“ Heterosis, Combining ability and Tester Identification of CIMMYT maize (Zea Mays L) adapted to low N conditions
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The production of high yielding single cross maize testers with good combining ability and the potential to discriminate materials under evaluation across diverse environments, is important in breeding programs. Testers have a lifespan and the continuous introduction of new and diverse germplasm into a breeding program renders the old testers irrelevant and obsolete in many cases. The objective of the study was to identify new single cross testers to replace CML312/CML442 (Heterotic group A) and CML395/CML444 (Heterotic group B), currently being used in the maize breeding program at CIMMYT Zimbabwe. Ten inbred lines comprising of 4 lines used in the current testers and 6 new elite lines were crossed in a diallel mating design to produce all possible F1 combinations excluding reciprocals. The resultant single cross hybrids were evaluated under optimum moisture and fertilization conditions and low nitrogen stress in Zimbabwe during the 2006/2007 main growing season. General combining abilities (GCA) and specific combining abilities (SCA) were calculated using Griffing’s Method 4 model 1. The single cross CML312/CZL04006 was identified as the best candidate that could replace CML312/CML442 for heterotic group A. CZL04006 had a positive and significant General Combining Ability (GCA) of 0.49 (±0.64) while CML312 had a GCA value of 0.21 (±0.26). The Specific Combining Ability (SCA) for this cross was –1.61 (±0.12) and there was relatively high intra group high parent heterosis for this cross (348 %). The single cross CML444/CZL068 performed better than the current B tester, CML395/CML44 for grain yield across the two environments. CZL068 had the second highest GCA estimate (1.24; SE ±0.31) after CML444 (1.71; SE ± 0.54). The SCA for the cross was –0.06 (±0.24). The intra group heterosis realized was 316 %. CML444 had the highest GCA value of 1.71 (±0.54). The two promising single cross testers identified in this study performed well under optimum and low nitrogen stress conditions, had good combining ability for yield across the tested environments, and had high heterosis. Further testing is needed particularly under drought stress to establish combining abilities and performance as well as to determine stability under a wide range of environments.