Heterosis and Combining Ability of CIMMYT's Tropical × Subtropical Maize Germplasm

Crossing maize (Zea mays L.) germplasm from different adaptation groups could result in better utilization of hybrid vigor. Such heterotic response is rarely exploited, due to problems in adaptation of some of these exotic materials. This study was conducted to determine the heterosis and combining ability between tropical and subtropical maize germplasm developed at CIMMYT. Six tropical and five subtropical populations were crossed in a Design-2 mating scheme; the 30 crosses and 11 parents were evaluated in six environments. Data were recorded on days to silk, plant height, and grain yield. Midparent heterosis was calculated for the crosses and general combining ability effects (GCA) estimated for grain yield. The six environments were classified into three mega-environments: tropical (TR), subtropical (ST), and transition and midaltitude (TM). Days to silk was lowest in TR environments (55.4 d), followed by ST (64.6 d) and TM (71.7 d) environments. Significant differences in grain yield were observed for entries, parents, crosses, and GCA effects under TR and ST environments. The three top-yielding crosses that also showed high heterosis were Population 43 × Population 42 (7.43 Mg ha⁻¹), Population 32 × Population 44 (8.31 Mg ha⁻¹), and Population 22 × Population 47 (7.97 Mg ha⁻¹) under TR, ST, and TM environments, respectively. Population 43 among tropical germplasm and Populations 42 and 44 among subtropical germplasm showed positive GCA for yield; both are ideal for hybrid development work. Population 47, which involves temperate germplasm, was a good combiner in TM environments. The results demonstrate the considerable potential that exists for improving the yield performance of subtropical germplasm by crossing to tropical materials; for the tropics, introgressing tropical with subtropical germplasm could be used to improve plant type and maturity. The well-known Tuxpeño × ETO heterotic pattern, observed in many crosses, could be successfully exploited in many of these materials for hybrid development.