Combining ability and heterotic grouping of early maturing quality protein maize inbred lines
Maize is the staple crop for most farmers in the arid and semi arid areas of the tropics. Drought and low soil fertility are major limiting factors in maize production in these areas. Research emphasis has been placed at developing early maturing quality protein maize (QPM) varieties that can yield well under water-limited conditions and low nitrogen stress experienced in drier mid-altitude areas of this region. The general objective of this study was to assess the general combining ability present in CIMMYT's early QPM maize lines and specific combing ability of single crosses under drought stress, low nitrogen and well-watered conditions and group the lines in known heterotic groups. Two studies were conducted. In the first study, 13 inbred lines were crossed in a diallel. Hybrids were evaluated under managed drought, random stress and low nitrogen stress and well watered conditions at six locations. The objective was to evaluate their performance, estimate GCA and SCA effects and determine the phenotypic correlations between grain yield and agronomic traits. Inbred line 1 and 2 had high, positive and significant (P < 0.05) GCA effects for grain yield under all well-watered conditions and managed drought stress. This suggesting that these inbred lines have the potential to be used as parents for developing good hybrids for well-watered conditions as well as drought-stressed environments. Entry 42 was consistent in its top performance for grain yield under all conditions tested. Phenotypic correlations between grain yield across well-watered conditions were positive and significant (P < 0.001) for days to anthesis, plant height and ear height. Under the three stress conditions, the correlation between anthesis silking interval and ears per plant was negative and significant (P < 0.05) indicating a reduced anthesis-silking interval which resulted in an increased number of ears under stress conditions. In the second study, lines were crossed with two inbred line testers. The FI hybrids were evaluated under six locations to estimate general combining ability and specific combining ability effects under drought stress and well watered conditions, and group them into known heterotic groups. Inbred line 45 had high, significant and positive GCA effect for grain yield under managed drought stress and across well-watered conditions. Lines with good GCA in each heterotic group may be used in recycling inbred lines. Based on SCA effects for grain yield, the two inbred line testers grouped 29 lines into heterotic group B, 25 inbred lines into heterotic group A and 11 into heterotic group AB. Entries 55, 60, 87 and 113 were the top performers in terms of grain yield both under managed drought and across well-watered conditions.