Revision summary
Allelopathy is the chemical effect of one plant on another through leachates, exudates or decaying residues. In irrigated rice-wheat it can suppress weeds such as barnyard grass and also cause residue autotoxicity if straw is mismanaged. Sorghum, sunflower, mustard and some mulches are used for weed suppression in irrigated field crops. Sugarcane, cotton and canal-side agroforestry also show allelopathic effects on ratoons and adjoining wheat. It supports, but does not replace, integrated weed and residue management.
Model answer
Copper italics in this answer — like this — are the key facts. Each one is unpacked in the Facts & figures rail.
Introduction
Allelopathy is the effect of one plant on another through chemicals released from leaves, roots, litter or decomposing residues. The effect may suppress weeds or neighbour crops, or, less often, stimulate them. In irrigated agriculture, where rice, wheat, sugarcane and cotton repeat on the same land, allelopathy is a tool for weed management and residue use, and a risk when residues or intercrops poison the next stand.
Body
What allelopathy is
- Plants release allelochemicals (phenolics, terpenoids, alkaloids, glucosinolates) by leaching, root exudation, volatilisation or decay.
- Unlike simple shading or nutrient theft, the pathway is chemical. Laboratory bioassays must be checked in the field, because irrigation water can dilute or move the compounds.
- Autotoxicity is a special case: a crop's own residues harm the next sowing of the same species, seen in some rice, lucerne and sugarcane systems.
Role in major irrigated systems
- Rice-wheat on the Indo-Gangetic plains: rice straw and wheat stubble contain phenolics. Poorly managed residues can delay the next crop's roots; well-timed incorporation or microbial breakdown can reduce that harm. Some rice cultivars show weed-suppressive allelopathy against Echinochloa (barnyard grass), which matters as herbicide resistance rises.
- Weed control: sunflower, sorghum, mustard, eucalyptus leaf mulch and some rice lines release compounds that suppress Phalaris, Avena and broad-leaf weeds. This can cut herbicide load in irrigated wheat and maize if density and timing are right.
- Intercropping and trap systems: mustard, sorghum or cowpea allelopathy can protect a main irrigated crop from weeds; the companion must be chosen so it does not also stunt the main crop.
- Sugarcane and cotton: residue and ratoon allelopathy, plus intercropped pulses or sunnhemp, can affect sprouting and early weed flush. Irrigation keeps soils moist, which can either speed breakdown of allelochemicals or spread them in the root zone.
- Green manures and oilseed cakes: mustard cake and some green-manure residues combine nutrient supply with suppression of soil-borne pests and weeds — a chemical-biological mix, not magic.
- Agroforestry on canal commands: poplar and eucalyptus belts can allelopathically affect adjoining wheat if leaf litter and roots are unmanaged; spacing and leaf removal are the farm response.
Practical caution
- Allelopathy is not a full substitute for water, nutrients and integrated weed management. It is one layer with crop rotation, stale seedbeds and judicious herbicides.
- Breeding weed-suppressive cultivars and using residue mulch under conservation agriculture are the scalable research paths for irrigated India.
Flow diagram
flowchart TD P[Donor plant] --> A[Allelochemicals] A --> W[Weed suppression] A --> C[Crop injury autotoxicity] R[Rice wheat sugarcane cotton] --> A W --> I[Lower herbicide load] C --> M[Residue timing rotation]
Conclusion
Allelopathy is chemical interference among plants. In irrigated rice-wheat, sugarcane, cotton and intercrops it can suppress weeds and also injure the next crop if residues are mishandled. Used with rotation, mulch and cultivar choice, it is a low-cost support to water-and-chemical-intensive irrigated systems.
Quick related
Students also ask
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Given the vulnerability of Indian agriculture to vagaries of nature, discuss the need for crop insurance and bring out the salient features of the Pradhan Mantri Fasal Bima Yojana (PMFBY).
Next question on this syllabus topic (2016 · Q10). View answer →
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Is allelopathy the same as competition for water and light?
No. Competition is resource capture. Allelopathy is chemical inhibition or stimulation, though both can occur together in the field.
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Can allelopathy replace herbicides in irrigated wheat?
Not fully. It can reduce weed pressure and herbicide cycles if cultivars, mulches and rotations are chosen with care.
Same topic · past papers
UPSC has asked this before
These previous-year questions sit on the same topic. Open one to practise the earlier ask.
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2017 · Q14 · GS III · 15 marks
What are the major reasons for declining rice and wheat yield in the cropping system? How crop diversification is helpful to stabilize the yield of the crop in the system? -
2017 · Q15 · GS III · 15 marks
How do subsidies affect the cropping pattern, crop diversity and economy of farmers? What is the significance of crop insurance, minimum support price and food processing for small and marginal farmers? -
2023 · Q13 · GS III · 15 marks
Explain the changes in cropping pattern in India in the context of changes in consumption pattern and marketing conditions. -
2018 · Q14 · GS III · 15 marks
How has the emphasis on certain crops brought about changes in cropping patterns in recent past? Elaborate the emphasis on millets production and consumption. (250 Words, 15 Marks).
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