Crop-Weed Association, Crop-Weed Competition and Allelopathy


Weeds possess many growth characteristics and adaptations that enable them to exploit successfully the numerous ecological niches left unoccupied by crop cultures.

Weeds compete with themselves and with crop plants.

Among the more important adaptations relevant to competitive advantage are properly synchronized germination, rapid establishment and growth of seedlings, tolerance to shading effects by the crop or by other weeds at the time of establishment, quick response to available soil moisture and nutrients, adaptation to the most severe climatic situations of the habitat, adaptations to the edaphic regime, relative immunity to post seeding soil disturbance, practices and resistance to herbicides that are used.

In the initial stages of an invasion by weeds of exposed ecological niches, only very limited competition for resources by the crop and weed may occur, but as the establishment of the crop-weed association is completed, competition for the available resources is more obvious.

Plant competition is a natural force whereby crop and weed plants tend to attain maximum combined growth and yield, with the development of each species being to some extent at the expense of the other.

It occurs when the demands of the plants for moisture, nutrients, light, and possibly carbon dioxide exceed the available supply.

Competition may develop between crop and weed plants and also between individual plants.

The outcome of competition usually results in the development of a characteristic crop-weed association.

Crop plants and weeds may grow and mature in the state of mutual suppression that is often found in crops where no suitable herbicide is available to control the weeds.

The weed suppresses the crop and results in a reduction in yield.

The crop also suppresses the weeds, a condition often found in row crop cultures.

This is a logical sequence in a crop habitat where both cultural and herbicide methods provide effective control.

A principle of plant competition is that the first plants to occupy an area have an advantage over latecomers.

This principle is of foremost consideration in practical weed control, where cropping practices are always directed to the establishment of the crop ahead of the weeds.

Competition and allelopathy are the main interactions, which are of importance between crops and weeds.

Allelopathy is distinguished from competition because it depends on a chemical compound being added to the environment while competition involves the removal or reduction of an essential factor or factors from the environment, which would have been otherwise utilized.

Crop weed competition

Weeds appear much more adapted to agroecosystems than our crop plants. Without interference from man, weeds would easily wipe out the crop plants. This is because they compete for nutrients, moisture, light, and space which are the principal factors in the production of crops. Generally, an increase in one kilogram of weed growth will decrease one kilogram of crop growth.

Competition for Nutrients

Weeds usually absorb mineral nutrients faster than many crop plants and accumulate them in their tissues in relatively larger amounts.

  • Amaranthus sp. accumulates over 3% N on a dry weight basis and is termed a “neutrophil”.
  • Achyranths aspera, a ‘P’ accumulator with over 1.5% P2O5
  • Chenopodium sp & Portulaca sp. are ‘K’ lovers with over 1.3% K2O in dry matter.
  • The associated weed is responsive to nitrogen and it utilizes more of the applied ‘N’ than the crop. E.g. The ‘N’ uptake by Echinochloa crusgalli is more than rice.
  • Nutrient removal by weeds leads to a huge loss of nutrients in each crop season, which is often twice that of crop plants. For instance, at the early stages of maize cultivation, the weeds were found to remove 9 times more of N, 10 times more of P, and 7 times more of K.

Competition for moisture

  • In general, to produce equal amounts of dry matter, weeds transpire more water than most of our crop plants.
  • It becomes increasingly critical with increasing soil moisture stress, as found in arid and semi-arid areas.
  • As a rule, C4 plants utilize water more efficiently resulting in more biomass per unit of water. Cynodon dactylon had almost twice as high a transpiration rate as pearl millet.
  • In weedy fields soil moisture may be exhausted by the time the crop reaches the fruiting stage, i.e. the peak consumptive use period of the crop, causing significant loss in crop yields.

Competition for light

  • It may commence very early in the cop season if a dense weed growth smothers the crop seedlings.
  • It becomes an important element of crop-weed competition when moisture and nutrients are plentiful.
  • In dryland agriculture in years of normal rainfall, the crop-weed competition is limited to nitrogen and light.
  • Unlike competition for nutrients and moisture, once weeds shade a crop plant, increased light intensity cannot benefit it.

Also, competition for space (CO2)

Crop-weed competition for space is the requirement for CO2 and the competition may occur under extremely crowded plant community conditions.

More efficient utilization of CO2 by C4-type weeds may contribute to their rapid growth over C3-type of crops.


Allelopathy is the detrimental effects of chemicals or exudates produced by one (living) plant species on the germination, growth, or development of another plant species (or even microorganisms) sharing the same habitat.

It does not form any aspect of crop-weed competition, rather, it causes Crop-weed interference, it includes competition as well as possible allelopathy.

Allelo chemicals are produced by plants as end products, by-products, and metabolites liberalized from the plants; they belong to phenolic acids, flavonoids, and other aromatic compounds viz., terpenoids, steroids, alkaloids, and organic cyanides.

Allelopathic Effect of Weeds on Crops


  • Leaves & inflorescence of Parthenium sp. affect the germination and seedling growth
  • Tubers of Cyperus esculentus affect the dry matter production


  • Stem of Solanum affects germination and seedling growth
  • Leaves and inflorescence of Parthenium affect germination and seedling growth


  • Seeds of wild oats affect germination and early seedling growth
  • Leaves of Parthenium affects general growth
  • Tubers of C. rotundus affect dry matter production
  • Green and dried leaves of Argemone mexicana affect germination & seedling growth


  • Seeds of Datura affect germination & growth

Allelopathic Effect of crop plants on weeds

  • Root exudation of maize inhibits the growth of Chenopodium album
  • The cold-water extracts of wheat straw when applied to weeds reduce germination and growth of Abutilon sp.

Allelopathic effect of weeds on weeds

  • Extract of leaf leachate of decaying leaves of Polygonum contains flavonoids which are toxic to germination, root, and hypocotyls growth of weeds like Amaranthus spinosus
  • Inhibitors secreted by decaying rhizomes of Sorghum halepense affect the growth of Digitaria sanguinalis and Amaranthus sp.

Factors influencing allelopathy

Plant density:

  • The higher the crop density the lesser will be Allelo chemicals it encounters

Life cycle:

  • If weed emerges later there will be less problem of allelochemicals

Plant age:

  • The release of allelochemicals occurs only at a critical stage. E.g. in the case of Parthenium, allelopathy occurs during its rosette & flowering stage.

Plant habit:

  • The allelopathic interference is higher in perennial weeds.

Habitat of plants:

  • Cultivated soil has higher values of allelopathy than uncultivated soil.

Climatic factors:

  • The soil & air temperature, as well as soil moisture, influence the Allelo chemicals potential

Soil factors: 

  • Physico-chemical and biological properties influence the presence of allelochemicals.

Stress factors:

  • Abiotic and Biotic stresses may also influence the activity of allelochemicals

Mechanism of action of allelochemicals

  • Interfere with cell elongation
  • Interfere with photosynthesis
  • Interfere with respiration
  • Interfere with mineral ion uptake
  • Interfere with protein and nucleic acid metabolism

Use of Allelopathy in biological control of weeds:

  • Use of cover crops for biological control
  • Use of allelopathic chemicals as bio-herbicides

Effect of weed competition on crop growth and yield

  • Crop growth and yield are affected
  • Crop suffers from nutritional deficiency
  • Leaf area development is reduced
  • Yield attributes will be lowered
  • Reduce the water use by the crop
  • Affects the dry matter production
  • Lowers the input response
  • Causes yield reduction
  • Pest and disease incidence will be more


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