Nitrogen Use Efficiency Differentially Influences Grain Yield in Old and New Era Maize Hybrids

Continued improvements in maize (Zea mays L.) nitrogen use efficiency (NUE) will benefit from understanding the physiological processes underlying NUE and the changes that these processes have accumulated over time. In this study, yield and NUE traits were examined in a diverse set of hybrids, consisting of 296 Old and New era inbreds crossed with a common male tester, grown at three nitrogen (N) rates for two years. The Old era inbreds were primarily Plant Variety Protection expired inbreds from the USA but also contained some inbreds originating from other continents. The New era inbreds consisted of pre-commercial inbreds from the USA. New era hybrids showed greater N content at maturity than Old era hybrids due to greater pre-flowering N uptake. While post-flowering N uptake was greater in Old era than New era hybrids, it was insufficient to compensate for the greater pre-flowering N uptake of the New era hybrids. The New era hybrids had greater NUE and yield at all N levels. New era hybrids were larger at flowering and remobilized more N to support grain production than Old era hybrids. Despite yielding less than New era hybrids as a group, some of the Old era hybrids germplasm yielded as well as the highest yielding New era hybrids. Overall, yield improvements over time shown by New era hybrids were due to greater pre-flowering N uptake and growth, compared to greater N accumulation during the reproductive stages in high-yielding Old era hybrids.