Losses Caused by Weeds

a. Reduction in crop yield

• Weeds compete with crop plants for nutrients, soil moisture, space and sunlight.
• In general an increase in one kilogram weed growth corresponds to reduction in one kilogram of crop growth.

• Among the pests weeds account for 45 % reduction in yield while the insects 30%, diseases 20% and other pests 5%.

b. Loss in crop quality

• If a crop contains weed seeds it is to be rejected, especially when the crop is grown for seed. For example, the wild oat weed seeds are similar in size and shape of the crops like barley, wheat, and its admixture may lead to rejection for seed purpose.
• Contamination by poisonous weed seeds is unacceptable and increases costs of crop cleaning.

c. Weeds as reservoirs of pests and diseases

• Weeds form a part of community of organisms in a given area. Consequently, they are food sources for some animals, and are themselves susceptible to many pests and diseases.
• However, because of their close association with crop they may serve as important reservoirs or alternate host of pests and diseases.

d. Interference in crop handling

• Some weeds can make the operation of agricultural machinery more difficult, more costly and even impossible.
• Heavy infestation of Cynadon dactylon causes poor ploughing performance.

e. Reduction in land value

• Heavy infestation by perennial weeds could make the land unsuitable are less suitable for cultivation resulting in loss in its monetary value.
• Thousands of hectare of cultivable area in rice growing regions of India have been abandoned or not being regularly cultivated due to severe infestation of nut grass (Cyperus rotundus) and other perennial grasses.

f. Limitation of crop choice

• When certain weeds are heavily infested, it will limit the growth of a particular crop.
• The high infestation of parasitic weeds such as Striga lutea may limit the growing of sorghum or sugarcane.

g. Loss of human efficiency

• Weeds reduce human efficiency through physical discomfort caused by allergies and poisoning.
• Weeds such as congress weed (Parthenium hysterophorus) causes itching. Thorny weeds like Solanum spp. restrict moment of farm workers in carrying out farm practices such as fertilizer application, insect and disease control measures, irrigation, harvesting etc.

h. Problems due to aquatic weeds

• Weed obstruction cause reduction in velocity of flow and increases stagnation of water and may lead to high siltation and reduced carrying capacity.
• Aquatic weeds form breeding grounds for obnoxious insects like mosquitoes.
• They reduce recreational value by interfering with fishing, swimming, boating, hunting and navigation on streams and canals.

i. Other problems

• Weeds are troublesome not only in crop plants but also in play grounds and road sides etc.
• Alternanthera echinata and Tribulus terrestris occurs in many of the playgrounds causing annoyance to players and spectators.