Optimizing Nitrogen Fertilizer and Planting Density to Improve Yield and Nitrogen Use Efficiency of Direct Seeded Shaga Rice in Ethiopia

Achieving high yield and quality rice with low environmental impact costs is the main driver of modern rice production systems. However, limited recommendations on optimal nitrogen (N) fertilizer and planting density for site-specific locations in Ethiopia are a challenge for achieving increased rice yield and nitrogen use efficiency (NUE). The purpose of this study was to optimize N fertilizer and planting density to improve the NUE, yield and yield attributes of Shaga rice. A factorial combination of N rates (N ₁₈₄ , N ₂₇₆ & N ₃₆₈ kgha ^− 1 ) and planting density (D ₆₀ , D ₈₀ & D ₁₀₀ kgha ^− 1 was presented as a randomized complete block design with three replications in the field experiments carried out between the 2020–2021 and 2021–2022 cropping seasons. The findings showed that the partial factor productivity of nitrogen (PFPn), NUE, grain yield, leaf area indices, number of filled spikelets, spikletes per panicle, and biomass accumulation of shaga rice were all significantly affected by the interaction between N rates and D. Grain yield and NUE increased by 49.2% and 0.342%, respectively, with optimized N184 kgha-1. Increasing N application rate by N ₃₆₈ kgha ^− 1 resulted in low PFPn (8.4%). N ₁₈₄ kgha ^− 1 application along with D ₁₀₀ kgha ^− 1 significantly increased grain yield (6.7tha ^− 1 ) for Fogera and N ₂₇₆ with D ₆₀ kgha ^− 1 combination attained better grain yield (6.2tha ^− 1 ) for North Achefer. Low temperature stress (14.4 ⁰ C) reduced the number of fertile spikelets. Days to 50% heading significantly delayed (102 days) by combination of D ₁₀₀ and with N ₃₆₈ kgha ^− 1 application rate. The application of a high N dose (N ₃₆₈ kgha ^− 1 ) delayed physiological maturity (157 days) by promoting vegetative growth and leaf photosynthesis. NUE, PFPn, and quality of high yielding Shaga rice with application of high N rate (N ₃₆₈ kgha ^− 1 ), was low and improved by reducing the N rate and increasing D. Low PFPn, 19.2% with application of N ₂₇₆ kgha ^− 1 shows attention should be given to the application of slow – release N fertilizer, to the application of slow-release N fertilizer, split application, and time of application to minimize loss of N. The economic analysis revealed that the combination of N ₁₈₄ and D ₁₀₀ kgha ^− 1 , and N ₂₇₆ and D ₆₀ kgha ^− 1 yielded the highest net income return of 2,722.43 and 2,460.91 US$ ha ^− 1 respectively, in Fogera as the optimal recommendation for achieving high profit with rice production in North Achefer.