Weeds Vs Plants. Weeds do not need cultivation to dominate a farm. How detrimental are weeds to crops?

A plant is often termed a “weed” when it has little or no recognized value (as in medicinal, material, nutritional or energy), Rapid growth and/or ease of germination and is Competitive with crops for space, light, water and nutrients. Weeds are a serious threat to primary production and biodiversity. They reduce farm and forest productivity, displace native species and contribute significantly to land and water degradation.

The costs of weeds to the natural environment are also high, with weed invasion being ranked second only to habitat loss in causing biodiversity decline. Despite considerable government and private sector investment, weed invasion still represents a major threat to both the productive capacity of land and water and the integrity of our natural ecosystems.

Characteristics of weeds

Weeds are also like other plants but have special characteristics that tend to put them in the
category of unwanted plants.

• Most of the weeds especially annuals produce enormous quantity of seeds, e.g. wild
  oats (Avena fatua), produces 250 seeds per plant, whereas wild amaranth (Amaranthus
  viridis) produces nearly 11 million seeds. It has been observed that among 61 perennial
  weeds, the average seed-production capacity was 26,500 per plant.

• Weeds have the capacity to withstand adverse conditions in the field, because they can
  modify their seed production and growth according to the availability of moisture and
  temperature. They can germinate under adverse soil-moisture conditions, have short
  period of plant growth, generally grow faster rate and produce seed earlier than most of
  the crops growing in association.

• Weed seeds remain viable for longer period without losing their viability, e.g. annual
  meadow grass (Poa annua) and scarlet pimpernel (Anagallis arvensis) remain viable foe
  about 8 years; creeping thistle (Cirsium arvense) for 20 years and field bind weed
  (Convolvulus arvensis) for about 50 years.

• Weed seeds have a tremendous capacity to disperse from one place to another through
  wind, water and animals including man. Many of times, weed seeds mimic with the crop
  seeds due to their size and get transported from one place to another along with them.

Harmful Effects of Weeds

• Competition for mineral nutrients: Being hardy and vigorous in growth habit; they soon outgrow the crops and consume large amount of water and nutrients. Thus causing heavy losses in yields. In general, weeds removed N and K from soil in much greater quantity than P.

• Competion for water: For producing equal amount of dry matter, the weeds, in general transpire more water than most crop plants. It is reported that wild mustard transpires about four times more water than a crop of oat.

• Competition for Solar Energy: About 99% of dry matter in plants is made up of organic matter that is dependent on solar energy. When plants are mutually shaded, their production potential is greatly reduced even though water and other nutrients are available to them in abundance.

• Competition for space: Weeds compete for space both in the rhizosphere and atmosphere. In the presence of weeds, crop plants also have limited space to develop their shoots, which amounts to reduced photosynthesis in them.

• Weeds reduce the crop quality: Weeds may reduce the quality of the crop produce in many ways. Weed seed like wild mustard, sweet clover, a Mexican poppy and bulblets of wild garlic and wild onion when threshed and ground with winter grains can results in serious consequences besides imparting objectionable odour to the flour. Khanna observed that striga (striga Spp.) reduced the quality of sugarcane juice by 3.9 to 8.9 percent.

• Weeds impair the quality of the animal produce: Many weeds in pastures and forage crops impart undesirable flavours to milk and meat of animals. For e.g. Pivali tilwan (Cleome viscosa) imparts undesirable flavour to milk. Gokharu or Landaga (Xanthium strumarium) get attached to the body of sheep and seriously impair the quality of wool.

• Weeds harm animal health: Several weeds of grasslands and forage crops contain high alkaloids, tannis, oxalates, gulcosides, and other substances that prove poisonous to animals when ingested. For e.g. Silky lupine ( Lupinus sericeus) is responsible for crooked calf disease.

• Weeds harbour insect pests or diseases; Weeds either give shelter to various insect pests and diseases or serve as alternate host. For e.g. Weed around paddy bunds harbour the gallfly.

• Weeds damage human health: Health, comfort and work efficiency of man are also affected by weeds directly or indirectly. For e.g. people in U.P. are plagued year after year with hay fever and asthma aggravated by pollens of regweeds bursage. Tsetse fly which cause African sleeping sickness.

• Weeds contaminate water bodies: Aquatic weeds change the flavour appearance and taste of drinking water. Aquatic weeds are a menace to fisheries too. Aquatic weeds on decomposition gives offensive odours and pollute atmosphere.

• Weeds cause quicker wear and tear to farm implements: Being hardy and deep-rooted, the tillage implements get worn early.

• Weeds reduce the value of the land: Agricultural lands heavily infested with perennial weeds like Kans (Saccharum spontaneum) always fetch less price.

• Less efficient use of land : In case of perennial weeds, the carrying capacity of the grazing lands is reduced and cause depreciation of land value.

• Increase in cost of cultivation: In fields of crops infested with weeds, the tillage operations require high cost.

• Disturbance in Public places: it is desirable that public places be kept clean of weeds. Presence of weeds around our living and working places makes the surroundings dull.

An efficient weed control program can only be developed after the weed has been properly identified. Weeds can be managed using many different methods. The most effective management of weeds is usually achieved through collaboration and cooperation, in partnerships between the community, land owners, agriculture, industry and the
various levels of government, using a combination of methods in conjunction with a thorough
follow-up campaign.

Weed management is an important component of plant protection improving the
production potential of crops. It includes management of the weeds in a way that the crop
sustains its production potential without being harmed by the weeds. Weed management is
done through the mechanical, cultural and chemical means. Use of biological control methods in
field crops is being considered, but still not much in use. Use of herbicides is an important
method in the modern concept of much in use. Use of herbicides is an important method in the
modern concept of weed-management technology. New hand-tools and implements have also
been designed to assist in wed-management programme.

Credit: http://eagri.org

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