Reduction in crop yields due to weeds result from their multiferous ways of interfering with crop growth and crop culture. Weeds compete with crops for one or more plant growth factors such as mineral nutrients, water, solar energy and space and they hinder crop cultivation operations.
1. 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. For e.g. Mani, found that on an average weeds growing in crop field during the kharif season removed 46.6 kg/ha N, 12.1 kg/ha P and 73.3 kg/ha K.
In general, weeds removed N and K from soil in much greater quantity than P.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
11. Weeds cause quicker wear and tear to farm implements: Being hardy and deep-rooted, the tillage implements get worn early.
12. Weeds reduce the value of the land: Agricultural lands heavily infested with perennial weeds like Kans (Saccharum spontaneum) always fetch less price.
13. 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.
14. Increase in cost of cultivation: In fields of crops infested with weeds, the tillage operations require high cost.
15. 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.
