Abstract
Stay green (SG) maize was found to have higher grain yield and post-silking nitrogen (N) uptake (PostN) compared with a non-stay green (NSG) hybrid.  To understand the effects of plant density on grain yield (GY) and N efficiency in modern maize hybrids, we compared two modern hybrids (SG hybrid DY508 and NSG hybrid NH101) with similar maturity ratings at three plant densities (45 000, 60 000, and 75 000 pl ha^–1) in 2014 and 2015.  GY, leaf senescence, dry matter (DM) accumulation, N accumulation, PostN, and post-silking N remobilization (RemN) were analyzed.  DY508 and NH101 had similar GY, but DY508 had higher thousand kernel weight (TKW) and lower kernel number (KN) than NH101.  Plant density significantly increased GY in the two hybrids.  On average, over the two years, plant density improved GY in DY508 and NH101 by 18.5 and 11.1%, respectively, but there were no differences in total dry matter (TDM) and post-silking DM (PostDM) between the two hybrids.  Plant density improved leaf N, stem N, and grain N at the silking and maturity stages in 2014 and 2015.  DY508 was lower in harvest index (HI), nitrogen harvest index (NHI), and grain N concentration (GNC) than NH101.  Grain N in DY508 was 2.61 kg ha^–1 less than in NH101, and this was caused by lower GNC and leaf RemN.  On the average, DY508 was 1.62 kg ha^–1 less in leaf remobilized N (leaf RemN) than NH101, but was similar in stem remobilized N (stem RemN; 2.47 kg ha^–1 vs. 3.41 kg ha^–1).  Maize hybrid DY508 shows delayed leaf senescence in the upper and bottom canopy layers in the later stages of growth.  The present study provides evidence that the NH101, which has rapid leaf senescence at the late grain-filling stage, has gained equivalent GY and higher leaf RemN, and was more efficient in N utilization.