Weed Control

Weeds

Weeds are unwanted plants growing naturally in vegetable cultivation all over India.

Weed control in Vegetable Cultivation

Weeds in vegetable cultivation fields are in different size, form and behaviour. They belong to many families varying in physiology, morphology and habits of growth. They compete with vegetable crops for water, nutrients, light and space and thus reduce yields. They also harbour insects and diseases. Weeds with larger and coarse leaves shaded vegetables and thereby restrict photosynthesis. Weeds are generally more vigorous in growth and absorb more mineral nutrients. Weed seeds germinate much earlier even under adverse conditions. They increase the expenditure on labours, equipments and chemicals for their control. Many weeds harbor fungus and bacterial diseases and insect pests. If the growth habits are similar the competition is very keen. The tillage operations are done constantly to remove the competition done by weeds. Weeds in vegetable fields may be annual, biennials or perennials.

The weeds in vegetable fields are generally removed by mechanical means such as cultivation with plough or tractor, mowing, burning etc. or by regular cultural methods. Each of these has advantages and disadvantages. With the discovery of herbicides the weeds are now controlled by chemical method. In home gardening weeds are usually removed with the help of spade, khurpi and hand pulling. In fact, hand weeding is the most efficient method. But due to high wages of labour all the manual methods are costly and time consuming. In large scale cultivation of weeds are controlled by tillage operations using various kinds of plough, harrow and cultivator.

The chemical method to control weeds is easier, less time consuming and less costly in comparison to mechanical method. The object of chemical weed control is to eliminate hand weeding and not to replace for more effective control of weeds. The chemicals used to kill weeds and non-selective herbicides. The selective herbicides kill certain weeds but not the crops. Most of herbicides used in vegetables are selective. However, before use one must know the mode of action and nature of tolerance of a particular herbicide. The amount of herbicide a particular vegetable will tolerate with no damage varies with season climate and soil type. Herbicides in vegetables must be applied very carefully as it is applied as spray. The rate of chemical is in terms of active material; therefore water used has no importance.

The application of weedicides on vegetables in India has not expanded very much as because much research on this aspect has not been done in different soil types, climate and situations. The kind of weeds and population also varies in different states and areas. What kind and how much herbicide should be used for different kinds of vegetables in recommendations. Generally in vegetable cultivation herbicide is applied or immediately after sowing before it has emerged as pre-sowing treatments or as pre-emergence treatments. The contact pre-emergence spraying is done to kill seedling weeds that have emerged before the crop. Pre-sowing treatments are done when a seed bed is prepared but for some reasons sowing is delayed. In such case usually a non-persistent herbicide is used to remove weeds before sowing or transplanting. As pre-emergence spraying the chemicals like Diquat, Paraquat, Dimexan, Simazine and many other similar products are used. Dalapon is very effective to kill grass weeds. As post –emergence MCPB – salt, Dinosebamine, Barban, 2-4 –DB, MCPB are quite effective. The manufacturer’s recommendations regarding quantity to be used, residual effect etc. should be followed.

There are other chemicals for use as pre-emergence spray like IPC, Dinitro amine, Chloro IPC, TCA, Karmax etc. In root crops, Aromatic oils are quite effective. There are many trade names with various formulations.

Weed control methods are grouped into cultural, physical, chemical and biological. A combination of these methods gives effective and economic control than a single method.

Cultural methods

Cultural methods, alone cannot control weeds, but help in reducing weed population. They should, therefore, be used in combination with other methods. Several cultural practices like tillage, planting, fertiliser application, irrigation etc., are employed for creating favourable condition for the crop. If used properly, help in controlling weeds. In cultural methods, tillage, fertiliser application and irrigation, selection of variety, time of sowing, cropping system, cleanliness of the farm etc.

Field preparation

Flowering of weeds should not be allowed. This helps in prevention of build up of weed seed population in the fields. Irrigation channels are the important sources of spreading weed seeds. It is essential, therefore, to keep irrigation channels clean. Deep ploughing in summer exposes underground parts likes rhizomes and tubers of perennial and abnoxious weeds to scorching summer sun and kills them.

Planting method

Sowing of clean crop seeds without weed seeds should be done. It is a preventive method against introduction of weeds. Sowing are taken up one to three days after rainfall or irrigation depending on soil type. Weeds already present in the soil start germinating within two or three days. Sowing operation with seed drill removes some of the germinating weeds. During this process, all the surface soil to a depth of 2 to 3 cm is disturbed uprooting germinating seeds.

Varieties

Weeds continue to germinate for long time in dwarf varieties resulting in high weed growth.

Planting density

Plants of one type do not generally allow germination of other plants near their vicinity. It may be due to allelopathy and competition for growth factors. Closer planting of crops suppresses germination and growth of weeds.

Fertiliser application

Crops like sorghum, maize, pearl millet and rice grow at a faster rate when nitrogenous fertilisers are applied and cover the soil earlier. Weeds like Cynodon dactylon, Cyperus rotundus do not respond to nitrogen application and they are suppressed by fast growing crops. If the weeds present are also responsive to nitrogen, then this method is not suitable.

Irrigation and drainage

Frequent irrigation or rain during initial stage of crop growth induces several flushes of weeds. Under submerged conditions, tubers of Cyperus and rhizomes of Cynodon are killed due to lack of oxygen. Therefore, a good method of reducing Cyperus rotundus is by converting the field to rice cultivation.

Cropping systems

The possibilities of a certain weed species or group of species occurring is greater if the same crop is grown year after year. Crop rotation can eliminate or atleast reduce difficult weed problems.

Weed management practices

1. Follow recommended agronomic practices for land preparation, stubble management, seed bed preparation, timely sowing etc. so as to have a desirable crop stand to start with.
2. The crop should be maintained weed free atleast 8-9 weeks after sowing till canopy starts closing in by resorting timely interculture and hand weedings.
3. A hoeing in between crop rows is given 15-20 days after emergence of cotton seedlings to control primary perennials weed vegetation.
4. For control of weeds, herbicides such as a pendemethalin, alachlor (1.5 kg/ha) and diuron (0.5 kg/ha) may be applied as pre-emergence and fluchloralin (0.75 kg/ha) as pre-plant incorporation.