Abstract: This research explored the effects of different nitrogen and phosphorus fertilizer rates and their interaction on productivity and nutritional value of alfalfa under drip irrigation in the oasis region of Xinjiang. A randomized block design of two nitrogen levels (105 and 210 kg·ha^-1) and four levels of P₂O₅ (0, 50, 100 and 150 kg·ha^-1) was used, making a total of 8 treatments (N₁P₀, N₁P₁, N₁P₂, N₁P₃, N₂P₀, N₂P₁, N₂P₂, N₂P₃). Growth traits, hay yield and the nutritional value of the alfalfa were studied. It was found that under the N₁ treatment, the plant height, stem diameter, growth rate, hay yield and crude protein of alfalfa in the P₂ treatment were greater than other treatments in the first three cuts. However, under the N₂ treatment these same responses were greater than other treatments in the P₁ treatment. The contents of acid detergent fiber and neutral detergent fiber in alfalfa leaves and stems in the P₂ treatment were lower than other treatments at each cut under N₁ and N₂ treatments. The plant height, stem diameter, growth rate, hay yield and crude protein of alfalfa in the N₁ treatment were higher than the N₂ treatment under P₀, P₂ and P₃ treatments. Under the same phosphorus application conditions, the contents of acid detergent fiber and neutral detergent fiber in the N₁ treatment were lower than the N₂ treatment in leaves and stems of alfalfa. Through grey correlation analysis between growth traits index and hay yield of alfalfa it was shown that growth rate and stem diameter had major contributions to hay yield of alfalfa, but the plant height and stem∶leaf ratio had lesser contributions. Through fuzzy similarity priority ratio, the optimal fertilization mode at each cut of alfalfa among the eight treatments under drip irrigation was identified as the N₁P₂ treatment. Under this regime alfalfa produced a high hay yield of 25103.19 kg·ha^-1, with a leaf protein content range of 23.60%-26.47% and a stem protein range of 10.57%-11.76%, together with lower acid detergent fiber contents (leaf: 13.28%-17.41%, stem: 38.63%-47.21%) and lower neutral detergent fiber contents (leaves: 18.18%-22.93%, stem: 49.53%-59.83%). Therefore, the combination of nitrogen (N) at 105 kg·ha^-1 and phosphorus (P₂O₅) at 100 kg·ha^-1 were optimal for high hay yield and improved nutritional quality of alfalfa under drip irrigation in the Xinjiang oasis area.

Key words: alfalfa, nitrogen and phosphorus interaction, drip irrigation, hay yield, nutritional quality