Allelopathy: An intercropping benefit
Intercropping (also known as poly culture) is growing two or more crops in order to maximize the beneficial interactions while minimizing competition. The resulting beneficial interaction can lower the need of external inputs. Intercropping can also provide increased biodiversity, stability, financial diversification and effective weed control. Intercropping can be done with field, vegetable and even tree crops. There are at least four basic spatial arrangements used in intercropping. Most practical systems are usually variation of these.
Row intercropping: Growing two or more crops at the same time with at least one crop planted in rows narrow enough to permit interaction, e. g., sugarcane can be intercropped with maize, tobacco and vegetables.
Strip intercropping: Growing two or more crops together in strips wide enough to permit separate crop production using machinery but close enough for the crops to interact. Alternative strips of soybean, sesame can be grown in cotton.
Mixed intercropping: Growing two or more crops together with no distinct row patterns. Seeds of Brassica spp. are sown in mixture with wheat.
Relay intercropping: Planting a second crop into a second crop at a time when the standing crop is at its reproductive stage but before harvesting. The planting of sugarcane in sugar beet, canola in sunflower and wheat in cotton are examples of this practice.
Intercropping is a common practice among the farmers of the developing countries for maximizing land resources and reducing the risks of single crop failure. Its advantages of improved crop yields and better soil conservation have been demonstrated over thousand years. Yield advantages from intercropping are often attributed to better weed control, mutual effects of components crops and better utilization of farm resources. Recent studies have suggested intercropping as an option for integrated weed management. The weed control advantages of intercrops over sole crops have been attributed to strong competitiveness in usurping resources from weeds or suppressing weed growth through allelopathy.
Allelopathy refers to interactions among organisms in soil-plant interface through biochemical pathways. It is possible to utilize allelopathic interactions in farming as a cost effective alternative of synthetic chemical inputs for controlling weeds, thus contributing towards sustainable agriculture. Allelopathy can be especially important for intercropping systems. The ability of some cover crops to suppress weeds is due in part to allelopathy. Winter rye, for example, suppresses weed growth as it actively grows and decays when residues are incorporated into the soil. Allelopathy is an intercropping benefit that can be exploited for managing weeds in agroecosystems.
Alternatively, intercropping may provide yield advantage without suppressing weed growth below levels observed in components sole crops if intercrops use resources that are not exploitable by weeds or convert resources to harvestable material more efficiently than sole crops. However, because of the difficulty of monitoring the use of multiple resources by intercrops/weed mixtures throughout the growing season, identification of specific mechanisms of weed suppression and yield enhancement in intercropping has so far proven elusive. Reduced weed incidence under intercropping depends in part on planting geometry, intercrops being used, cultural practices and growing season among other factors. A cover crop with over 50 per cent soil coverage is capable of intercepting most of the incident radiation, thereby preventing the same from reaching weed seeds lying on or underneath the soil between the main crop rows.
Rapid canopy closure by cover crop between the rows of main crop suppresses weed seedling emergence and subsequent growth. Emergence of weed seedlings is suppressed either due to light unavailability or allelochemical secretion. Nevertheless, after weed seedling establishment, resource competition becomes the most obvious weed suppressing mechanism.
Utilizing intercrops, that are selectively allelopathic to weeds but do not appreciably interfere with main crop growth, can provide a cost effective alternative to the use of synthetic chemicals for the management of weeds, thereby contributing to a more sustainable agriculture. Such companion plants can be effectively incorporated into an intercropping system. All the intercropping systems (single or double rows of sorghum, soybean and sesame planted within cotton rows) significantly inhibited purple nutsedge (Deela) density and dry biomass relative to control.
Intercropping of maize with tall and dwarf sorghum cultivars furnished significant suppression of purple nuts edge as compared to maize planted alone. This suppression was greater for tall sorghum cultivars. Sorghum intercropped with maize suppressed weeds density and resulted in low biomass of purple nut sedge, field bind weed (Lehli) and horse purslane (Itsit) compared with other treatments comprising of sunflower and mungbean. These encouraging results suggest that intercropping can be opted as a sustainable weed management tool in agroecosytems.
The writers are associated with Weed Science and Allelopathy Lab, Department of Agronomy, University of Agriculture, Faisalabad. They can be reached at