Delaying Application and Reducing the N Rate Enhances Grain Yield and N Use Efficiency in No-Tillage, Direct-Seeded Hybrid Rice

The no-tillage, direct-seeded (NTDS) rice cropping system has attracted considerable attention because of its social, economic, and environmental benefits. However, very limited research has been conducted on optimizing nitrogen (N) management practices to enhance grain yield and N use efficiency (NUE) of rice grown in NTDS. An N fertilization field experiment with two rice hybrids was conducted in 2020 and 2021; the experiment consisted of three N rates (180, 153, and 0 kg N ha(-1); N-180, N-153, and N-0, respectively) and two N-application ratios split among the basal, seedling (three-leaf stage), mid-tillering, and panicle initiation stages (50%, 0%, 30%, and 20% and 0%, 30%, 40%, and 30%; R-1 and R-2, respectively). Although the N rate was 15% lower in the N-153 treatment than in the N-180 treatment, grain yield for N-153 was equal to or slightly higher than that for N-180. N-153 had a higher agronomic efficiency of applied N (AE(N)), partial factor productivity of applied N (PFPN), and recovery efficiency of applied N (REN) compared to N-180 by 10.1-24.7%, 15.0-20.1%, and 1.8-12.6%, respectively. Grain yield, AE(N), PFPN, and REN in R-2 were higher than those in R-1 by 0.8-4.6%, 2.4-19.7%, 0.7-4.6%, and 3.5-30.0%, respectively. The increase in grain yield was due to improvement in the sink size that resulted from larger panicles, higher biomass production at maturity, which was partially attributable to increased biomass accumulation after heading, and a higher harvest index. Our results suggest that it is feasible to simultaneously improve grain yield and NUE in hybrid rice under NTDS through delayed and reduced N application rate, and current breeding programs need to target large panicle size as a primary objective for NTDS rice.