Objective This study aims to investigate the effects of regulated deficit irrigation, phosphorus fertilizer, and organic fertilizer on the yield and quality of alfalfa hay. Additionally, it seeks to determine the optimal water and fertilizer ratio for achieving high yield and quality of alfalfa under irrigation conditions in arid regions.

Methods This study established two irrigation methods: quota irrigation (QI) and regulated deficit irrigation (RDI), alongside two organic fertilizer treatments: organic fertilizer (OF) and no organic fertilizer (-OF). Additionally, three phosphorus fertilizer gradients were implemented: P₀ (0 kg/hm²), P₁ (90 kg/hm²), and P₂ (180 kg/hm²).

Results Regulated deficit irrigation, phosphate fertilizer and organic fertilizer all had significant effects on the yield and quality of alfalfa hay (P < 0.05). The hay yield and crude protein content of alfalfa initially increased and then decreased with increasing phosphorus fertilizer levels, while neutral and acid detergent fibers exhibited an initial decrease followed by an increase (P < 0.05). Compared with the quota irrigation (QI), regulated deficit irrigation (RDI) treatment significantly reduced the hay yield (decreased by 3.43 %), crude protein content (decreased by 7.08 %), dry matter digestibility (decreased by 2.08 %) and relative feeding value (decreased by 6.95 %) of alfalfa (P < 0.05), but the water use efficiency increased by 29.02 %.The application of organic fertilizer (OF) significantly increased the hay yield, crude protein content, and relative feeding value of alfalfa (P < 0.05).

Conclusion Under QI + P₁ + OF treatment, the hay yield of alfalfa reached the highest, and the nutritional quality was the best, followed by RDI + P1 + OF treatment.