Combined application of blended NPSB fertilizer and cattle manure improves yield and economic returns of sweet potato (Ipomoea batatas (L.) Lam) at Bako, West Shewa Zone, Ethiopia

Sweet potato ( Ipomoea batatas (L.) Lam) is an economically important food security crop in Ethiopia, but its productivity remains low due to poor soil fertility and lack of site-specific fertilizer recommendations. This study evaluated the combined effect of blended NPSB fertilizer (0, 50, 100, and 150 kg ha⁻¹) and cattle manure (0, 5, and 10 t ha⁻¹) rates on yield and yield components of sweet potato (variety Tola) at Bako, Ethiopia, during the 2019/2020 season under furrow irrigation. The experiment was laid out in a randomized complete block design with factorial arrangement and three replications. Treatment effects were highly significant ( P < 0.001), accounting for 99.3% of total yield variability. The highest tuber yield (23.66 t ha⁻¹), marketable yield (23.33 t ha⁻¹), and net benefit (81,440 Birr ha⁻¹) were recorded for T12 (150 kg NPSB ha⁻¹ + 10 t CM ha⁻¹). Both NPSB (estimate = 0.062, P < 0.001) and cattle manure (estimate = 0.401, P < 0.001) showed significant positive effects, with additive rather than synergistic interaction ( P = 0.972). Response surface methodology predicted an optimum of 162.5 kg NPSB ha⁻¹ and 11.2 t CM ha⁻¹ (yield: 24.15 t ha⁻¹), but this extrapolation lies outside the tested range and requires experimental validation. The 150 kg NPSB ha⁻¹ + 10 t CM ha⁻¹ treatment is preliminarily recommended for on-farm validation trials in Bako and similar zones. However, these findings are limited to a single season and location; multi-season, multi-location validation under both irrigated and rainfed conditions is recommended to further strengthen these findings.