Yield and Quality Trade-Offs in Sugar Beet Cultivars Under Deficit Irrigation for Bioenergy Production

Water scarcity in semi-arid regions limits the sustainable production of sugar beet (Beta vulgaris L.), a crop grown for sugar and increasingly used as a bioethanol feedstock. This two-year field study (2020–2021), conducted at the Agricultural Research and Application Station of Bilecik Şeyh Edebali University (Türkiye), evaluated the effects of full (well-watered, WW) and deficit irrigation (DW) on root yield and soluble solids content (Brix) of 12 commercial cultivars in a split-plot design with three replications. Data were analyzed with a combined mixed model, and cultivar × environment patterns were described with GGE biplots. Irrigation was the only factor with a significant effect on both traits (Brix, p = 0.001; root yield, p < 0.001); year and cultivar effects were not significant. Deficit irrigation reduced root yield from 58.3 to 49.0 Mg ha⁻¹ (−16.0%), with a similar reduction in both years, and increased Brix from 20.07% to 21.72%. Because the yield loss exceeded the gain in concentration, the indicative soluble solids output (root yield × Brix) was about 9% lower under deficit irrigation. Cultivar yield reductions ranged from 7.9% to 24.1%, but the Irrigation × Cultivar interaction was only marginal for root yield (p = 0.079), so cultivar-specific tolerance could not be confirmed. The deficit regime applied therefore did not preserve root yield, and the higher Brix reflected a concentration effect and not a higher sugar output per hectare. Irrigation water-use efficiency, sucrose content, and ethanol yield were not determined; the bioenergy implications are therefore discussed as feedstock potential only.