Optimizing irrigation regimes for foxtail millet (Setaria italica L.) in Northwest China: Effects of irrigation quota and frequency on yield and crop water productivity

Optimizing irrigation schedules is essential for improving crop yield and crop water productivity (WP c ) in arid and semi-arid regions. A field experiment was conducted to investigate the growth, yield and WP c responses of foxtail millet ( Setaria italica L.) in response to three irrigation quotas (W1–3) with 150, 187.5, or 250 mm and with three different irrigation frequencies (T1–3) (seven, eight or nine irrigations) per season over each of two consecutive growing seasons in Ningxia, China. The results indicated that plant height, stem diameter, tiller number, panicle length and WP c increased and then decreased with increasing irrigation quota, while biomass increased steadily with increasing irrigation quota. The highest grain yields were recorded under W3T2 (8.40 t ha −1 ) in 2022 and W2T2 (8.37 t ha −1 ) in 2023, with no significant difference between yields of these two treatments. Yield was positively correlated with biomass and straw yield but negatively with panicle length and 1000-grain weight. The highest harvest index value occurred in W2T2 in both growing seasons. After a comprehensive evaluation based on grain yield, straw yield, 1000-grain weight, yield stability and WP c , the optimal combination of irrigation quota and irrigation frequency was the W2T2 treatment for millet using drip irrigation under plastic film in Northwest China. • Optimal millet yield and WUE were achieved under 187.5 mm and eight irrigations (W2T2). • Moderate irrigation improved growth traits and aboveground biomass accumulation. • Grain yield correlated positively with biomass and straw yield, negatively with panicle length and grain weight. • WUE peaked under moderate irrigation quota and frequency across two seasons. • TOPSIS analysis confirmed W2T2 as the optimal regime for millet in arid Northwest China.