Enhancing irrigation water use efficiency to reinforce sugar beet competitiveness in Northern Italy

In the quest for higher yields to support sugar beet competitiveness in Northern Italy (spring sowing), a complete dataset of irrigation trials was worked out, looking for clues to enhance irrigation yield response, more specifically irrigation water use efficiency (IWUE). To this aim, four treatments were compared: rainfed control (CTRL); 50% restoration of water deficit, calculated through the AcquaFacile software (50 AF); 100% restoration based on the software (100 AF) and on a tensiometric method (100 TENS). Two harvest times, early (E) and late (L), were cross-combined with the four treatments over 11 fields in the years 2004 to 2006. A net deficit, i.e. including the contribution of the water table, of 193 ± 80 mm in E and of 244 ± 104 mm in L was observed. Raw sugar yield (SY) response to irrigation was always significant, averaging 1.3 Mg ha-1 in 50 AF; 2.3 in 100 AF and 100 TENS. In a multiple linear regression, SY response to irrigation was inversely related to the yield level in CTRL; directly related to the seeding lateness, to net seasonal deficit and to available water; the amount of explained variation (R2) attained 50%, indicating an equally-strong background effect. IWUE averaged 1.6 kg raw sugar m-3 of supplied water, showing a good improvement over the recent past, thanks to the restraint of the losses in sugar content associated with irrigation. In a multiple linear regression, IWUE was inversely related to the seasonal irrigation volume; directly related to the seeding lateness and to the seasonal ETC; a R2 slightly below 40% indicates a stronger environmental effect with respect to the irrigation parameters. Nonetheless, the significant equations for SY response and IWUE allow to interpret crop behaviour and to predict irrigation effects according to simple crop and water-balance parameters.