AGRONOMIC MODIFICATION OF RESOURCE USE AND INTERCROP PRODUCTIVITY

Suitable land areas for food production remain fixed or are diminishin g, yet farmers and agronomists are faced with the task of increasing p roduction. Raising productivity, through a more effective use of natur al (e.g. light) and added (e.g. fertilizer) resources, is possible thr ough intercropping, provided component crop demands for resources are well understood. Management of intercrops to maximize their complement arity and synergism, and to minimize competition between them follows simple natural principles, and its practice is limited only by the ima gination of farmers and agronomists. Successful crop mixtures extend t he sharing of available resources, over time and space, exploiting var iation between component crops in such characteristics as rates of can opy development, final canopy width and height, photosynthetic adaptat ion of canopies to irradiance conditions and rooting depth. Occasional ly, commensalism is effected. Loosely defined as one organism gaining benefits from another without damaging or benefitting it, it is exempl ified when one crop modifies the microenvironment to suit another. Pri me examples are the benefits of shading during crop, particularly tran splanted crop, establishment under hot or dry conditions, the supply o f nitrogen and solubilization of phosphorus by legumes for companion c rops, and the suppression of weeds through direct competition or allel opathic effects. The onset of competition between intercrops can be de layed by judicious choice of relative planting dates. The differential influence of weather (in particular temperature) on component crop gr owth and development can be modified through reasoned planting dates, and relative proportions of crop component yields can be targeted. In general, to ensure its high yield the main crop should be planted firs t. Choice of plant population density and crop geometry, including row orientation, permits a planned sharing of natural resources and manip ulation of competitiveness to suit targeted yields. Increases in recta ngularity in the crop geometry of the main crop tends to enhance trans mission of light to shorter crops for longer periods before canopy clo sure. Crops harvested for their vegetative yield appear less sensitive to supra-optimum Population densities within mixtures than do seed cr ops. The period over which intercrops compete for resources can be sho rtened by the supply of external inputs, in as much as they permit gre ater exploitation of the finite supply of light. Supplementary irrigat ion has been shown to raise total productivity in various intercrop sy stems, but little research effort has been turned towards mineral nutr ients. Addition of N fertilizer to legume intercrops reduces the relat ive over-yielding, i.e. compared to mixtures without N fertilizer, but not without overall improvement in total yield. Benefits of residual N on succeeding crops following legume intercrops are also not unsubst antial, and deserve attention when evaluating the merits of intercropp ing. In order to sustain enhanced productivity from intercrops, it wil l become increasingly more important to substitute natural resources w here feasible for purchased inputs. Since the major focus of intercrop research has been on small-scale resource-poor systems, a serious gap in our knowledge on high input intercrop systems will hinder their ra pid spread.