PROCESSES DETERMINING INTERCROP PRODUCTIVITY AND YIELDS OF COMPONENT CROPS

This review examines how intercrop productivity is determined, by analysing several key physiological processes which affect yields of component crops. Availability of environmental resources to each of the component crops is important in determining combined intercrop productivity, and hence the analysis is based on capture of environmental resources and efficiency of conversion of captured resources into growth of harvested organs of the component crops. It is emphasized that the competitive abilities of component crops, which determine their biomass production and often yields, vary greatly according to growth environment, and hence cultural manipulation can adjust the balance of their yields. Intercrops are most productive when their component crops differ greatly in growth duration so that their maximum requirements for growth resources occur at different times. For high intercrop productivity, plants of the early-maturing component should grow with little interference from the late-maturing crop. The latter may be affected somewhat by the associated crop, but a long time period for further growth after the harvest of the first crop should ensure good recovery and full use of available resources. Compared with a sole crop, the reduced size of non-harvested organs of the late-maturing crop can result in improved assimilate partitioning to the harvested organ during the later part of the growth period and consequently a higher harvest index. Because of the differences in growth rhythm between component crops, there tends to be little interaction between relative performance of component crops and growth environment and hence productivity of this type of intercrop is often insensitive to management interventions. In contrast, when growth durations of component crops are similar the crops compete more intensely for available resources. Their relative performances can then be greatly affected by small changes in growth environment. 'Additive' intercrops of this type may nevertheless be productive, particularly where growth resources are more completely captured than in corresponding sole crops. However, if non-replenished growth resources are utilized too rapidly, the less-competitive component may suffer greatly. 'Replacement' intercrops of this type are not so productive in high-yielding environments. When the growth environment is not favourable however, their total lower plant populations compared to additive intercrops may allow yields of replacement intercrops to be less depressed. Where similar-duration crops are grown in variable environments, replacement intercrops may therefore be preferred due to their greater yield stability. Where a dominant crop uses available resources excessively and inefficiently, agronomic manipulation in favour of the usually suppressed component seems most likely to improve the productivity of the whole intercrop. Intercrop productivity depends on the genetic constitution of component crops, growth environment (atmospheric and soil) and agronomic manipulations of microenvironment. The interaction of these factors should be optimized so that the limiting resource is utilized most effectively in the inter crop. An understanding of the sharing of resources among component crops will help identify more appropriate agronomic manipulations and cultivars for intercrops.