Hybrid era and management practices like nitrogen (N) rate and plant density influence canopy traits and their correlations with grain yield, kernel number, and kernel weight in maize (Zea mays L.). A four site-year study was conducted employing two N rates (55 and 220 kg N ha(-1)) with three plant densities between 54,000 and 104,000 plants ha(-1) for two newer hybrids (2005) and two older hybrids (1967 and 1975). Hybrids varied in anthesis-silking interval, specific leaf nitrogen (SLN), specific leaf area, and leaf greenness (soil plant analysis development [SPAD] readings), but not in leaf area index (LAI) at silking. Consistently higher kernel weight in newer hybrids (15% in 2012, 23% in 2013) across management treatments was related to their higher SLN at silking and green leaf number retention during grain filling. The threshold LAI at silking for maximum kernel number for 2005 hybrids (averaging 3917 kernels m(-2)) occurred at 4.0 m(2) m(-2), compared with 3.28 m(2) m(-2) for the 1975 hybrid (3893 kernels m(-2)), and kernel number declined when LAI exceeded 3.43 m(2) m(-2) for the 1967 hybrid. Higher leaf biomass, leaf N content, and therefore higher SLN at silking in newer hybrids contributed to greater green leaf retention during grain filling. Applying more N weakened correlations between grain yield and canopy attributes, including leaf retention in both years and yield correlations with leaf biomass, leaf N content, LAI, and SLN in 2013 (more favorable weather). Grain yields for all hybrids in 2013 had higher correlations with LAI at silking and during grain fill when grown at higher densities.
