Yield, quality and water productivity of autumn-sown sugar beet under early and late season irrigation regimes in semi-arid Iran

Optimizing irrigation scheduling for autumn sown sugar beet under semi-arid conditions is critical to improving water productivity and sustaining yields. Therefore, this study aimed to determine an irrigation strategy that maximizes yield, sugar quality, and water productivity by evaluating different early- and late-season irrigation schedules for autumn-sown sugar beet under semi-arid conditions. This field experiment, conducted over 2 years (2022–2024) in Khuzestan, Iran, employed a randomized complete block design with eight surface irrigation treatments. Treatments primarily varied late-season irrigation (LSI) strategies, alongside a consistent early-season irrigation (ESI) threshold based on 50% of the maximum allowable depletion, including rainfed and deficit irrigation scenarios. Results showed that ESI was a dominant factor in yield formation, whereas additional LSI provided limited benefits and could decrease sugar quality. The highest root yield (68.3 t ha −1 ) was obtained under full irrigation (T1), while the water-saving treatment (T2) produced 56 t ha −1 with approximately 50% less irrigation and achieved the highest root water productivity (17.5 kg m −3 ). Water productivity declined with increasing irrigation depth. The yield response factor (Ky) averaged 0.74, indicating moderate sensitivity to water stress. Crucially, the findings demonstrate that autumn-sown sugar beet can achieve high yields and quality with a minimum irrigation depth of 300–350 mm, emphasizing the practical implication of utilizing early-season moisture and rainfall for water-saving irrigation.