Crops are being raised from times immemorial without damaging the environment but the use of herbicides during the last 50 years have raised serious doubts about their continuous use. Allelopathic suppression of weeds is receiving greater attention in recent years as a possible alternative for weed management.

Present understanding of the plant biochemistry, physiology, morphology, interactions and chemistry of natural products have shown that the crop residues and their allelochemicals may be used for weed control, overcoming the above problems associated with the herbicides.

Allelopathy is a natural and environment-friendly technique which may prove to be a unique tool for weed management and thereby increase crop yields. Allelopathy involves direct or indirect (harmful or beneficial) effects of one plant on another through the production of secondary chemical compounds that escape into the environment. Chemicals with allelopathic potential are present in virtually all plants and in most tissues, including leaves stems, flowers, roots, seeds and buds. Under appropriate conditions these chemicals may be released into the environment (generally the rhizosphere) in sufficient quantities to affect the neighbouring plants. Crop allelopathic interactions may provide weed control in the crops by various ways such as (a) use of phytotoxix crop residues as mulches and cover crops (b) use of allelopathic plants in crop rotations (c) crop mixtures and intercropping (d) germplasm selection (e) use of allelopathic crop water extracts. Phytotoxic mulches and cover crops: Allelopathy may be utilized in weed management systems through the manipulation of allelopathic cover crop residues in annual and perennial cropping systems.

So by the use of this cultural approach, the produces of corn, soyabean and small and large fruit crops can get maximum benefits. The cover crop residues on the soil surface provide many benefits including the conservation of soil and water, higher population of bacteria and pseudomonas in soil surface, rapid release of water soluble inhibitors. The physical presence of the cover crop mulch on the soil surface also has profound effects on weed density and distribution. It was found that the residues of wheat and rye reduced the weed growth upto 38 per cent. The weed infestation levels were reduced by the wheat straw which decreased the light interception and soil temperature.

Soil incorporation of sorghum stalk greatly reduce weed growth which is due to its allelopathic effects on other crops. Phenolic compounds from decomposing residues and soils were probably responsible for the growth and the yield reduction associated with the allelopathy of sorghum. In a field study it was noted that the soil incorporation of sorghum stalks @ 2, 4 and 6 mgha reduced weed dry weights by 42, 48 and 56 per cent, respectively. The suppression in weed dry weight by soil incorporation of sorghum stalks suggested the presence and effectiveness of allelochemicals. Similarly soil incorporation of sunflower residues significantly reduced the weeds by 66 per cent compared with the control. The residues of several annual legumes have also shown weed suppression. So the use of allelopathic crop residues as mulches to control weeds is simple and easy technique and improves the soil organic matter.

Crop rotations: Crop rotation means growing of crops one after the other on a piece of land without disturbing its fertility. The crops like sorghum, maize, barley, rye, sudangrass, sweetclover, sunflower, rapeseed, soyabean, alfalfa, cowpeas and hemp smothered various weed species through crop interference. Farmers should include these crops in their rotation to suppress the growth of weeds. These crops exert weed suppression through competition for growth resources and growth inhibition through allelopathic interactions. For example the living plants and the residues of sorghum can reduce the common purslane greatly. The allelopathic crops may affect the germination of subsequent crops, therefore, those crops should be included which are tolerant.

Crop mixtures and intercropping: The role of allelochemicals in interference between the inter-crop components and weeds may be useful to control the weeds. Intercropping is superior than monoculture in weed suppression but their effectiveness varies greatly. For allelopathy to be effective for weed control in intercropping, there should be selectivity in the toxins released by the crops and weed species must be more susceptible to such phytotxin than crops. To make this concept successful, more extensive research is needed on (a) applicable crop sequences (b) the role of soil and climatic factors (c) long-term impacts on the weed seed populations (d) to integrate allelopathic crops with the weed control practices. In a research study it was found that the intercropping of some legumes in main crops reduced the weed growth and increased the crop yield. In selecting crops for mixture or intercropping, their allelpathic effects on the component crops must be considered because certain crops might have adverse effects on the component crops. Germplasm selection:One potential technique of exploiting allelopathy in weed management is the transfer of allelopathic characteristics from wild types or unrelated plants into the commercial crop cultivators. If the new allelpathic character does not have undesirable effects, this technique could increase the ability of the crop to compete naturally against the weeds. Very few attempts have been made to enhance the weed suppressive potential of crop plants through conservation or non-traditional breeding programmes, even though this is a logical way to integrate the biorational approaches to pest control in the current production systems. It has been established that the potential for suppression of weeds could be enhanced through the selection of crop cultivators for allelopathic characters. The superior weed suppressing genotypes have been reported in cucumber, oat, rice, sunflower, soyabean, sorghum, pearlmillet and brassica. The first step in such a process would be to select plants with greater potential for allelopathy. Several scientists believed that present cultivators have low allelopathic capacity except in sunflower and several varieties of sunflower inhibited the germination and growth of the associated weeds greatly. Similarly some cultivators of sorghum could reduce the requirement of herbicides. In some studies, seeds of sorghum cultivators were exposed to 0.5, 1.0 and 1,5 K rad of gamma rays. The seedlings from these seeds produce more inhibitory root exudates, aqueous extract and decaying residues against the growth of certain weeds. These results have opened a new possible approach for increasing the allelopathic potential of crops without causing genetic damage.

Use of crop water extracts: The interest in natural products as sources of herbicide chemistries has increased in the recent years. Plants and microorganisms produce hundreds of secondary compounds. Many of these compounds (allelchemicals) are phytotoxic and have potential of herbicides or as templates for new herbicide classes. These allelchemicals offer great potential for the pesticides because they are free from problems associated with the present pesticides. Therefore, allelochemicals are current area of research for the development of new herbicides. These could be used for weed control directly or their chemistry could be used to develop new herbicides. The water extracts of many crops e.g. sorghum, sunflower, brassica, sesamum, eucalyptus, tobacco etc, contain a number of allelochemicals which are more effective and economical to control the weeds of many crops. In mature sorghum plants nine water soluble allelochemicals have been identified which are phytotoxic to the growth of certain weeds. The effects of sorghum allelchemicals are selective and concentration dependents. By continuous research and experimentation for many years, researchers at the Weed Science Allelophathy Laboratory, Department of Agronomy, University of Agriculture, Faisalabad, have succeeded to use sorghum water extract (sorgaab) alone or in combination with other water extract for controlling the weeds without damaging the environment. A single spray of the concentrated sorgaab applied 30 DAS (day after sowing) suppressed the weed biomass by 20-40 per cent and could substitute chemical herbicides. The use of sorgaab (sorghum water extract) in combination with other water extract may reduce the need for chemical herbicides and provide economic benefits. For example the application of sorgaab with sunflower water extract can increase the wheat yield upto 46 per cent and reduce the weed biomass by 44 per cent. While the mixture of sorgaab and eucalyptus extract can reduce the weed biomass by 30 per cent and increase the wheat yield up to 39 per cent. Sorgaab and other crops water extracts reduced the weed population by inhibiting the germination of weeds after their application.

Future research: Allelopathy is a very young field of science and has wide application in various discipline of agriculture, therefore, the focus of future research may be on areas involving:

1) The possible use of certain allelochemicals as herbicides or as structural models for herbicide development and reducing the herbicide dose and use.

2) To develop the biotechnology techniques for incorporating a controlling gene into the crops for their own production of allelochemicals. To breed the pest resistant varieties of crops based on their high allelochemical contents.

3) Crop-to-crop relationship need to be investigated in greater depth to determine which crop can follow others with the least inhibitory or having the stimulatory effects.

Conclusion: The use of crop mulches may be beneficial if the crop mulches are cheap and have no other use, while inclusion of allelopathic crops in crop rotations, would be highly desirable for reducing the populations of certain difficult to control weeds such as the cyperuss rotundas however, the adverse effects on subsequent crop may not be ruled out. Mixed cropping or intercropping generally suppresses weed population, the allelopathic crops, might have adverse effect on the companion crops, therefore, this aspect should be kept in view. The use of water extracts of allelopathic crops particularly, sorhum alone or in combination with other water extracts provide an economical, environmentally safe and effective weed control techniques as an alternation of herbicides.