Relationships Among Agronomic Traits of Tropical Early Maturing Maize Cultivars Under Low-N and High-N Environments.

Tolerance to low-N is crucial to reduce maize losses from low-N stress and increase productivity in West and Central Africa. Secondary traits can improve the precision with which low N tolerant genotypes are identified compared to measuring only grain yield under low-N. Therefore breeders selecting genotypes for tolerance to low soil nitrogen use a base index that combines high yield (YLD) with increased ears per plant (EPP), improved plant aspect (PASP), ear aspect (EASP), short anthesis-silking interval (ASI) and good stay green characteristics (LDTH). However, the value of these traits in an index for selecting early maturing genotypes under low soil nitrogen has not been assessed. Experiments were thus conducted under low-N and high-N in 2008 and 2009 at two locations in Nigeria to examine the performance of early maturing genotypes based on interrelationships among multiple traits. In Genotype-by–Trait biplot analysis 2TY was the top-ranking genotype, while T5A was the lowest ranking genotype identified under low-N. Under high-N, TW4 was the best ranking genotype, while BB was the worst. Eleven of the 28 genotypes were above-average under low-N, while 12 were above-average under high-N. Under low-N, 7 genotypes were closest to the ideal genotype. The best traits for selecting for tolerance to low-N were YLD, PLHT, EASP and PASP. Under high-N, the best traits were YLD, PASP and EASP. Under low-N, traits were ranked based on discriminating power and representativeness for selection for tolerance to low soil nitrogen and it was found that PLHT and HUSK should be included in the base index for selecting tolerant genotypes.  However, since PLHT is usually correlated with maturity and YLD, and in order not to compromise maturity, a restricted index that pegs both maturity and PLHT would be ideal in selecting for tolerance among early maturing genotypes.