ALLECHEMIALLELOPATHY is a biological phenomenon in which an organism releases one or more bio-chemicals that influences the other organisms. These biochemicals are recognized as allelochemicals and can have positive or negative effects on the target organisms. At present, allelopathy has been considered to be the most important factor influencing the invasion and spread of exotic plants.
The importance of allelopathy in natural agro-ecosystem has attracted researcher’s attention with the main goal of using the phenomenon in natural control of weeds. Currently, active involvement of scientists from different disciplines made allelopathy a multidisciplinary subject, and transformed the research from basic to applied, enabling use of allelopathy in agriculture.
Screening accessions of allelopathic crops and natural vegetation for their ability to reduce weeds is the basic approach for utilizing the phenomenon. From a holistic viewpoint, research prospective and use of allelopathy in an agro-ecosystem is wide. The richness of agricultural techniques, cover cropping, crop rotation and related practices allow scientists to assess and make use of allelopathic plants for weed management in agricultural systems. The science of allelopathy has advanced its descriptive and basics to offer a foundation to assist crop production.
Currently, allelopathy is employed in research relating sustainable agriculture, and resource conserving, low input, or organic agriculture. Allelopathy has been used in agricultural practices as crop rotations, intercroppings, weed control, nutrient recycling and low-external input farming practices.
The allelopathic plant products offer a vast range of secondary metabolic compounds which have the potential to be used directly as alternative of herbicide or can be utilized for synthesis of new herbicides. Investigations into allelopathy started with the observations of crop phytotoxicity problems in field by preceding crops. As far as agricultural production is crucial to carry on, the need for new herbicides is inevitable.
At present, about two-third by volume, of the agro-chemicals used in agriculture are herbicides. The possible undesirable environmental pollution from herbicides and there is a need for environmentally safe herbicides that are equally or more effective and selective than currently available synthetic herbicides. There is a strong feeling that allelopathic research can be applied to so many current weed problems. The present emphasis on reduced- or no-tillage agriculture will depend on herbicides for weed control. On the other hand, the increasing incidence of herbicide resistance is creating a demand for new herbicides with unexploited mechanism of action. Thus, the need for new herbicides becomes obvious to solve the dilemma of the continued demand for herbicides.
Natural plant secondary metabolic compounds released by crops, weeds, or their residues may offer solutions to some of these needs.The recognized importance of allelopathy in agricultural practices has been increased with the main objectives of using this phenomenon in natural control of weeds. One approach to utilize this phenomenon is suggested to screen allelopathic crops for their ability to reduce weeds, and a few crops have been evaluated in this aspect. Sorghum is known to provide a good weed killing capacity and on the other hand it is also autotoxic and should be rotated with other crops for maximum yield.
Complementarity of allelochemicals and herbicides can also be exploited as means of weed control. Herbicides applied along with allelopathic conditions could have supportive action, affecting the same or different weed species. A reduced level of herbicide may be feasible to provide weed control when it operates simultaneous with allelopathic conditions. Manipulation of timing, formulation and application rate of a herbicide used to kill or supplement a cover crop can enhance phytotoxicity, improving weed control.
Ample information is available and agricultural production needs are causes for exploiting allelopathy to benefit production systems. Current farming is in a transition of reduction of tillage and less chemical use. Reducing tillage restricts weed seeds to poor germination sites and by utilizing natural phytotoxins leaching from plant residues, the germination of seeds and growth of many weeds can be inhibited. There are many crop species known for their allelopathic potential. Herbicides can be used to supplement cover crops in reduced tillage practices.
Crop plant breeding for genetic manipulation and allelopathic potential against weeds is believed to solve, in part, weed problems. Use of proper type and amount of crop and weed mulch should be considered for weed management. Residue management can be beneficial to control weeds. In this respect residues of the allelopathic crops may be utilized by incorporating into the soil or it may also be valuable residue mulch restricting the emergence of weeds. Crop rotation can be employed in order to include the allelopathic crops in rotation. Timing of cultural operations and proper agronomic practices needs to be identified for specific areas of production to make use of allelopathic conditions.
Social and environmental concerns as well as a desire to improve weed control efficiency have led to increase emphasis on integrating several cultural methods, although herbicides, screening trials still in force in conventional weed research. Manual weeding is still the dominant weed control method in many parts of Asia, since management options for weed control are limited under diverse agro-ecological conditions. Mechanical method is considered cheaper as compared to manual method but it alone is suitable in certain conditions. These factors plus the increasing cost and difficulty of developing synthetic new herbicides indicate that new and safe weed control strategies need to emerge. The alternative to this situation is adoption of integrated weed management (IWM).
Integrated weed management involves the judicious use of all possible weed control methods. Two or more than two methods can be integrated to get sustainable weed control. This is more effective because the left over weeds with one method can be controlled with other method. A long-term integrated weed management plan, that considers all available management control techniques or tools to control weeds, can be developed for a particular area. Any integrated weed management plan or strategy should focus on the most economical and effective control of the weeds and include ecological considerations.
The long term approach to integrated weed management should reduce the extent of weeds and reduce the weed seed stock in the soil. It should consider how to achieve this goal without degrading the desirable qualities of the land, such as its native ecology or agricultural crops. Integration of chemical, mechanical, cultural, allelopathy or even biological method can be made for effective and long term weed control.
Integration of allelopathic crop water extract with the reduced doses of herbicide alone and in combination has been studied and found promising. Similarly inclusion of allelopathic crops in crop rotation and intercropping showed encouraging results regarding weed control. Allelopathic crop residue amendment in soil is also another aspect of using allelopathic phenomenon in integrated weed management. Allelopathy is natural and low cost but effective measure that can be integrated with one or more weed control measures for sustainable integrated weed management.
Author: Javed Akhter 

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