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.
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.
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.
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.
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
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.
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
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.