Weeds are defined in many ways, but most definitions emphasize behavior that affects humans. The most basic concept of weed science is embodied in the word ‘weed’ itself. Each weed scientist has a clear understanding of the term, but there is no universal definition that is accepted by all scientists. Weeds are the plants, which grow where they are not wanted (Jethro Tull, 1731).
Characteristics Features of Weeds:
To develop appropriate weed control, the characteristic features of weeds have to be known
1. Prolific seed production
One of the most pernicious habits of weeds is their vigorous reproduction. In general, annual weeds are characterized by the production of very large number of seeds. Seed production varies from 40 to 1,96,000 no. of seeds / plant
2. Vegetative reproduction
The vegetative reproduction is another way by which the weeds were able to spread. Some of the perennial weeds multiplied by various means like tubers and nuts (nut grass), rhizomes and root stock (bermuda grass & Johnson grass), bulbs (wild onion and garlic), roots (Canada thistle), stems (dodder) and offsets (aquatic weeds)
3. Seed dormancy
It is an efficient survival mechanism of weeds. It is defined as a state in which a viable seed fails to germinate even under favorable conditions for plant growth. Seeds of most of the weeds exhibit one or another type of dormancy like inherent or innate or natural dormancy or induced or secondary dormancy or enforced dormancy by wild mustard, black mustard and wild oats respectively. Avena fatua exhibit all the three kinds of dormancy
4. Longevity and viability of seeds
Lotus seed collected from the bottom of the Manchurian lake bed have viability even after 1000 years. Where as wild mustard and Chenopodium remain viable for 30 and 20 years, respectively.
5. Weed seed dispersal and germination
Weed seed dispersal has no geographical boundaries. Common agents of weed dispersal are wind, water, animals, birds, organic manures, agriculture implements and human beings
6, Wider adaptability
They have the ability to thrive under adverse conditions due to morphological and physiological conditions Eg Trianthema (carpet weed), Sorghum halepens and Saccharum spontaneum
Weed menace in agriculture
Reduction of crop yield has a direct correlation with weed competition. Weeds compete for water, light, nutrients and space. Weeds compete for water in dry land and for nutrients in irrigated crops. It includes reduction in crop yields and production efficiency and erosion of crop quality
Reduction in crop yields and production efficiency is a direct effect due to weeds. It varies from 34.3% to 89.8% depending upon the crop. In rice (30-35%), wheat (15- 30%), Maize, sorghum, pulses, oilseeds (18-85%), sugarcane (38.8%), cotton (47.5%), sugar beet (48.4%) and onion (90.7%). Beside the direct reduction in crop yields there are many indirect ways by which the weeds may be troublesome in agriculture. For example in weedy fields management practices become cumbersome. Harvest may be difficult when the field is invade with wild safflower (Carthamus oxycantha), Canada thistle (Cirsium arvense), cocklebur (Xanthium strumarium). Cowage (Mucuna pruriens) cause itching to labor. Harvesting becomes troublesome when the field bindweed (Convolvulus arvensis) and morning glory (Ipomoea sp) bind the crop plants together.
Erosion of crop quality
Weeds may reduce the quality of farm produce in many ways. In dry land agriculture weeds cause severe moisture stress and force the food grains to shrivel. The vegetables and fruits are disclored and de shaped in the presence of weeds. Contamination of food grains with poisonous weed seeds fetches low price. Foundation or certified seed is rejected if weed seeds exceed 2% and also the market value is reduced. The quality of the sugarcane crop is also reduced due to the presence of the parasitic weed striga litura. Where as leaves of loranthus (Dendrophthoe falcate) impair the quality of tea. Leaves of Mikania micrantha create problem in tea plantation. In cotton the dry weed fragments adhere to the lint and hinder its spinning process. In India Cocklebur (Xanthium srtumarium) reduce the quality of wool in sheep.
Weed menace in animal husbandry
Milk gives odd smell when animal fodder crops are mixed with wild onion and wild garlic, Cichorium intybus, Argemone mexicana. Certain weeds cause illness and death of animals due to high levels of alkaloids, tannins, oxalates, glucosites or nitrates. Death of herds of sheep occurs due to its high oxalate content of. Halogeton (H. glomeratus) .It is dominant weed of arid and semi arid region. Leaves of lantana cause acute photosensitivity and jaundice in animals was due to the toxic principle of “Lantradene”. Puncture wine (Tribulus terrestris) a weed of dryland induces extra sensitivity to light in sheep and puncture of the animal skin. In Kashmir rhododendron cause diarrhea and showed blood strains in milk. Crotalaria spp.
Weeds as alternate hosts to crop pests and diseases
In rice crop Echinochloa and Panicum act as alternate host for stem borer, whereas in maize Chenapodium album acts a host for stalk borer and beetles & cut worms in case of tomatoes. Crotalaria spp. act as a main host for hairy caterpillar which effect castor crop.
Reduction in land value
Land unsuitable for economic crop production when land infested with Cyperu rotundus, Cynadon dactylon. Thousands of hectares of cultivable area in rice growing regions of India have been abandoned or not being regularly cultivated due to severe infestation of nutgrass (Cyperus rotundus) and other perennial grasses.
Weed menace to human health
Hay fever and asthma aggravated by pollens of Ambrosia artemissifolia. Parthenium hysterophous, Poison ivy (Rhus sp), common rag weed (Ambrosia artemissifolia) are responsible for respiratory problems and skin allergies (dermatitis). ) act as alternate host and vectors of malaria, yellow fever, encephalitis, dengue fever and filariasis. Wheat flour contaminated with seeds of cocklebur gives bitter taste to the bread and irritates the gastric tract of the consumer. When the Mexican poppy seeds (Argemene mexicana) crushed with mustard seeds cause death and blindness of people. Milk from animals feed on the Mexican poppy weed can cause „Glaucoma‟ in humans.
Weed menace to aquatic ecosystems
Aquatic weeds make the appearance of water bodies repulsive and decline their recreational value. Weeds hinder the navigation, fishing and slow down the flow in irrigation channels Aquatic weeds upon decomposition emit offensive odors and pollute the drinking water bodies.
Weed menace to forest and pasture lands
In Karnataka and Andhra Pradesh large forest area come under the grip of Parthenium hysterophorus. In West Bengal 11% of high forest area and 38% of plantation forest have been affected by Mikania micrantha (mile-a-minute weed)
CLASSIFICATION OF WEEDS
Classification based on origin Indigenous weeds:
All the native weeds of the country are coming under this group and most of the weeds are indigenous. Eg. Acalypha indica, Abutilon indicum, Sorghum halepense, Cynodon dactylon and Echinochloa colonum Introduced or Exotic weeds or Alein: These are the weeds introduced from other countries. These weeds are normally troublesome and control becomes difficult. Eg. Parthenium hysterophorus, Acanthospermum hispidum, Eichhornia crassipes, Argemone mexicana, Lantana camara and Croton bonplandianus When man aids in its introduction such Weeds are called as anthrophytes
Classification based on association When two plants are living together ie called association. Based on association they are season bound weeds, crop bound weeds and crop associated weeds. Crop bound weeds: Weeds which usually parasite the host crop partially or fully for their nourishment ie parasitism also called as parasitic weeds. Those parasites which attack roots are termed as root parasites and those which attack shoot of other plants are called as stem parasites
Mimicry If weeds look exactly like crops morphologically & complete their life cycle, Echinochloa colonum (Jungle rice) mimic the rice crop. Avena fatuva (wild oat) and Phalaris minor (canary grass) both mimic the wheat and Loranthus in tea gardens. For example Avena fatua (wild oats) tends to grow to the height of winter grains and adjusts its ripening period to the crop over a wide varietal range and this type of mimicry is called phenotypic mimicry
Biennial weeds: Complete their life cycle within two years / two seasons, 1st year vegetative growth – Rosette stage. 2nd year produced inflorescence called bolting. They may propagate either by seeds or vegetative parts or by both. Biennials generally do not come up in annual crop fields but they infest perennial crop fields
Perennial weeds: Grow more than two years. Reproduce vegetatively from underground and specialized organs. First time they come to flowering in 2nd year and there after flowering every year. Difficult perennial weeds also known as pernicious weeds. Eg. Cynodon dactylon (Bermudagrass) and Agropyron repens (quackgrass) are shallow rooted perennials where as Cyperus rotandus. (Purple nut sedge) Sorghum halepense (Johnsongrass) are deep rooted Perennials Simple perennials: These reproduce solely by seeds but when roots or crown are cut, the cut pieces may produce new plant Eg. Ipomea carnea and Lantana camera
Parasitic weeds: The parasite weeds are either total or partial which means, the weeds that depend completely on the host plant are termed as total parasites while the weeds that partially depend on the host plant for minerals and capable of preparing its food from the green leaves are called as partial parasites. Those parasites which attack roots are termed as root parasites and those which attack shoot of other plants are called as stem parasites
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