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Plant water use strategies are crucial for fodder production in semi-arid environments, with implications for food supply, livestock products and social sustainability in a global warming environment. This study aimed to determine the effect of phenology and sowing date on water use in four annual forage crops on the Loess Plateau of China. Hydrogen and oxygen isotope ratios (delta D and delta O-18) measured soil water in different layers (0-1.2 m), xylem water at jointing, flowering and maturity, and rainwater. The IsoSource model was used to calculate the relative contribution of soil water to plant water use in the shallow (0-0.1 m), middle (0.1-0.6 m) and deep (0.6-1.2 m) layers. As the forage crops grew, more water was absorbed from deeper layers. Delaying sowing up to four weeks could promote forage maize (Zea mays), Sudan grass (Sorghum sudanense Stapf.) used water from deep layers at all measured growth stages (jointing, flowering, and maturity). Foxtail millet (Setaria italica) and Japanese millet (Echinochloa crus-galli var. frumentacea) absorbed deep water at flowering and maturity. Further, maize, Sudan grass, and foxtail millet used more water at flowering while Japanese millet used more at maturity. The distinct water use patterns of the four crops at different growth stages and sowing dates showed that plant water use in a semi-arid environment is controlled by both environment and growth stage. Delaying sowing might be an efficient way of coping with water stress caused by unpredictable rainfall by using water from deeper layers to reduce the risk of yield losses. Our results provide a reference for agricultural water management under global warming and sustainability in rainfed farming systems.