Heterosis and Combining Ability among Subtropical and Temperate Intermediate‐Maturity Maize Germplasm

Information about combining ability and heterotic patterns among CIMMYT's maize ( Zea mays L.) gene pools and populations should assist research programs in their hybrid development activities. A nine‐parent diallel was formed to (i) study the combining ability of CIMMYT's subtropical and temperate intermediate‐maturity maize gene pools and populations and (ii) evaluate the potential of CIMMYT's subtropical and temperate materials as exotic germplasm sources for temperate breeding programs. Parents and crosses were evaluated in 5 environments in Mexico and 11 in the USA. Highly significant, positive general combining ability (GCA) effects for yield were observed with Populations 42, 47, and 34 in Mexico. The Population 42 × Population 47 cross ranked first in yield (7.87 Mg ha −1 , with 9.1% high‐parent heterosis). Population 33 x Population 45 was the only cross with a significant, positive specific combining ability (SCA) effect for yield. Average yields in U.S. environments were low (3.49 Mg ha‐I), due partially to adaptation problems and late plantings. Only Pool 41 had a significant, positive GCA effect for yield in the USA. The four highest yielding combinations included Pool 41 with Populations 42, 47, 34, and 45. In Mexican subtropical environments, superior crosses included Population 42 × Population 47 (white grain) and Population 33 × Population 45 (yellow grain). Though the CIMMYT materials did not generally perform well in U.S. environments, Pool 41 and Population 42 showed the best potential as exotic germplasm sources for temperate breeding program