One‐Time Top‐Dressing of Panicle Nitrogen Fertilizer Maintains High Grain Yield and Nitrogen Use Efficiency of Rice in South China

Optimized nitrogen (N) management for rice production typically involves multiple split applications of N fertilizer to enhance rice yield and nitrogen use efficiency (NUE). However, China's rural labor shortage highlights the need to reduce the frequency of fertilizer applications to minimize labor inputs and improve production efficiency. A simplified N management involved less application frequency at critical stages in rice production, but its effectiveness was still uncertain. Thus, a field experiment was conducted in both early and late seasons of 2018 with 5 treatments: N0 (no N fertilizer), TC (optimized N management involving application at tillering, panicle initiation (PI), and grain filling stages); N1 and N2 (one‐time N top‐dressing at uppermost internode elongation (IE) or PI with the same total N input as TC); and RN1 and RN2 (one‐time N top‐dressing at IE or PI with a reduced total N input compared to TC). The effects of N management on grain yield, yield components, agronomic traits, biomass production, N uptake, and NUE were evaluated. In both seasons, N2 achieved grain yield and NUE comparable to TC, whereas N1 exhibited significantly lower grain yield, agronomic N use efficiency (AE N ), partial factor productivity (PFP N ), and physiological N use efficiency (PE N ). Furthermore, RN1 and RN2 resulted in significant yield reductions in the late season relative to TC. These suggested that tiller and grain N fertilizers might be omitted, and one‐time N top‐dressing at PI stage could effectively support high grain yield and NUE. The comparable performance of N2 and TC was primarily attributed to an optimal number of panicles and filled grains, as well as similar total biomass production and N uptake, resulting from the appropriate timing of panicle fertilizer application. Overall, N2 represented a labor‐saving N fertilizer management strategy that sustained high grain yield and NUE under current labor‐constrained production systems.