Defoliation timing and severity in sugar beet

Defoliation produced by different biotic and abiotic factors can reduce sugar beet (Beta vulgaris L.) sugar content and final yield production. Four trials were conducted to study the effect on sugar content and yield production of systematic defoliations carried out during the sugar beet growing cycle, in the Central Ebro Valley in Spain. In two trials, defoliation was achieved using shears, a largely untraumatic defoliation method, and in the other two by blasting with a pressurized water jet, producing a traumatic defoliation. Defoliations were performed at seven different growth stages (defined by heat sums and days from sowing). For each growth stage, four defoliation levels (control, slight, moderate, and heavy) were imposed. The results revealed a relationship between yield loss, level of defoliation, and growth stage when defoliation took place. The most critical growth stage for the effect of defoliation on yield was between 1700 and 1800 degree-days, when 100% defoliation produced a 42% yield loss. The higher the level of defoliation, the higher the yield loss. This study produced a set of regression equations that can be used to assess final yield loss caused by defoliation of sugar beet by such different agents as hail storms, wind, insects, and other mechanical damage.