A weed may be defined as any plant that is growing where it is not wanted. Thus, in a crop field, all plants other than the crop(s) in the field are weeds.
- Weeds compete with crops for nutrients, water, light and space.
- Some weeds can be poisonous to humans and livestock.
- Weeds can harbour pests and diseases that may attack crops, and interfere with harvesting.
- If weeds are not controlled crop yields will be reduced.
The most critical period of weed competition is during the first four to six weeks after emergence of the crop. If crops are kept weed-free during the early stages, yields will not be affected significantly. At later stages the maize plants will be well established and out-compete the weeds. Although early weeding is critical to producing a good yield, late control is also important in preventing the weeds from flowering and producing seeds, which would affect the crop and increase weed load in subsequent seasons. Harvesting will also be made easier if the crop is weed free.
Farmers should become familiar with the types of weeds present in their fields. For instance, broadleaf weeds should be distinguished from grasses. The farmer needs to learn which weeds are annuals (i.e. live for only one season) or perennials (i.e. live for more than one year). This will help in the control of the weeds, especially in the application of chemicals.
In conventional farming, farmers were advised to till the soil in order to control weeds. Farmers would plow in order to clean their fields of weeds and prepare the seedbed before planting. Plowing buries some seeds, but as the soil is turned other seeds are brought back to the surface. In some cases farmers burn the crop residues to control weeds, but this can stimulate germination of other seeds from the so called ‘soil seed bank’.
Plowing is also detrimental to the structure of the soil and it exposes the soil to erosion. The use of plowing is therefore no longer recommended to control weeds, except in extreme cases.
Conservation agriculture is the preferred option for crop production, not only because it conserves the soil and stabilizes yields, but also because it helps to reduce weed numbers in a number of ways:
Prevention - Practices that prevent the introduction, propagation, and spread of weeds. For example: destroying the weeds before they set seeds, planting weed-free seed, using clean equipment on the farm, and keeping the field margins clean to prevent weed invasion.
Cultural - Practices such as crop rotation, intercropping and mulching.
Manual - Practices whereby human energy is directly utilized to remove the weeds. This involves hand weeding using hoes or pangas, hand pulling, hand slashing and push-type weeders.
Mechanical - Appropriate tillage equipment used for weed control, with animals or fossil fuel as the source of energy, e.g., drawn by donkeys, oxen or a tractor.
Chemical - The use of herbicides selected for the soil, crop, weeds and stage of crop development. For instance, after the maize seed has been planted, atrazine can be applied to kill the weeds before they emerge. Atrazine, applied as a pre-emergence herbicide, controls most annual broadleaf weeds and some annual grasses. It is only recommended for use on soil with more than 35% clay.
For successful weed control it is best to carry out the procedure when the weeds are still small, before competition sets in. It is recommended to use a combination of the methods to reduce weeds (Integrated Weed Management) as no one weed control method can meet the needs of any crop all the time.
Cultural methods of weed control use practices common to good land and water management. These include planting weed-free crop seed, planting crops at the right spacing, planting cover crops, using mulch, rotating crops, and intercropping.
Crop spacing - Plants spaced closely together will develop cover quickly and shade the weeds that try to grow. However, crop spacing should not be too close so as to cause negative competition between the crop plants. The ideal spacing will depend upon conditions in which the crop is grown. Closer spacing is possible in favourable growing conditions, whereas in drought-prone environments, wider spacing is advisable.
Cover crops - Growing cover crops that develop quickly will help to suppress weeds before they grow. Some cover crops (such as black oats) control weeds by producing chemicals that prevent weeds from growing.
Mulching - Covering the soil with mulch makes it hard for weeds to grow because they do not have enough space or light. Mulch should, however, be selected carefully and not include flowers and seeds, otherwise they may introduce more weed seeds into the field.
Crop rotation - Rotating crops helps to break the life cycle of certain weeds common to a particular crop.
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