Agronomic, yield and yield component responses of lowland rice (Oryza sativa L.) variety (Shaga) to nitrogen and phosphorus fertilizer rates in Dembia District, northwestern Ethiopia

Abstract

Rice (Oryza sativa L.) is an important food security crop in Ethiopia, particularly in the Dembia Plain; where productivity remains below its potential because of the lack of site-specific fertilizer recommendations. This study aimed to determine the agronomic and economic optimum rates of nitrogen (N) and phosphorus (P₂O₅) fertilizers for lowland rice production in the Dembia District of northwestern Ethiopia. Field experiments were conducted during the 2021 and 2022 main cropping seasons at two sites using a factorial combination of four N rates (0, 92,184, and 276 kg N ha^-1) and four P₂O₅ rates (0, 23, 46, and 69 kg P₂O₅ ha^-1) arranged in a Randomized Complete Block Design (RCBD) with three replications. Growth, yield, and yield-related parameters were subjected to analysis of variance using R software (version 4.4.2). Grain yield response was evaluated using regression analysis, while economic feasibility was assessed through partial budget analysis. Nitrogen significantly (p < 0.01) influenced all recorded parameters, whereas phosphorus significantly affected effective tiller number, thousand-seed weight, aboveground biomass yield, and grain yield. Grain yield exhibited a significant quadratic response to the interaction of N and P₂O_5, indicating increasing returns up to the optimum level followed by diminishing returns. The highest observed grain yield (4576 kg ha⁻¹) and predicted (4239 kg ha⁻¹) were obtained at the application of 276 kg N ha^-1 combined with 46 kg P₂O₅ ha^-1. However, the partial budget analysis identified the application of 92 kg N ha^-1 combined with 23 kg P₂O₅ ha^-1 as the most economically profitable treatment, providing the highest net benefit (69,760 Birr ha^-1) with an acceptable marginal rate of return (118%). Therefore, the combined application of 92 kg N ha^-1 and 23 kg P₂O₅ ha^-1 is recommended for lowland rice production in the Dembia District and other similar agro-ecologies. Further validation across locations, soil types, and growing seasons is recommended to improve wider applicability.