Objectives The efficient summer maize production relies on academic management and machinal grain harvesting. The optimum plant density and nitrogen application for efficient summer maize production in North China plain were studied.

Methods Summer maize cultivar Jingnongke728 was used as the research materials. The split-plot experiment design was adopted: the main plots were two plant densities 7.5×10⁴ plants/hm² (D_7.5) and 9.0×10⁴ plants/hm² (D_9.0), while five N application rates (0, 180, 240, 300 and 360 kg/hm² tagged as N₀, N₁₈₀, N₂₄₀, N₃₀₀ and N₃₆₀, respectively) were the sub-plots. The plant’s leaf SPAD value, leaf area index (LAI), dry matter accumulation (DM), grain filling, grain yield and its components, nitrogen partial factor productivity (PFP_N) and agronomic efficiency (AE_N) of maize under different planting densities and N application were measured.

Results Compared with D_7.5, LAI and DM of D_9.0 treatments at V6 and R1 stage were increased by 5.0%–26.3% and 3.7%–34.8% respectively. The maximum grain filling rate (G_max) of D_9.0 treatment was 0.35–1.33 g/(100-grain·d) higher than that of D_7.5, and the date of reaching the G_max (T_max) was 4.4 d earlier in D_9.0. The grain filling duration in D_9.0 were shortened by 6.9–12.2 days, but ear grain number reduced by 15.0–51.3 grain. The grain yield of D_9.0 was 2.4%–28.3% higher than D_7.5, which was 7.36×10³–12.22×10³ kg/hm². The D_9.0 significantly increased PFP_N and AE_N by 10.1%–17.2% and 72.0%–94.4%, respectively. Compared with N₀, summer maize leaves SPAD values increased significantly with increase in N application. The LAI and DM also increased by 3.5%–171.3% and 5.0%–177.7%, respectively, with increase in N application. The G_max reached the highest 1.33–1.39 g/(100-grain·d) at N₂₄₀–N₃₆₀ levels, and the T_max of N treatments were 4.1–4.6 days earlier than N_0. The G_max weight (W_max) also increased by 5.7%–9.4% with increase in N application. Therefore, with increase in N application, 100-grain weight increased by 0.5%–18.4%, grain number per ear increased significantly by 62.8–79.2 grain, and yield increased by 3.2%–115.7% (i.e., 10.10×10³–11.33×10³ kg/hm²). With increase in N application, PFP_N, AE_N were reduced by 24.2%–46.6% and 21.2%–43.1%, 21.1%–32.5% and 13.0%–32.9%, respectively, under D_7.5 and D_9.0 treatments, respectively.

Conclusions Higher density of summer maize can effectively improve the dry matter accumulation, reduce grain moisture content to 15.4%–24.8%, and satisfied grain mechanical harvesting. Controlling nitrogen application rate at 180–240 kg/hm² could further reduce the moisture content at grain harvest, improve leaf SPAD value, LAI and dry matter accumulation, improve grain filling and increase grain weight. The yield was stable at 10.5×10³–11.2×10³ kg/hm², and the PFP_N and AE_N were 47.0–59.7 kg/kg and 27.6–30.9 kg/kg. Therefore, the suitable density of summer maize for machinal grain harvesting was 9.0×10⁴ plants/hm², and the nitrogen application level was 180–240 kg/hm².