Maize nitrogen use efficiency improved due to density tolerance increase since the 1950s.

Meeting maize (Zea mays L.) production targets of high grain yields and nitrogen use efficiency (NUE) requires the development of stress‐tolerant cultivars and corresponding cultivation practices that maximize efficiency. Therefore, our objectives were to (i) determine the impacts of N application rate, planting density, and maize cultivar (which have improved with each successive release date) on the N partial productivity factor (PFPN) and N agronomic efficiency (AEN), and to (ii) characterize the responses of maize cultivars indexes to different cultivation conditions. We found that maize PFPN and AEN values were affected by N application, planting density, and cultivar. The PFPN and AEN declined with increasing N but increased with increasing planting density or genetic improvement (i.e. later year of release). The responses of PFPN and AEN to planting density and N application, taken either as single factors or as interacting factors, were greater for recently released cultivars than for older cultivars. The PFPN and AEN values were not automatically associated with maize yields, indicating that the genetic improvements that led to increases in PFPN and AEN were due to unconscious selection. Thus, improvements in NUE resulted from improvements in cultivars' tolerance to planting density stresses and from cultivation practices that employed higher planting densities from the 1950s to 2010. [ABSTRACT FROM AUTHOR]