Heterotic orientation of tropical maize inbred lines towards populations ZM523 and Suwan-1 under downy mildew infestation

Development of top cross varieties with downy mildew (DM) resistance is one approach to enhance maize productivity in tropical lowland environments. The objective of this study was to determine heterotic orientation of 18 advanced maize inbred lines towards popular open pollinated synthetic populations ZM523 and Suwan-1 under the prevalence of DM. The 36 top crosses, four hybrid check varieties and two testers, ZM523 (Z) and Suwan-1 (S) were evaluated in a 6 × 7 α-lattice design with two replications across three environments. General combining ability effects were significant (P ≤ 0.05) for DM resistance and grain yield, suggesting that genes with additive effects were important in controlling the traits. Specific combining ability (SCA) effects were not significant for DM suggesting small influence of DM resistance by the genes with non-additive effects; but SCA effects were significant for grain yield, indicating that non-additive gene effects played a significant role in governing the grain yield. Based on the SCA data, ten lines were grouped with Suwan-1 and eight lines with ZM523. Using the heterosis data, the lines were fitted into three groups that were designated as S, Z and SZ orientation. The lines ML2, ML30 and ML42, which displayed positive heterosis with both testers for grain yield, were allocated to the SZ-group. The lines ML8, ML10, ML25, ML45, and ML48 exhibited positive heterosis with Suwan-1 and were therefore, classified in the Z-group, and line ML19 that showed positive heterosis with ZM523 was fitted in the S-group. The remaining eight lines did not show any significant and positive heterosis with both testers hence they could not be classified based on heterosis data, suggesting that hybrid breeding efficiency could be improved by expanding the number of testers. Line ML42 displayed the highest level of heterosis with both Suwan-1 (32 %) and ZM523 (29 %) and outperformed all the standard check varieties qualifying it as a potential candidate for further testing. Generally, there was consistency of heterotic grouping of the lines using SCA and heterosis data.