Beneficial Effects of Weeds and Factors Effecting Crop Competitive Ability against Weed
Beneficial effects of weeds: Although weeds appear to have many undesirable features, there are some that are useful. Weeds provide a vegetative cover that protects the soil surface against soil erosion by rain or wind.
Weeds play an important role in nutrient recycling. Roots of weeds absorbed nutrients from the lower soil depth and return these to the top soil surface as litter when the weeds shed their leaves or when the entire plant dies and decays.
Weeds add organic matter to the soil. Many weeds are used directly as food for humans e.g. Talinum triangulare, and Pannisetum purpureum, are commonly used as vegetable in some parts of the world.
Weeds play an important role as a source of drugs used in public health. Weeds provide food and cover for animals. Weeds serve as important source of genetic materials for crop improvement.
Weeds serve as host to beneficial insects and at the same time a source of nectar for bees. Many weed species help to beautify the landscape.
Factors Effecting Crop Competitive Ability against Weed
1. Stage of crop development
Usually, the competitive ability of crop is low during the seedling and early vegetative stages of development and are particular sensitive to weed competition.
Once the stages have been identified, the farmer ensures maximum weed control at those times, while permitting less stringent weed control during the other less critical stages.
In yam, for example, the crop is particularly sensitive to weed competition during the first two or three months after emergence and failure to control weeds during that period result in lower yields than failure to control weeds during the latter part of the season (Kasasian and Seeyave, (1969).
2. The density of the crop plant
This can also influence its competitive ability against weeds. A high crop density, by providing a dense canopy, may control weeds by depriving them of light.
For example, closely spaced cassava experience little weeds problem after the first three months, whereas widely spaced cassava plantings suffer from weeds problems throughout their field life.
3. Persistence of weeds
Weeds usually are able to survive in competition with crop plants because of the following:
A wide range of adaptability to adverse environmental condition;
Effective means of propagation, such as possessing of underground parts;
Many weeds produce large amount of seeds per plant;
Many weed seeds remain viable in the soil for many years;
Many weed seeds exhibit dormancy.
4. Dissemination of Weeds
The main agents of seed dispersal for weeds are wind, animals and water.
Dispersal by wind: The possessions of feathery structures as in milkweed, or of wings, tend to facilitate dispersal of weed seeds by wind.
5. Dispersal by water
Adaptations for water dispersal include a low density of the seed as well as the possession of membranous structure or cork on the testa.
These adaptations enable the seeds to float on water and be carried for long distances.
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6. Dispersal by animal
To aid dispersal by animal, many weed seeds have hooks or barbs with which they can stick to the skin of animal and the clothing of humans.
For example seeds of Bermuda grass are spread in this way;
Weed seeds eaten by animal may pass through the digestive tract uninjured. Cactus is spread principally by jack rabbit that eat the fruit that contain indigestible seeds.
Man-made agencies of weed seeds dispersal;
Weeds are widely spread in impure seeds. Some of the most serious weed pests have been introduced from foreign countries in crop seeds.
Spreading of fresh farm yard manure on cultivated fields may disseminate weed seed.
Farm machinery may spread weed seeds. Plows, harrows and cultivators may drag roots or seed-bearing portion of perennial plants to other parts of a field.
Irrigation water may carry along with it seeds of weeds that grow on the bank of reservoir, canals and ditches to cultivated field.
In summary, weeds are plants growing out of place. Weeds bring about a lot of harmful and beneficial effect to the farmer. They could be classified as annuals, biennials, perennial, common or noxious weeds.
They possess some special features that aid their adaptability, competitiveness and dispersal. Clear understandings of weeds characteristic, classification, harmful and beneficial effects as well as some special adaptive features of weeds and dispersal methods of weed seeds are necessary if we are to effectively tackle them and minimize their harmful effects.
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