Expression of tolerance to drought and low nitrogen levels in maize inbred lines and hybrids in southern Africa
The increased incidence of drought and low fertility challenges in southern Africa emphasizes the continued need for innovations and technologies to improve the productivity of the maize-based production systems in the region. This region depends mainly on maize for food security, and thus breeding for drought and low N tolerance has been, and will continue to be a focal point and major objective in sub-Saharan Africa. The hybrids tested in this study were developed from CIMMYT maize inbred lines selected for drought and low N tolerance, for the tropical and sub-tropical regions. Trials were conducted in Zambia, Zimbabwe and South Africa under managed drought, low N stress and optimum conditions. This study was conducted to determine the combining ability of lines and testers and heritability of yield and agronomic characteristics of early and late maturity maize hybrids generated from a line x tester crossing design, to determine the testcross performance of the developed hybrids and to determine yield stability of early and late maturity hybrids using AMMI and GGE models. Yield reduction due to stress conditions of 28.6-79.0% was observed for hybrids grown under low N and drought conditions. Late maturity maize hybrids face a larger risk of exposure to drought conditions that eventually reduces yield drastically. The reduced yields under random drought and low N stress observed in this study indicated the potential threats to maize-based production systems in the southern African region. Combining ability studies indicated the importance of both additive and non-additive effects across the stress environments. GCA of lines and testers and SCA effects for hybrids were significant across all locations, which indicates the importance of both additive and non-additive gene action in the genotypes evaluated. Lines which showed positive GCA effects for grain yield across all the environments can be successfully utilized as potential sources for hybrid breeding programmes across areas where drought and low N stresses are a challenge. Testcross performance results indicated that several hybrids (both early and late maturity) performed better than the local commercial checks evaluated and warrants further evaluation for stability and consistency, and can be recommended for use as hybrids across varied environments in southern Africa. AMMI and GGE models were efficient in differentiating the performance of maize hybrids across the test environments. Several hybrids performed better than the local commercial checks, indicating their suitability as potential cultivars under stress and non-stress environments. Entries 46, 82, 32, 15, 100, 6, 21 and 83 (early maturity hybrids) and entries 109, 115, 22, 63, 1, 24, 21, 20, 2, 19, 5, 6, 10, 14, 25, 9, 108 and 114 (late maturity hybrids) performed better than all commercial check hybrids and were consistently identified by the AMMI and GGE biplots as performing above average in terms of yield and stability, and warrants recommendation as hybrids under both stress and non-stress environments in southern Africa. The results indicated the success story of the developed drought and low N stress hybrid breeding programme in reducing the effects of these stresses, which will help to sustain and improve the efficiency of the maize-based production systems in southern Africa, and other regions of sub-Saharan Africa where these stresses are intense.