In this article we will discuss about:- 1. Meaning of Weed 2. Criteria of Weed 3. Classification of Weed 4. Crop – Weed Competition 5. Characteristics of Weed 6. Multiplication and Dissemination of Weeds.
Contents:
- Meaning of Weed
- Criteria of Weed
- Classification of Weed
- Crop – Weed Competition
- Characteristics of Weed
- Multiplication and Dissemination of Weeds
1. Meaning of Weed:
The term ‘weed’ was firstly used by Jethrotull. Weed is such undesirable plant grown in association with crop which snatches major parts of nutrients, water, light, place and CO2 available to the crop. Due to tough competition crop suffers adversely and the production is reduced.
Followings are the main points:
(i) Unwanted plant.
(ii) Plant growing where it is not required i.e. a plant out of place.
(iii) Extremely noxious, useless, unwanted or poisonous plant.
(iv) Any plant or vegetation excluding fungi interfering with the objectives or requirements of people.
Thus weed may be defined as unwanted and undesirable plant which interferes with the utilisation of land and water resource and adversely affects human welfare.
2. Criteria of Weed:
Whether a plant is weed or not depends upon:
(i) Characteristics and habit of plant
(ii) Relative position
(iii) Time of occurrence of plant.
Therefore all plants may become weeds in a particular situation.
3. Classification of Weed:
A. On the Basis of Site of Predominance:
(a) Obligate Weeds:
Such weeds are grown only in association with man and his agriculture. It is never found in wild form e.g. Chenopodium, Anagallis etc.
b) Facultative Weeds:
Such weeds are grown both as wild and in cultivated habitats e.g. Argemone mexicana, Euphorbia hirta.
B. On the Basis of Dependence on Host:
(a) Total Root parasite/Holo root parasite e.g. – Orobanche
(b) Total stem parasite/Holo stem parasite e.g. – Cuscuta
(c) Semi-root parasite e.g. – striga
(d) Semi-stem parasite e.g.- Loranthus (birdvine)
C. On the Basis of Botanical or Morphological Characters:
(a) Grasses:
Cylindrical and hollow stern having nodes and internodes, Leaf emerges from node, fibrous root, graminae family e.g. Echinochloa, Phalaris etc. Most of the weeds belong to this group.
(b) Sedges:
Triangular stem, no node, very large internode and leaf at top, Cyperaceae family, no branch, usually three leaves at top, pith present e.g. Cyperus (Motha).
(c) Broad Leaf:
Broad leaf alternately arranged on stem, cylindrical stem, usually tap root e.g. all dicot weeds like- Camellina, Chenopodium album (Bathua).
D. On the Basis of Season:
(a) Kharif Weeds:
Example:
(i) Amaranthus viridis (Junglee chaulai)
(ii) Boerhauia diffusa (Vishkhopra)
(iii) Cassia occidentalis (Kasaundhi)
(iv) Cyperus rotundus (Motha)
(v) Euphorbia thymiloia (Chhoti duddhi)
(vi) Euphorbia dracunculoides (Titali)
(vii) Digera arvensis (Lahsua)
(viii) Datura festoosa.
(b) Rabi Weeds:
Example:
(i) Anagallis arvenis (Krishna nil)
(ii) Argemone Mexicana (Satyanashi)
(iii) Convolvulus arvenis (Hiran Khuri)
(iv) Chenopodium album (Bathua)
(v) Desmodium trifolium (Tinpatia)
(vi) Cuscuta reflexa
(vii) Asphodelus tenuifolius (Vanpyaji)
E. Other Basis:
(a) Relative weed e.g. rice in wheat field.
(b) Absolute weed e.g. – Such weeds are due to its characteristics e.g. Cyperus rotundus.
(c) Rogue – The off type crop variety in the field of certain crop variety is called rogue.
(d) Mimicry weeds – Such weeds are similar in external morphology with the crop e.g. Phalaris in wheat field and wild rice in the rice field.
(e) Volunteer weeds- Such weeds are grown from the fallen seeds of previous or preceding crop in the field.
(f) Noxious weeds – Such weeds are declared undesirable, extremely noxious and poisonous by the law e.g. Parthenium.
4. Crop – Weed Competition:
Weeds are the naturally grown plant which are able to extract nutrients & water from the soil and are component enough to harvest light energy. In the growing plants, leaf area index (LAI) and root density increase which lead to mutual interference in the utilisation of growth factors. The population of weeds crossing over the threshold level increases the competition for growth factors resulting in reduction of crop production.
The weed and crop compete for such common growth factors whose availability is less. In the normal cases the weed competes with the crop mainly for nutrients, water, light, place and CO2 but in some instances weeds show the allelopathic effect on crops.
(i) For Nutrients:
Naturally grown weeds absorb more nutrients from the soil than the crop. Nutrient analysis of weed and crop shows that the concentration of nutrients is far more in weeds than the associated crop.
In the beginning of the crop growth, weed absorbs excess nutrients because majority of the annual weeds complete its life cycle within 50-60 days where as the crops take 100-130 days. It is experimentally proved that in the highly infested rice field with Echinochloa, there is the uptake of 60- 80% of the available nitrogen from the soil by weeds itself.
In some cases weed takes up nutrients directly from the host plant. Some weeds activate denitrification, for example, Nutsedge releases a type of organic scented chemicals in the soil which increases the population of dentrifying bacteria and thus leaves very less amount of nitrogen for the crop.
(ii) For Water:
There is a tough competition between crop and weed for water in unirrigated and dryland areas. In such area, scarcity of soil water is the most common feature. The water uptake is comparatively more by weeds. The amount of water required to produce unit amount of dry matter is called transpiration ratio.
The transpiration ratio of weed is comparatively high. It means in comparison to crop, to produce one tonne biomass of weed requires more water than that of crop. Under dryland areas, soil water is absorbed quickly by weeds depriving the crop of water and the crop suffers adversely.
The weed is capable to extract soil moisture even available at above 0.3 atmospheric metric tension. Therefore weed thrives well at permanent wilting point of the crop (PWP for most of the crop is 15 atm).The transpiration ratio of Cynodon dactylon is around 813 compared to 450 for Bajra and 430 for sorghum.
Amaranthus depletes water from deep soil layers and maintains its higher leaf water potential and turgor potential than cotton throughout the growth period. Water hyacinth transpires 140-170% more water in comparison to normal evaporation from the open water surf ace. Therefore weed control is utmost important in dryland or barani agriculture.
(iii) For Light:
The vast growth and rapid biomass production of weeds cause shading and smothering effect on the associated crop. Majority of photosynthetically active radiations (PAR) is harvested by weeds which results in reducing photosynthesis in the crop.
Thus it reduces the life of the smothered and shaded crop. In the groundnut, weeds like Celosia argentia and Amaranthus viridis cause shading effect and interfere in harvesting of light. It is reported that Amaranthus hybridis reduced the photon flux density by 90% reaching the cotton canopy at mid-day.
(iv) For CO2:
When weed infestation is more, competition for CO2 may set in. Most of the weeds are C4 plants therefore weeds deplete CO2 to a very low concentration in comparison to crops.
(v) Allelopathic Effect:
Some of the weeds releases some chemicals which have allelopathic effect on crops. Such weeds are Agropyron repens, Sorghum halepense, Lantana camara, Abutilon theophrasti, Cyperus rotundus, Euphorbia maculata, Ambrosia psitostachya etc. The extracts from the rhizomes of Ambrosia reduce germination and seedling growth of several crops like wheat, oats, tomato and lucerne.
Exudates of roots of wild oat at 2 and 4 leaf stage are toxic to wheat crop. The extract from Abutilon theophrasti decreased water status of soyabean and this induced water shortage in soyabean, leads to stomatal closure and breaking down of chlorophyll.
The critical period for crop=weed competition is around 30 days for most of the crops. The average annual loss due to weeds is 30-40% in different crops. The rapid and vast growth of the crop reduces the length of this critical period. Such crops are sorghum, maize, sunflower and cowpea etc. The length of the critical period for dwarf varieties is shorter and for taller traditional varieties is longer.
5. Characteristics of Weed:
1. They thrive well even under adverse conditions.
2. They spread vegetatively.
3. Able to regenerate lost parts.
4. Compete with cultivated plants.
5. Produce enormous number of seeds.
6. Its seed remain viable for many years.
7. All seeds of weed mature simultaneously.
8. Some weed seeds have hairy appendages.
9. Weeds have allelopathic effects on crops.
10. It produces seeds in flushes.
11. Seed production through apomixis, spore etc.
12. Weed seeds have dormancy either inherited or induced.
6. Multiplication and Dissemination of Weeds:
Weeds are prolific with high fecundity.
It multiply profusely by:
(i) Bearing seeds or
(ii) Propagules or by
(iii) Both
Weeds are disseminated by three ways:
(i) Weed seeds either fall near the mother plant or
(ii) Travel a long distance and short distance or
(iii) Travel with agricultural crops.
Most of weeds travel by various agencies:
(a) Structural modification like siccative fruits, winged, parachute fruits, plumed fruits, hook or spiny appendages.
(b) Introduction from outside like Johnson grass (Sorghum halepense).
(c) Impurities in crop seeds and in feed stuff.
(d) Threshing and cultivating machines spread rhizomes.
(e) Manuring with partially decomposed FYM & compost.
(f) Wind e.g. Canada thistle seeds disseminate by wind.
(g) Water/irrigation.
(h) Animals e.g. – xanthium.
(i) Human activities e.g. Phalaris minor.