combining ability studies of arex and cimmyt, maize (zea mays L.) inbred lines under stress and non stress conditions

Maize (zea mays L.) is the staple food crop in Zimbabwe with a per capita consumption of 103kg. It is grown in a wide range of environments with 80% being produced by smallholder farmers who occupy more than 90% of the marginal areas of the country. Marginal area production has seen a high hybrid variety turnover on the market hence the need of inbred lines as testers has had shortcomings in the early identification of good inbred lines, resulting in slow variety development, poor seed devalopment and eventual delayed variety release for farmer use. The objectives of this study were to develop single-cross hybrid testers among Agriculture Research and Extension (AREX) and CIMMYT lines as well as determining the hetorotic relationship among the two sets of inbred lines. In the study, testcross development was done using ten elite inbred lines each from AREX and CIMMYT programs. Using North Carolina Design II, the resultant 100 hybrids were evaluated under optimum and stress (low N and drught) environments. An Alpha (0,1) lattice desighn was used in the evaluation process with traits such as flowering dates, standability, disease tolerance scores, plant heights and grain weight being recorded.An across site analysis was done and results showed that there were significant differences (p<0.05) for environments, genotypes, and genotype x environment interactions. Significant general combining ability (GCA) effects for all the traits (p<0.05) measured except for plant heights and stem lodging were observed, with five lines being identified as having good (positive) GCA effects for grain yield. Non addictive genes were also predomonant in most trais except for anthesis dates, anthesis silking interval and ear heights. A total of testcrosses were assigned heteronic groups basin on the N and SC hetoronic groups. Tester Identificatiohn was observed on good GCA for grain yield, stability under diverse environments and maturity genptype. in the N genotype goup, genotype LT52 (NAW5885/CMML442) was identified as a potential single tester in the intermediate maturity group while in the SC heteronic group genotype LT26 (SC5522/ZM621A-BBBB) was identified as another intermediate maturity group tester. In the early maturing category LT99 (RS61P/CML508) which is in the SC heterotic group was identified. The study also showed that there were heterotic group overlaps of the N and SC groups in relation to CIMMYT's A and B heterotic groups as some genotype combinations had to be assigned new heterotic groups or had their group unidentified resulting in the need for further evaluation.