Economic Importance of Weeds
Weeds have certain effects in agriculture, which are mostly in the form of different harmful results but somehow there are also some beneficial effects.
Direct Losses by weeds
- Weeds cause reduction in crop yield through competition for light, nutrient, water and space. They can also reduce the yield of crop through the release of toxic substances or exudates which inhibit crop growth. This is called allelopathy. Uncontrolled weed infestation can lead to 95% yield loss in cassava, 40% in maize and 53% in cowpea, soybean and pigeon pea.
- Weeds can reduce the quality of harvested agricultural products.
- Weeds interfere with harvest operations and increase the cost of harvesting in both small holder and large scale farms.
- Weeds may poison animals e.g. Amaranthus spp can adversely affect livestock because of the high nitrate content of the shoots.
- The cost of controlling weeds is high.
- The presence of weeds can impede water flow in irrigation canals.
- The presence of weeds in lakes and reservoir can increase loss of water by transpiration.
Indirect Losses caused by weeds
- Weeds serve as alternate hosts to many plant diseases and animal pests e.g insects, rodents, birds etc that attack crops.
- The presence of weeds imposes a limit on farm size.
- The presence of weeds can also reduce the economic value of lakes by preventing or limiting fishing activities.
- Weeds such as Imperata cylindrica become fire hazards in the dry season throughout the savanna vegetation zone.
Non Agricultural Losses
- Weeds affect health of humans, stinging nettle can cause skin rashes and the flowers of some other weeds can be associated with allergies in humans
- Weeds impair visibility along roads and railway lines.
- Uncontrolled weed growth reduces the value of real estates.
- In situations where farmers depend on human labour for weeding, children have to miss school at peak of weeding periods. This reduces the quality of education that these children can get during their early years.
Beneficial Effects of Weeds
- Weeds provide a vegetative cover that protects the soil surface against erosive action of rain and wind.
- Weeds play an important part in nutrient recycling. Roots of weeds tap nutrients from the lower soil depths and return these to the soil surface as litter when the weeds shed their leaves or when the entire plant plants dies and decays.
- Weeds add organic matter to the soil both from the roots and from the above ground parts.
- Many plants that are designated weeds are used as potherbs e.g Talinum triangulare.
- Weeds are sources of pesticides e.g Chrysanthemum cinerariifolium which provides insecticide pyrethrum.
- Weeds provide food and cover for animal. Wildlife generally depends on weeds for survival as food and shelter.
- Weeds serve as an important source of genetic materials for crop improvement such as breeding for resistance to pests and diseases which are made possible by genetic materials provided by wild species of the crop plants.
- Weeds serve as hosts beneficial insects, and at the same time provide nectar for bees.
- Many weeds help to beautify the landscape. e.g a good ground cover of Cynodon dactylon beautifies the home.