Introduction to Weeds, Harmful and Beneficial Effects Of Weeds & Weed Management

Introduction to Weeds

Origin of weeds

Weeds are no strangers to man. They have been there ever since farmers started to cultivate crops about 10,000 BC and undoubtedly recognized as a problem from the beginning. Any plant in the field other than his crop became a weed. Again the characters of certain weed species are very similar to that of wild plants in the region. Some of the crops for example including the wheat of today are the derivatives of wild grass.

Man has further improved them to suit his taste and fancy. Even today they are crossed with wild varieties to transfer the desirable characters such as drought and disease resistance.

So the weeds are, to begin with essential components of native and naturalized flora but in course of time these plants are well placed in the new environment by the conscious and unconscious efforts of man. Hence, it is considered that many weeds principally originated from two important and major arbitrarily defined groups.

  1. By man‟s conscious effort
  2. By invasion of plants into man created habits

In the world, there are 30,000 weed species, out of these 18,000 species cause damage to
the crops.

Definition

Weeds are the plants, which grow where they are not wanted. Weeds can also be referred to as plants out of place. Weeds are unwanted or undesirable plants that compete with crops for water, soil nutrients, light, and space (ie CO2) and thus reduce crop yields.

Weeds are competitive and adaptable to all the adverse environments. It has been estimated that in general weeds cause a 5% loss to Agricultural production in most developed countries. 10% loss in less developed countries and 25% loss in least developed countries.

Harmful effects of weeds

Of the total annual loss of agricultural produce from various pests in India, weeds account for 45%, insects 20%, diseases 20% & others 5%.

The losses due to weeds depends on

  1. Type of weed
  2. Severity of Infestation
  3. Duration of infestation
  4. Competitive ability of the crop plants with weeds
  5. Climatic conditions which affect the growth of the crop and the weed

The following are the harmful effects of weeds on various activities of human beings, crops,
livestock and related aspects.

  1. Weed menace in agriculture
  2. Weed menace in animal husbandry
  3. Weed menace to human health
  4. Weed menace to aquatic ecosystems
  5. Weed menace to forest and pasture land
  6. Weeds as alternate hosts to crop pests and diseases
  7. Weeds reduce land value.
  8. Weeds limits choice of crops

Beneficial effects of weeds or economic uses of weeds:

Several weeds have been put to certain economic uses for ages. Typha and Saccharum species used for making ropes and thatch boards. Cichorium intybus roots are used for adding flavor to coffee powder. Amaranthus Viridis, Chenopodium album, and Portulaca species used as leafy vegetables. In north India Saccharum spontaneum used in a breeding program for developing the noble canes.

Incorporation of Crotalaria, Parthenium, Calotropis, and Eichornia reduced root-knot nematode population in the soil as they exhibited nematicidal properties. Hariyali grass (Cynodon dactylon) and Cenchrus ciliaris, Dichanthium annulatum Eclipta alba weeds of grassland serve as food for animals.

People in China and Japan consume Chlorella pyrenoids (algae) as a protein supplement. Weeds act as alternate hosts for predators and parasites of insect pests which feed on the weeds. For example Trichogramma chilis feed upon eggs of caster semi looper which damages the castor plants.

Some weeds are useful to identify the metals (Indicator geobotany) through satellite imageries Eg Commelina species (Copper), Eichornia crassipes (Copper-Zinc, lead, and cadmium in water bodies. Several species of weeds Tephorsia purpurea and Croton sparsiflora in South India used as green manures. Whereas Eichornia crassipes and Pistia stratiotes are used for composting. Argemone mexicana used for the reclamation of alkali soils.

Some weeds have medicinal properties and used to cure snakebite (Leucas Aspera), gastric troubles (Calotropis Procera), skin disorders (Argemone mexicana), and jaundice (Phyllanthus nirur ) and Striga orobanchioides to control diabetes. In addition to the above agarbathis (Cyperus rotundus), aromatic oils, (Andropogan species & Simbopogon species) are prepared from weeds. Air pollution determined by wild mustard and chickweed respectively. Aquatic weeds are useful in paper, pulp, and fiber industry. Chenopodium album used as mulch to reduce evaporation losses, whereas Agropyron repens (quack grass) used to control soil erosion because of its prolific root system. weeds like Lantana camera, Amaranthus Viridis, Chenopodium album, Eichhornia crassipes used for beautification. Agropyron repens used for soil conservation, whereas Dicanthium annulatum stabilizing field bunds. Opuntia Fellini used as a biological fence. Maize field infested with Striga and field bindweed.

Weed management is the application of certain principles and suitable methods that will improve the vigor and uniform stand of the crop. At the same time ignore or discourage the invasion and growth of weeds.

Methods of weed management

Weed control and weed management are the two terms used in weed science. Weed control is the process of limiting the infestation of the weed plant so that crops can be grown profitably, whereas weed management includes prevention, eradication, and control by regulated use, restricting invasion, suppression of growth, prevention of seed production and destruction. Thus weed control is one of the aspects of weed management.

Principles of Weed Management are

  1. Prevention
  2. Eradication
  3. Control

Prevention

Prevent the entry and establishment of weeds into uninfected areas.

It encompasses all measures taken to prevent the introduction and/or establishment and spread of weeds. Such areas may be local, regional, or national in size. No weed control program is successful if adequate preventive measures are not taken to reduce weed infestation. It is a long term planning so that the weeds could be controlled or managed more effectively and economically than is possible where these are allowed to disperse freely.

Arresting the movement and denying them entry in an uninfested area. Crop hygiene can also be considered as prevention. Weeds producing seeds in the current season are the inoculum for next season. If somehow they do not produce seeds in the current season, this is also one of the preventive measures. Thus controlling weeds at the flowering stage, which will prevent contamination in other seasons, is also a preventive measure.

Important weed prevention practices are

  1. Use clean crop seeds/ weed-free crop seed
  2. Avoid feeding of screenings, grain or hey containing weed seeds to livestock without
    destroying their viability by grinding, cooking, and ensiling.
  3. Use well rotten\decomposed organic manure. Avoid reaching of weed seeds into the
    compost pit.
  4. Prevent the movement of weeds with other farm resources.
  5. Keep non-crop area clean.
  6. Use vigilance.
  7. Follow legal & quarantine measures.

Eradication

It is complete removal of all live plant parts and seeds of the weed from an area. It may be a field/farm/village/geographical region depending upon the need.

In general eradication of common weed seeds is not practiced as these weeds harbor crop pests or secretes soil nematicides. They may be useful to hold the soil nutrients against leaching losses during the fallow period.

However weed eradication is justified against weeds like Striga, Cuscuta, Lantana to prevent their dispersal to new areas of useful land and water bodies. Weed eradication programs should begin when the weed growth is limited. If the weed occupied large and continuous areas eradication is not economical. It should be carried out for more than one year. It requires intensive initial efforts to destroy all plant parts and followed by many years of vigilance to prevent the new weed seedlings from establishing into adult plants.

Control

Weed infestations are reduced but not necessarily eliminated. Weed control includes

  • Cultural
  • Biological
  • Chemical

INTEGRATED WEED MANAGEMENT

System approach also called as integrated weed management. “Integrated method is a system that brings all feasible methods of weed control harmonizing them into a single and co-ordinated system designed to maintain weeds below those levels at which they cause economic loss”.

Principles of Integrated weed management

  1. IWM places the crop in a competitive advantage over the weeds by manipulating the crop habitat by utilizing some biological differences between crops and weeds.
  2. IWM measures should be directed to reduce the survival mechanism of weeds in the soil.
  3. Crop cultural practices should be incorporated to discourage the establishment of the perennial and parasitic weeds. Eg: Crop rotation
  4. Any individual element of the weed management should be eco friendly and it should not be harmful to the environment.
  5. Weed management practices should be flexible to accommodate possible innovations and experiences of progressive farmers.

Advantages

  • It shifts the crop-weed competition in favor of crop
  • Prevents weed shift towards perennial nature
  • Prevents resistance in weeds to herbicides
  • No danger of herbicide residue in soil or plant
  • No environmental pollution
  • Gives higher net return
  • Suitable for high cropping intensity