A weed is a plant that grows where it is not wanted, including plants that were not intentionally sown or those that are more competitive and interfere with human activity.
Weeding is the process of removing weeds from a field, which is an efficient approach to crop protection and agricultural production management.
Weeds compete with other cultivated crops in the field for sunlight, water, and essential nutrients. Examples of weeds include Amaranthus, Chenopodium, and Gajar Ghas.
Methods for weed control include pulling, digging, disking, plowing, and mowing. The effectiveness of these mechanical control methods depends on the life cycle of the target weed species.
Classification of Weeds in Agriculture
Classification of weeds can be based on various factors, including their life cycle, habitat, dependence on other hosts, soil type, and plant families.
Weeds can be classified based on their life cycle as annuals, seasonal, biennials, or perennials. Annual weeds complete their life cycle within a year, while seasonal weeds and monsoon annuals complete their life cycle during specific seasons. Winter annuals, also known as Rabi season weeds, complete their life cycle during the winter season. Two-seasonal weeds include biennials, which require two years to complete their life cycle, and perennials, which continue their life cycle for years.
Weeds can also be classified based on their habitat or place of occurrence, such as croplands, pastures and grazing lands, water channels, roadsides, wastelands, lawns and orchards, and forest lands.
Another classification is based on the weed's dependence on other hosts. Stem parasites naturally graft to their host plants to extract water and nutrients, while root parasites such as Striga attach themselves to sugarcane roots. Independent parasites do not require a host.
Weeds can also be classified based on the type of soil they grow in, such as black soils, sandy loam soils, ill-drained soils, and tank weeds that may be submerged, immersed, or floating, such as water hyacinth and calcium.
Finally, weeds can be classified based on their plant families, including Graminae, Commelinaceae, Cyperaceace, Amaranthaceace, Euphorbiaceae, Composite, Leguminous, Malvaceae, Tiliaceae, Cruciferae, Chenopodiaceae, Solanaceae, Papaveraceae, Portulacaceae, Orobanchaeceae, and Cactaccae. These are primarily vascular plants that produce most of their vegetative growth beneath the water surface and have true roots, stems, and leaves.
Let's Talk About The Downsides of Weeds
Reduction in crop yield: Weeds compete with crops for water, nutrients, and light, and since they have a vigorous growth habit, they can outgrow crops and consume large amounts of water and nutrients, leading to significant reductions in crop yield. For example, groundnut yield can be reduced by 40%, and chili yield can be reduced by 66%. Weeds also cause a loss of nitrogen, with around 150kg/ha being lost.
Increase in the costs of cultivation: Controlling weeds is one of the main objectives of tillage, and it incurs approximately 30% of the total expenditure. In heavily infested areas, the cost of weed control through weeding or chemical control may increase even more, reducing the margin of net profit.
Reduced quality of field produces: Weed seeds can be harvested and threshed along with crop production, lowering the quality of produce. This can result in lower prices in the market for products like leafy vegetables and grain crops.
Reduced quality of livestock products: Weeds can impart an undesirable flavor to milk and impair the quality of wool in sheep, and the poisonous nature of weed seeds can cause the death of animals.
Blockage of irrigation channels: Weeds can block drainage and impede the flow of water in irrigation canals and field channels, leading to increased seepage losses and losses due to overflowing, thereby reducing irrigation efficiency.
Harmful secretions: Certain weeds, such as quack grass and lavala weed, can secrete phytotoxins that can lower germination and reduce the growth of many crop plants.
Harmful to humans and animals: Weeds can cause skin allergies and poisoning in humans, and they can also be fatal to livestock.
Quicker wear and tear of farm implements: Weeds have deep roots and are hardy, which can cause tillage implements to wear out early and work less efficiently unless they are properly sharpened or mended.
Reduced land value: Lands heavily infested with perennial weeds fetch less money since it requires significant expenditure to bring them under cultivation.
Harboring insects and crop diseases: Certain weeds, such as mustard-family weeds, can carry over canola diseases, making crop rotation less effective for disease management. Immature weeds can also interfere with harvesting operations, and weed seeds in harvested crops can cause dockage and increase the risk of spoilage.
Related:
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Benefits or Advantages of Weeds
Weeds, through their ability to shade the soil surface, reduce evaporation and the sun's harmful effects while also decreasing wind speeds at the soil surface. In winter, they trap snow, which adds to the soil's moisture content, making them important agents of soil conservation.
When plowed under, weeds add nutrients or organic matter to the soil and can also check winds or water erosion due to the soil-binding effect of their roots. They can be used as food for cattle and vegetables by human beings.
Additionally, some weeds have medicinal value. For instance, Leucas aspera is used against snake bites, while the oil of satyanashi seed is useful against skin diseases, and the nuts of Kavala are used in making scented incense sticks.
Weeds also have economic importance. For example, Saccharum species are used for making thatches and for the reclamation of alkali lands. They can serve as ornamental plants, be used for fencing, and function as mulch to check the evaporation losses of water from the soil.
Weeds are used in green manuring and composting, and some can even fix atmospheric nitrogen (Blue-green algae).
How Weeds Are Spread
Weed dispersal, weeds dissemination, and the spread of weeds are caused by various agents. These agents include:
- Wind: Weed seeds, which may be equipped with a parachute-like arrangement, plumes, or fuzz, can be blown long distances by the wind.
- Water: Weed seeds can be carried by irrigation canals, drainage channels, surface runoff, and flood water from rivers and streams.
- Animals: Weeds with hooks, twisted awns, and spines can attach to wild and domestic animals, which then disperse the seeds. This is common among weeds of the Graminae family.
- Humans: Weeds can be indirectly dispersed by humans through compost that contains partially decomposed weed plants, feeding cattle with hay or fodder that contains weed plants, and using uncleaned farm machinery such as Ghaneri.
- Crop harvesting: Weed seeds can also be inadvertently mixed with crop production, such as wheat, during harvesting.
Principles of Weed Control
Habits of weed plants: An xerophytic weed (Alhagi camelorum) that thrives under dry conditions will die if the fields are flooded with water. Similarly, weeds that thrive under marsh or poorly drained soil conditions can be controlled by improving drainage.
The life cycle of the weed: Annuals and biennials can be effectively controlled if the land is cultivated before the seedling stage of weeds. Perennials require deep plowing to remove their rhizomes and bulbs, which are the vegetative parts by which they propagate.
Susceptibilities: Certain chemicals can be effective in controlling some weeds while others are not susceptible. For example, dicotyledons are susceptible to 2,4-D herbicides, while monocotyledons are not. Therefore, 2,4-D is used to control broad-leaved weeds in monocot crops.
Dormancy period: When controlling dormant weeds, the period must be considered as some weeds have a long dormancy period.
Resistance to adverse conditions without losing viability: Some weed seeds have hard seed coats, which allow them to remain viable for a long time. Therefore, they should be controlled before seed formation.
Methods of reproduction: Weeds propagate either by seeds, vegetative parts, or both. Seeded weeds should be removed or smothered before seed formation. Vegetatively propagated weeds such as rhizomes, bulbs, and stolons should be exposed to sunlight and heat to dry and die, for example, by deep plowing.
Frequent cultivation leads to destroying green leaves and thereby exhausting the food reserves and starving the plants, which may have to be restored. In weeds propagated by both mechanical and chemical methods, both may have to be followed.
Dispersal of seeds: Weeds can be controlled or kept in check if the different ways weed seeds disseminate are known, and countermeasures are undertaken.
Weed Control Methods
Weed control methods can be broadly classified into two groups: Preventive Measures and Curative or Control Measures. Preventive measures aim to prevent weed multiplication and introduction by nipping off buds and using clean seeds, well-decomposed farm yard manure, and compost manure.
Additionally, it involves cutting weeds before seeding, removing weed growth, and keeping irrigation and drainage channels free from seeds.
It is important to avoid feeding animals grain screenings, hay, or fodder containing weed seeds without destroying their viability by grinding or cooking, using sand or soil from weed-infested areas to clean or cultivated areas.
Other preventive measures include cleaning farm implements and machinery after use in infested areas and before use in clean areas, keeping farm fences, roads, and bunds free from weeds, and watching seedlings in nurseries carefully to prevent weed seedlings from being carried to fields.
Curative measures, on the other hand, aim to remove or smother weed growth and multiplication. It includes mechanical methods such as hand pulling, hand weeding, burning, flooding, hoeing, tillage, and moving, as well as smothering with non-living material (mulching).
Burning of seed beds is called 'rubbing'. Cropping and competition methods (cultural) are also used, with practices aimed at helping crop plants establish early and grow faster than weeds.
Crop rotation checks the free growth of weeds due to changes in crops from season to season, and the use of certain crops like groundnut-covering crops like legumes will smother weed growth.
Fertilizer application of optimum doses of fertilizers to crops will also help them grow faster, and the date and rate of planting or sowing of crops at the proper time with optimum seed rate will help the crop to cover the ground and will make the weeds deprived of light.
Biological methods involve using living organisms like plants, animals, or microorganisms for suppressing or controlling weeds. Bio-agents that feed only on weeds and not on crop plants can be used, such as Prickly pear or Nagphana weed in South India which was controlled by Cochineal insects (Dactlopius tomentosus).
In Australia (Hawaii Islands), several kinds of moths were used to control Lantana Camara which eats flowers and fruits. This method is efficient and economical provided the right type of predators, parasites, or pathogens that will not feed upon cultivated crops are found and introduced.
Chemical methods are effective in certain cases and have a great scope provided that the chemicals used for weed control are cheap, efficient, and easily available. Chemicals used for weed control are called herbicides and can either help in killing the weeds or inhibit their growth, such as 2,4-D, Atrazine, and Glyphosate, among others.
Conclusion
In conclusion, weeds pose numerous challenges and drawbacks in the field of agriculture. They compete with crops for vital resources like water, sunlight, and nutrients, leading to reduced crop yields and increased cultivation costs. Weeds can also have a negative impact on the quality of both field produces and livestock products. Moreover, they can cause blockages in irrigation channels, release harmful secretions, and even pose risks to human and animal health. Ultimately, weeds can diminish the land value and contribute to the spread of crop diseases.
However, it is important to note that weeds also have some benefits and can serve certain purposes. They aid in soil conservation by reducing evaporation, preventing erosion, and adding organic matter to the soil. Some weeds possess medicinal properties, while others have economic uses such as for thatching, fencing, and mulching. Additionally, weeds can be utilized in green manuring and composting processes.
Controlling weeds requires understanding their habits, life cycles, susceptibilities, and methods of reproduction. Various methods can be employed, including preventive measures like clean farming practices and crop rotation, curative measures like mechanical and cultural methods, biological approaches using natural predators, and chemical control using herbicides.
