WHAT ARE WEEDS?
There are numerous definitions of a weed, including:
- a plant out of place and not intentionally sown
- a plant growing where it is not wanted
- a plant whose virtues have not yet been discovered.
- plants that are competitive, persistent, pernicious, and interfere negatively with human activity.
- There are approximately 250,000 species of plants worldwide; of those, about 3% or 8000 species behave as weeds. In an agricultural context, plants are weeds if they compete with crops for resources or reduce the ability of livestock to forage. Their presence means lower crop yields.
BENEFITS OF WEEDS
- Weeds when ploughing under, add nutrients, organic matter.
- Weeds check winds or water erosion by soil binding effect of their roots (underhand).
- Useful as fodder for castles (Hariyali) & vegetable by human beings (Ghol, Tandulja).
- Have medicinal value, Leucas aspera is used against snake bite, oil of satyanashi seed is useful against skin diseases, nuts of lavala are used in making scents (Incense sticks).
- weeds can also be valuable indicators of growing conditions in a field, for example of water levels, compaction and pH.
- Have economic importance e.g.: saccharum spp used for makingthatches.
- Reclamation of alkali lands (Satyanashi).
- Serve as ornamental plants (Ghaneri).
- Used for fencing (Cactus, Nagphana).
- Used as mulch to check the evaporation losses of water from the soil.
- Used as green manuring & composting.
- Fix atmospheric ‘N’ (Blue-green algae, Tarota, Unhali, etc.)
LOSSES CAUSED BY WEEDS
1. Reduction in crop yield: Weeds compete for water, nutrients & light. Being hardy & vigorous in growth habit, they soon outgrow the crops & consume large amounts of water & nutrients, thus causing heavy losses in yield. E.g.: 40% reduction in yield of groundnut & 66% reduction in yield of chilli. The loss of N through weeds is about 150 kg/ha.
2. Increase in the cost of cultivation: One of the objects of tillage is to control weed on which 30% expenditure is incurred and this may increase more in heavy infested areas & also cost on weed control by weeding or chemical control. Hence, reduce the margin of net profit.
3. The quality of field produce is reduced: Weed seeds get harvested & thrashed along the crop produce which lowers the quality. Such produce fetches fewer prices in the market. E.g.: Leafy vegetables, grain crop.
4.Reduction in quality of livestock produce: Weeds impart an undesirable flavour to the milk (Ghaneri), impair quality of wool of sheep (Gokhuru, Aghada), and cause the death of animals due to poisonous nature of seed (Dhatura).
5. Harbour insect-pests & disease pathogens: Weeds either give shelter to various insect pests & disease pathogens or serve as alternate hosts & thus helps in perpetuating the menace from pests & diseases. E.g.: Gall fly of paddy, midge fly of Jowar, leaf minor of soybean & Groundnut, rust of Wheat, tikka of Groundnut, Black rust of wheat.
6. Check the flow of water in irrigation channels: Weeds block drainage & check the flow of water in irrigation canals & field channels thereby increasing the seepage losses as well as losses through over through over flowing, so reduce the irrigation efficiency.
7. Secretions are harmful: the Heavy growth of certain weeds like quack grass (Agropyon repens) or lavala lowers the germination & reduce the growth of many crop plants due to the presence of certain phytotoxins secreted by weeds.
8. Harmful to human beings and animals: Weeds cause irritation of skin allergy & poisoning to human beings, also the death of castles.
9. Cause quicker wear & tear of farm implements: Being hardy & deep rooted; the tillage implements get worn out early & cannot work efficiently unless they are properly sharpened or mended.
10. Reduce the value of the lands: Heavily infested lands with perennial weeds fetch less price as require heavy expenditure to brought under cultivation.
CAUSES OF WEED
The main cause of weed problem in the cropped areas is due to the presence of following characteristics in them:
- Reproduce in many ways; sexual (seed) and asexual (parts of the plant re-sprout)
- Produce many seeds
- Produce small seeds
- The seeds have fancy ways of getting carried around(float on air or water, stick to animals, many others)
- If you try to pull them they break off and re-sprout
- Roots or stems from rhizomes or runners
- They are hardy “generalists” and can live almost anywhere
- They grow fast (compared to crop plants)
- Their seeds may stay dormant in the soil for long periods
Weeds are also successful in giving competition to crop plants due to:
- no special environmental requirements for germination
- self-controlled, discontinuous germination and great longevity of seed
- only a short time spent in vegetative period before beginning to flower
- continuous seed production maintained for as long as growing conditions permit
- self-compatible, but not obligatorily self-pollinated or apomictic
- cross pollination may be achieved by a nonspecialized flower visitor or by wind
- very high seed output in favorable environmental circumstances
- production of some seed in a wide range or environmental circumstances; high tolerance or and often plasticity in face of climatic and edaphic variation
- special adaptations for short- and long-distance dispersal
- specialized features for competition, ex. rosette formation, etc.