Losses Caused by Weeds
Weeds compete with crop plants for nutrients, soil moisture, space, and sunlight. Depending on type of weed, intensity of infestation, period of infestation, the ability of crop to compete and climatic conditions the loss varies. Among the pests weeds account for 45 % reduction in yield while the insects 30%, diseases 20% and other pests 5%.
Loss in crop quality
If a crop contains weed seeds it is to be rejected, especially when the crop is grown for seed. Contamination by poisonous weed seeds is unacceptable and increases costs of crop cleaning. The leafy vegetables much suffers due to weed problem as the leafy weed mixture spoil the economic value.
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.
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.
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 nutgrass (Cyperus rotundus) and other perennial grasses.
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.
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.
Problems due to aquatic weeds
The aquatic weeds that grow along the irrigation canals, channels and streams restricts the flow of water. 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.
Factors affecting the competitive ability of crops against weeds
Increase in density of weed decrease in yield is a normal phenomena. However, it is not linear as few weeds do not affect the yields so much as other weed does and hence, it is a sigmoidal relationship.
Increase in plant population decreases weed growth and reduce competition until they are self-competitive. Crop density and rectangularity are very important in determining the quantum and quality of crop environment available for the growth of weeds.
Type of weeds species
The type of weeds that occur in a particular crop influences the competition. Occurrence of a particular species of weed greatly influences the competition between the crop u0026amp; weed. For eg. E. crusgalli in rice, Setaria viridis in corn and Xanthium sp. in soybean affects the crop yield. Flavaria australasica offers more competition than the grasses
Type of crop species and their varieties
Soil type, soil fertility, soil moisture and soil reaction influence the crop weed competition. Elevated soil fertility usually stimulates weeds more than the crop, reducing thus crop yields. Fertilizer application of weedy crop could increase crop yields to a much lower level than the yield increase obtained when a weed free crop is applied with fertilizer. Weeds are adapted to grow well and compete with crops, in both moisture stress and ample moisture conditions. It is therefore specific weed species suited to different soil reactions exist with us, our crops grow best only in a specified range of soil pH. Weeds would offer more intense competition to crops on normal pH soils than on normal pH soils.
Adverse weather condition, Eg. drought, excessive rains, extremes of temperature, will favour weeds since most of our crop plants are susceptible to climatic stresses.
Time of germination
Time of germination of crop coincides with the emergence of first flush of weeds, it leads to intense Crop-Weed interference. Weed seeds germinate most readily from 1.25 cm of soil and few weeds can germinate even from 15cm depth.
Cropping practices, such as method of planting crops, crop density and geometry and crop species and varieties have pronounced effects on Crop-Weed interference.
Maturity of the crop is yet another factor which affects competition between weeds u0026amp; crop. As the age of the crop increases, the competition for weeds decreases due to its good establishment. Timely weeding in the early growth stages of the crop enhances the yield significantly.
Critical period of weed competition
Critical period of weed competition is defined as the shortest time span during the crop growth when weeding results in highest Economic returns.
The critical period of crop-weed competition is the period from the time of sowing up to, which the crop is to be maintained in a weed free environment to get the highest economical yield.