Changes in Nitrogen Use Traits Associated with Genetic Improvement for Grain Yield of Maize Hybrids Released in Different Decades

ABSTRACT Further enhancement of maize (Zea mays L.) N‐use efficiency (NUE) will benefit from a thorough understanding of how genetic improvement has shaped N use parameters. Since selection for grain yield has occurred at high N fertilizer rates, our hypothesis was that modern hybrids would have a greater response to supplemental N than hybrids from earlier eras. In 2009 and 2010, 21 single‐cross maize hybrids released between 1967 and 2006 were characterized for grain yield and N use traits. While the ability to acquire mineralized soil N did not change over era, the utilization increased with decade of introduction (0.24 kg kg–1 of plant N [kgplantN–1] yr–1; R2 = 0.37). Increases of grain yield at high N (86 kg ha–1 yr–1; R2 = 0.68) over era were accompanied by increases at low N of 56 kg ha–1 yr–1 (R2 = 0.69). Grain yield improvements at all levels of N were associated with decreased barrenness and increased kernel number expressed on a per‐plant and per‐area basis. Fertilizer N response, NUE, increased at a rate of 0.16 kg kg–1 of fertilizer N (kgN–1) yr–1 (R2 = 0.40). Increased NUE was positively correlated with improved N‐uptake efficiency (r = 0.76, P ≤ 0.001), due to the greater postflowering N uptake of more recent hybrids. The response of grain yield to fertilizer N in current hybrids is more dependent on uptake of fertilizer N than the efficiency of fertilizer N utilization, and approximately two‐thirds of genetic gain for grain yield at high N can be explained by improvements in grain yield at low N.