Heterotic patterns and combining ability of stress tolerant maize lines under optimum and low nitrogen conditions

Locally adapted S sub(2) lines developed from introduced stress tolerant S sub(1) lines were evaluated for heterotic patterns and combining ability under optimum and low nitrogen levels. One-hundred twenty-four F1 test-cross hybrids obtained by crossing 62 S sub(2) African Maize Stress (AMS) lines to two Ethiopia testers (Gutto LMS- 5 and SC-22) were used for the study. Data were recorded on ear height, plant height, days to maturity and grain yield. General combining ability (GCA) and specific combining ability (SCA) effects were estimated by line x tester analysis. Under optimum nitrogen, the effects of entries, crosses, lines and testers were highly significant for all traits. Line x testers interaction was highly significant for days to maturity, indicating the variations among crosses in their specific combining ability. In case of low nitrogen, mean squares due to entries, crosses and testers were highly significant for all traits. Differences among lines were significant for days to maturity and grain yield. Line x tester interaction was significant for plant height and grain yield. A number of crosses outsmarted the checks (BH-140, BH-540 and Kuleni) in agronomic performances and grain yield under both fertility levels. Estimates of GCA effects showed that lines CML221 /[AC8342 /IKENNE\{1\} 8149SR //PL9A]C1F1-500-4-X=Sn]-B-B2 and [[[NAW5867/P30-SR]-25-1]-9-2-3B-2-B/CML-388]-B-1 were good combiners for early maturity while line [CML312/[M37W/ZM607[hash]BF37SR-2-3SR..]]-B-4 was the best combiner for grain yield under both fertility conditions. Only few cross combinations showed significant SCA effects. So, the results of this study did not clearly exhibit the heterotic grouping of the lines. This might be either the two testers could not provide the best discriminations among genotypes or the lines used were genetically broad based as they were at early stages of inbreeding. This suggests the need for development of two genetically diverse populations. Heterotic patterns of the lines have also to be re-evaluated at advanced generations of inbreeding.