Abstract: The effects of water and nitrogen supply on the growth, yield and water and nitrogen use efficiency of alfalfa were investigated to find out the best combination of water and nitrogen in the production of alfalfa and to provide scientific reference data for actual production under drip irrigation in the Yellow River irrigation region of Ningxia. A field experiment using a split-plot design was used. There were four irrigation levels (W₁, 480 mm; W₂, 550 mm; W₃, 620 mm; W₄, 690 mm) in main plots and four nitrogen levels (N₀, 0; N₁, 60 kg·ha^-1; N₂, 120 kg·ha^-1; N₃, 180 kg·ha^-1) in sub-plots. The results showed that water and nitrogen supply had different effects on the plant height and stem diameter of alfalfa for each harvest. The plant heights of first and second cutting of alfalfa increased with increased nitrogen application rate and with increased irrigation supply, and the stem diameter at the first cutting was increased with increased irrigation rate. The yields of alfalfa hay at the first and second cutting increased with increased irrigation rate. Applied nitrogen significantly increased the alfalfa hay yield at the first and fourth cuttings and over the whole growth season. The irrigation effect, the nitrogen application effect, and the irrigation and nitrogen interaction for hay yield were all extremely significant (P<0.01). The irrigation water use efficiency (IWUE) and water use efficiency (WUE) were decreased with increased irrigation rate, and were increased by nitrogen application. The treatment W₄N₀ had the lowest IWUE and WUE.Alfalfa nitrogen use efficiency (ANUE) showed different patterns with different nitrogen fertilizer and irrigation rates. In the W₁, W₂ and W₃ treatments, ANUE was greatest in the N₂ treatment and reduced at higher or lower N application rates, while in the W₄ treatment, ANUE was decreased with increased nitrogen application rate. Nitrogen partial factor productivity (PFPN) was decreased significantly with the increase of nitrogen application rate. ANUE was decreased by irrigation at lower water application rates but was increased by irrigation at higher application rates, while PFPN showed the opposite pattern with highest PFPN at intermediate irrigation rates, indicating that irrigation at an appropriate rate can improve the ANUE and PFPN of alfalfa. From these results, to optimize hay yield while conserving water and fertilizer resources, W₃N₂ (620 mm, 120 kg·ha^-1) was indicated as the best irrigation and fertilizer treatment combination for alfalfa under drip irrigation in this region. The study provides valuable information for improving management of alfalfa in the Yellow River irrigation region of Ningxia.

Key words: alfalfa, water and nitrogen supply, production performance, water and nitrogen use efficiency