Optimizing N Application for Forage Sorghum to Maximize Yield, Quality, and N Use Efficiency While Reducing Environmental Costs

Investigating the responses of forage crop yield, quality, and nitrogen (N) use efficiency to different N application rates is beneficial for guiding proper N fertilization regimes and for reducing reactive N environmental pollution. A field experiment was conducted to investigate the effects of different N application rates on above-ground dry matter yield, forage quality, crop N uptake, N use efficiency (NUE), and ecosystem economic benefits (EEBs) of forage sorghum cultivated on the Longdong Loess Plateau in 2019 and 2020. Five N application rates were tested, namely 0, 80, 160, 240, and 320 kg·ha−1 (referred to as N0, N80, N160, N240, and N320, respectively). The maximum above-ground dry matter yield (22.3 t·ha−1 in 2019 and 18.0 t·ha−1 in 2020) was obtained at an N application of 160 kg·ha−1. Forage sorghum crude protein (CP) content increased significantly with increasing N application rates (the CP content at N320 was 7.4% and 8.6% in 2019 and 2020, respectively). In contrast, neutral detergent fiber (NDF) and acid detergent fiber (ADF) were only affected by high N application rates (NDF and ADF were significantly higher in N320 compared with N0 and N90). The relative feed value (RFV) was significantly higher in N0 compared with N320. Crop N uptake was significantly higher in N160 compared with N0 (25.7% increase to 249.4 kg·ha−1 in 2019 and 40.5% increase to 247.4 kg·ha−1 in 2020, respectively). NUE decreased linearly as N rates increased, but NO3−–N residue (0–200 cm), reactive N loss (Nr loss), and greenhouse gas (GHG) emissions increased. Private profitability and EEB were the largest at N160 (private profitability at N160 was 514.2 USD·ha−1, and EEB at N160 was 392.7 USD·ha−1). Above-ground yield and optimum forage quality must be maximized, while simultaneously safeguarding farmer income and reducing environmental pollution from N fertilizers. Therefore, the optimum N application rate for forage sorghum cultivation in the dry areas of the Loess Plateau is recommended at 160 kg·ha−1.

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