Agronomic modification of resource use and intercrop productivity

Abstract

Suitable land areas for food production remain fixed or are diminishing, yet farmers and agronomists are faced with the task of increasing production. Raising productivity, through a more effective use of natural (e.g. light) and added (e.g. fertilizer) resources, is possible through intercropping, provided component crop demands for resources are well understood. Management of intercrops to maximize their complementarity and synergism, and to minimize competition between them follows simple natural principles, and its practice is limited only by the imagination of farmers and agronomists. Successful crop mixtures extend the sharing of available resources, over time and space, exploiting variation between component crops in such characteristics as rates of canopy development, final canopy width and height, photosynthetic adaptation of canopies to irradiance conditions and rooting depth. Occasionally, commensalism is effected. Loosely defined as one organism gaining benefits from another without damaging or benefiting it, it is exemplified when one crop modifies the microenvironment to suit another. Prime examples are the benefits of shading during crop, particularly transplanted crop, establishment under hot or dry conditions, the supply of nitrogen and solubilization of phosphorus by legumes for companion crops, and the suppression of weeds through direct competition or allelopathic effects. The onset of competition between intercrops can be delayed by judicious choice of relative planting dates. The differential influence of weather (in particular temperature) on component crop growth 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 first. Choice of plant population density and crop geometry, including row orientation, permits a planned sharing of natural resources and manipulation of competitiveness to suit targeted yields. Increases in rectangularity in the crop geometry of the main crop tends to enhance transmission of light to shorter crops for longer periods before canopy closure. Crops harvested for their vegetative yield appear less sensitive to supra-optimum population densities within mixtures than do seed crops. The period over which intercrops compete for resources can be shortened by the supply of external inputs, in as much as they permit greater exploitation of the finite supply of light. Supplementary irrigation has been shown to raise total productivity in various intercrop systems, but little research effort has been turned towards mineral nutrients. Addition of N fertilizer to legume intercrops reduces the relative 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 unsubstantial, and deserve attention when evaluating the merits of intercropping. In order to sustain enhanced productivity from intercrops, it will become increasingly more important to substitute natural resources where 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 rapid spread.