Infertile soil mainly due to low nitrogen (N) has become an important abiotic stress affecting maize production. Progress in breeding for varieties that tolerate low-N stress has been slow due to complex genetic control. Breeders have therefore resulted to the use of secondary traits in multiple traits selection indices for selection of best genotypes under low-N condition. The objectives of this study were to identify promising inbred lines and single cross hybrids that possesses tolerance to low-N stress, and to validate the reliability of multiple traits selection indices using secondary traits and their relationship with combining ability in improving selection efficiency under low-N stress environment. Five parental inbred lines TZEEI-6, TZEEI-21, TZEEI-29, TZEEI-38, and TZEEI-76 were crossed in a complete diallel mating design, and the resulting F1 hybrids were evaluated under low (45 kg/ha) and optimal nitrogen (90 kg/ha) environments. Non-additive gene action controlled grain yield under low-N and optimal-N environments. Additive gene action controlled anthesis-silking interval, leaf senescence, leaf chlorophyll content, and ears per plant under low-N stress and optimal N environment. The inbred lines TZEEI-76 and TZEEI-21 were the best combiners for grain yield under low nitrogen stress. Multiple traits selection indices showed 10 hybrid combinations having tolerance to low nitrogen stress as against 15 hybrid combinations shown by specific combining ability for grain yield alone. Both combining ability and multiple traits selection indices showed that TZEEI-6×TZEEI-29, TZEEI-6×TZEEI-38, TZEEI-21×TZEEI-38, TZEEI-21×TZEEI-76, TZEEI-29×TZEEI-38, TZEEI-29×TZEEI-76, and TZEEI-38×TZEEI-29 combined high yield with tolerance to low nitrogen stress. The secondary traits anthesis-silking interval, leaf senescence, leaf chlorophyll content, and ears per plant should be used in multiple traits selection indices with grain yield to improve selection efficiency of maize genotypes under low nitrogen stress condition.

Keywords: Additive gene, combining ability, diallel mating, low nitrogen tolerance, multiple traits selection indices, secondary traits