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Combining ability of inbred lines is important information in maize hybrid breeding programs incorporating materials from various germplasm sources. This study was conducted with germplasm varying in their maize streak virus (MSV) reaction, genetic, and geographic origin. In a half diallel cross of six parents, general and specific combining abilities were determined for grain yield, days to silking, ear height and MSV disease resistance. The grain yields of the single crosses were significantly higher from those of the hybrid checks, H614 and H513, which are popular varieties. The importance of both GCA (53%) and SCA (47%) for grain yield was observed, but a preponderance of GCA existed for days to silking, ear height, and MSV disease resistance. The best general combiners for grain yield and MSV resistance were CML197 (0.92, p<0.001) and C92 (-0.61 p<0.001) respectively. The prediction of hybrid performance for grain yield and MSV resistance could be based on GCA alone, but this should be within specific niches since GCA interacted highly with environments.

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The spotted stem borer, Chilo partellus, Swinhoe (Lepidoptera, Crambidae) is one of the most destructive insect pests of maize in tropical environments. However, the combining ability and heterotic orientation of the germplasm for grain yield and borer resistance is limited. The objective of this study was to determine combining ability and heterotic orientation of new maize inbred lines under C. partellus infestation. Sixty six (66) inbred lines were crossed to two single cross testers in a line x tester mating scheme. The 132 testcross hybrids and four checks were evaluated at three locations in Kenya under C. partellus infestation. General and specific combining ability effects were significant for C. partellus stem borer resistance traits and grain yield, suggesting the importance of both additive and non-additive gene effects for these traits. Heterotic classification of lines was done based on both heterosis and specific combining ability data. Based on heterosis for grain yield data at Embu, 15 lines were allocated to group A, 18 to group B and 12 to group AB. At Kakamega, 26 lines were oriented towards group A, 19 to group B and 9 to group AB. At Kiboko, 15 lines were inclined towards group A, 18 to group B and 11 to group AB, whilst the remainder could not be classified. Based on the SCA estimates, at Embu, 10 lines revealed positive SCA effects with both testers and were considered to be AB-oriented while 8, and 1 lines were oriented towards A, and B, respectively. A similar trend was detected at Kakamega and Kiboko. The identified lines and heterotic groups would be used by hybrid maize programs where C. partellus stem borers occur exclusively or in league with other stem borers.