Optimizing Water–Nitrogen Coupling to Improve Yield, Nutritional Quality, and Nitrogen Use Efficiency of Sudangrass in Southern Xinjiang

Water and nitrogen (N) are the most critical limiting factors for sudangrass (Sorghum sudanense (Piper) Stapf) growth under drip irrigation in arid oases of southern Xinjiang, yet the quantitative interaction mechanism governing yield–quality–efficiency trade-offs remains unclear. This study employed a quadratic orthogonal regression design to generate 11 water–nitrogen treatment combinations (irrigation: 1800–4200 m3·ha−1; nitrogen: 240–720 kg·ha−1). Agronomic traits, dry matter yield, nutritional quality, and nitrogen use efficiency (NUE) were measured through field experiments in 2023–2024, and quadratic models were fitted to identify optimal water–nitrogen bundles maximizing multi-objective performance. Irrigation volume was the dominant factor controlling the plant height, SPAD index, and stem–leaf ratio, whereas stem diameter depended on the water × N interaction (p < 0.01). The “medium-water × moderate-nitrogen” regime (3000 m3·ha−1 + 480 kg·ha−1) maximized dry matter yield (~28 t·ha−1), NUE (~44%) and forage quality (Relative Feed Value > 135, crude protein ≥ 8.8%). This climate-adaptive precision strategy reduces water use by 30% and nitrogen by 20% while increasing yield by 15% and quality by 15%, providing a sustainable production framework for arid and semi-arid regions of Xinjiang.