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

Comparison of old and new hybrids (era studies) is a classic approach to understanding the physiological differences associated with genetic improvement for agronomic traits like grain yield and stress tolerance. N use has been a largely unexplored trait for era studies. Further improvement of maize N use efficiency (NUE) will benefit from a thorough understanding of how past and present genetic improvement has shaped N use parameters. In 2009, we characterized 21 maize hybrids released in different years ranging from 1967 – 2006 for grain yield and N use traits. These hybrids were grown under three different levels of N availability in an N responsive evaluation nursery at Champaign, IL. Several traits were evaluated including plant biomass and N content at physiological maturity, grain yield, kernel weight, kernel number, and derived N use measures like NUE, N uptake efficiency (NUpE), N utilization efficiency (NUtE), and genetic utilization (GU). Our hypothesis was that continued selection for grain yield under high levels of N fertilizer has resulted in modern hybrids with a larger response to fertilizer N than hybrids from earlier eras.

The results of this study show that the response of grain yield to fertilizer N has remained relatively constant over the past 30 years. Increases in grain yield at high N (0.9 Mg ha^-1 decade^-1) have coincided with increases in grain yield at low N (0.6 Mg ha^-1 decade^-1). Genetic utilization which describes the physiological efficiency of N utilization for grain production at low N increased with decade of introduction (3.3 kg_grain kg_plantN^-1 decade^-1), while NUE which is reflective of fertilizer N response did not significantly differ between modern and historical hybrids. Most of the historical improvement in grain yield at all levels of N resulted from increasing kernel number per plant although individual kernel weight also trended higher. It is not clear how selection for improved grain yield at high N might also indirectly improve grain yield at low N, but selection for stress tolerant hybrids with yield stability across multiple environments might be one explanation.

Although NUE has remained relatively unchanged over time, its component traits appear to be affected by genetic improvement. N uptake efficiency (NUpE) trended higher with decade of hybrid introduction while N utilization efficiency (NUtE) trended lower. These results suggest that the response of grain yield to N in the most current hybrids is more dependent on uptake of fertilizer N than the efficiency of utilization. Further efforts to develop maize hybrids for high grain yield under lower N inputs will require a greater understanding of the physiological mechanisms and genetic architecture underlying N utilization efficiency.