A weed is a plant growing where it is not desired. They not only compete with crop plants for plant nutrients, moisture, space and sunlight but also interfere with agricultural operations increasing the cost of labour and tillage. And ultimately affect the yields and quality of farm produce adversely.
Weeds are often excellent at surviving and reproducing in disturbed environments and are often the first species to colonise and dominate in these conditions. Weeds are one of the major threats to the natural environment. They are destroying native habitats, threatening native plants and animals and choking our natural systems including rivers and forests.
Weeds may out-compete native plants because:
- they may not be affected by the pests or diseases that would normally control them in their natural habitats
- the disturbed environment provides different conditions that better suit the invading weed.
As a result the weed may:
- grow faster than native plants and successfully compete for available nutrients, water, space and sunlight
- reduce natural diversity by smothering native plants or preventing them from growing back after clearing, fire or other disturbance
- replace the native plants that animals use for shelter, food and nesting.
The damages caused by weeds include:
- Increase in the cost of cultivation
Tillage operations are done to control weeds and it is generally estimated that on an average about 30 percent of the total expenditure for crop production is on tillage operations and more labour is employed for weeding. This results in increasing cost of cultivation and reducing the margin of net profit.
- Reduction in crop yield
Weeds compete with crops for water, nutrients and light. Being hardy and vigorous in growth habit, they grow faster than crops and consume large amount of water and nutrients, thus causing heavy losses in yields.
- Weeds reduce the value of the land
Agricultural lands which are heavily infested with perennial weeds always fetch less price, because such lands can not be brought under cultivation without incurring heavy expenditure on labour and machinery
- Competition for mineral nutrients
Being hardy and vigorous in growth habit; they soon outgrow the crops and consume large amount of water and nutrients. Thus causing heavy losses in yields.
- Competition for space
Weeds compete for space both in the rhizosphere and atmosphere. In the presence of weeds, crop plants also have limited space to develop their shoots, which amounts to reduced photosynthesis in them.
- Weeds as reservoirs of pests and diseases
Weeds form a part of community of organisms in a given area. Consequently, they are food sources for some animals, and are themselves susceptible to many pests and diseases. However, because of their close association with crop they may serve as important reservoirs or alternate host of pests and diseases.
- Problems due to aquatic weeds
The aquatic weeds that grow along the irrigation canals, channels and streams restricts the flow of water. Weed obstruction cause reduction in velocity of flow and increases stagnation of water and may lead to high siltation and reduced carrying capacity. Aquatic weeds form breeding grounds for obnoxious insects like mosquitoes. They reduce recreational value by interfering with fishing, swimming, boating, hunting and navigation on streams and canals.
- Increases in production costs
Production costs are increased since the farmer has to manage additional activities in order to control the weeds in the field. The additional activities can be:
- Spending money on labour to remove weeds during the growing season
- Pay for chemicals to control weeds
- Install additional equipment to remove seeds and leaves from the harvested crop.
- Apply other technologies such as mulches in order to control weeds.
The consumer pays for the higher cost of production at the end.