Impacts of Nitrogen Fertilization and Row Spacing on Forage Nutritive Value and Seed Yield of Eragrostis nigra

Despite the potential of Eragrostis nigra in alleviating forage shortages and controlling soil erosion, there is little information about how nitrogen fertilization and row spacing affect its forage and seed production. Thus, a two-year field experiment was conducted to explore the effects of nitrogen (N) fertilization (0, 45, 90, 135 kg hectare−1 (ha−1)) and row spacing (20 cm, 40 cm) on its forage yield, nutritive value, seed yield and quality. Results showed that forage dry matter yield (DMY) and harvested seed yield (HSY) increased with higher N rates; however, no further enhancement was observed at 135 kg per ha−1, making 90 kg ha−1 optimum. This may be because excessive nitrogen disrupts the carbon–nitrogen metabolic balance. N fertilization increased the relative feed value by enhancing crude protein content and decreasing acid detergent fiber. Relative to the 20 cm spacing, the 40 cm spacing averagely raised PSY and HSY by 34.7% and 21.3%, respectively. The quadratic regression model yielded maximum predicted HSY values of 27.6 and 26.7 kg ha−1, achieved at optimal N rates of 135 and 98.6 kg ha−1 for the 20 cm and 40 cm spacings, respectively. Thousand-seed weight (TSW), germination index, and vigor index were significantly improved by row spacing and N application (p < 0.05). Seed yield enhancement from N was positively correlated with increases in number of fertile tillers (FT), number of spikelets per tiller and seed set (p < 0.05). Furthermore, FT and TSW had the strongest direct effects on seed yield, with standardized path coefficients of 0.362 and 0.390. Our findings highlight that an application rate of 90 kg ha−1 N combined with 40 cm row spacing could be recommended as the efficient agronomic practice for maximizing forage yield, nutritive value, and seed yield in E. nigra. Importantly, this regime also enhances economic returns while reducing N leaching risks, thereby supporting more sustainable forage production systems.