Utilization of water and light resources by annual crops under semi-arid conditions when planted between Leucaena leucocephala (Lam.) de Wit hedgerows

Abstract

Agriculture for rural communities in semi-arid areas is a risky exercise. The adverse weather conditions (high evaporative demand and low rainfall) of these regions, coupled with comparatively limited mechanization and often soil of poor chemical and physical constitution make crop production difficult and seldom sustainable. To some this problem represents a conundrum to be solved, to others an inconvenience to be avoided. Reduction of agricultural risk in these communities can be addressed in numerous ways, but should always begin with a thorough investigation of the agricultural imperatives of the community targeted, with the recommended system then tailored specifically to meet these imperatives. Agroforestry (combining trees and crops in agricultural systems temporally or spatially simultaneously) has been recommended for, and often been very successful in, many rural communities in tropical Africa and Asia. Adoption in semi-arid Africa has been less widespread, and often less successful once implemented. Nevertheless, the risk reduction possibilities that agroforestry offers through crop complementarity and yield diversification warrant further investigation into the practice in these regions. These studies sought to quantify yield advantage or disadvantage of hedgerow intercropping or alleycropping systems (combining hedgerow Leucaena leucocephala with various annual alley crops of importance to rural South African communities) compared to pure stands, and to investigate the use and distribution of, and competition for, water and light in these systems. Although alley-cropped annuals seldom outperformed pure stand annual crops, the diversification of yield, enabled by combining trees and crops, compensated for lower yields of annual crops in dry seasons or when yield losses occurred through disease. Whether or not this compensation for yield loss would be to the ultimate benefit of the target community would depend on the relative value of the different products resulting from the intercropping system. Using modifications of the renowned Land Equivalent Ratio and energy contents of products of the system, attempts were made to quantify possible benefits of intercropping in terms valid for the community targeted. Studies of water and light distribution in these systems illuminated often-severe water shortages in alley rows close to hedgerows and showed little benefit in altering row orientation for improved light distribution. Water emerged, as expected, as the primary concern to possible adopters of these systems in semi-arid zones. Crop modeling with mechanistic, biophysically sound models should be considered as an extremely important tool in combining needs of communities with environmental constraints of the area targeted.