning is the cheapest method of eliminating mature unwanted vegetation from uncropped areas like rangelands, field bunds, roadsides, ditch banks, etc.

7) Mulches

Mulches when applied on the soil surface, do not allow weeds to germinate or to grow as light does not reach the soil.

8) Intercultivation

Inter-cultivation is a very effective and cheap method of weed control in line-sown crops. Inter-cultivation implements have a blade that cuts the weeds just below the soil surface and thus kills weeds. It also makes the surface soil loose and dry so that subsequent germination of weeds is avoided unless irrigation or rain follows.

 

C) CHEMICAL METHOD

Chemicals that are used to kill plants or weeds are called herbicides.

a) Soil application

1) Soil surface application: Herbicides are usually applied to soil surface to form a uniform herbicide layer. The applied herbicides, due to their low solubility may penetrate only a few centimeters into the soil. Weeds germinating in the top layers are killed due to incidental absorption of herbicides. eg. triazines, ureas and anilide

   

2) Soil incorporation: Some herbicides are applied to the soil surface and incorporated into the soil either by tillage or irrigation for their effectiveness. eg. volatile herbicides viz., aniline and carbamate

3) Sub-surface application: Perennial weeds like Cyperus rotundus and Cynodon dactylon are controlled by injecting herbicides into the lower layers of the soil at several points.

4) Band application: Herbicides are applied to narrow bands over or along the crop row. The weeds in between the crop rows can be controlled by inter cultivation or band application of herbicide. This method is useful where labour is expensive and inner cultivation is possible. eg. Weeds in maize can be controlled effectively by spraying atrazine on seed row at the time of sowing.

 

b) Foliar application

1) Blanket application: Application of herbicide over the entire leaf area. Selective herbicides are applied by this method.

2)Directed Application: Herbicides are applied directly to weeds between crop rows, avoiding the crop foliage. Care is taken to avoid spray fluid falling on the crop. 

3) Spot application: Herbicides are applied or poured on small patches of weeds, leaving the relatively wee free patches untreated. It minimizes the herbicide usage per unit area.

Benefits of chemical method:

1. Herbicides can be applied for weed control in crop rows and where cultivation is impossible.
2. Pre-emergence herbicides provide early-season weed control.
3. Cultivation & manual methods of weed control may injure the root system.
4. Herbicides reduce the need for pre-planting tillage. They are extremely useful in minimal\zero tillage.
5. Herbicides can control much perennial weeds which cannot be controlled by other methods. Eg: Cyperus sp.,

 

D) BIOLOGICAL METHODS OF WEED CONTROL

1. Bioagents like insects, pathogens, etc., and other animals are used to control weeds.
2. Insect and pathogens infest weeds and they either reduce growth or kill weeds.
3. The biological control method can reduce weeds but it is not possible to eradicate weeds.

Examples of Biological Weed Control

Bioagent Weeds Insect Two beetles:   Octotoma scabripennis and Uroplata giraldi Lantana camara Scale insect Dactylopius tomentosus. Prickly-pear weed – Opuntia Flea beetle Agasicles hygrophyla Alligator weed –  Alternanthera philoxeroides Fish Common carp (Cyprimus carpio) Chinese carp Aquatic weeds Mammals Manetee or sea-cow Water hyacinth Snails     Marisa sp and other fresh water snails   Submerged weeds like coontail and algae Fungi Rhizoctinia blight. Hyacinth Mites Tetranychus sp Prickly pear Plants Cowpea as intercrop in sorghum Effectively reduces the growth of weeds in sorghum