Effect of defoliation intensity at different growth stages on the root yield and quality of sugar beet

To evaluate the effect of artificial defoliation intensity at different growth stages on the root yield and quality of sugar beet (Beta vulgaris L.) a field experiment was carried out at Miyandoab Agricultural Research and Natural Resources Field Station in 2007 growing season. Experimental treatments were arranged as factorial (4×5) in RCBD with three replications. Defoliation were practice at four growth stages (S) of sugar beetincluding: plant establishment to 10% ground cover (S1), from 10 up to 70% ground cover (S2), from the S2 to root maturity (S3), and from the S3 to harvest time (S4). Five levels of defoliation intensities (I) were: blade removal up to 25% (I1), 50% (I2), 75% (I3), 100% (I4) and no defoliation as check (I5). At final harvest, sugar beet roots were harvested and root yield, sugar content, white sugar content, white sugar yield and extraction coefficient of sugar were determined. Results showed that the main effect of growth stage was significant on quality traits of sugar beet such as sugar content, white sugar content and sugar extraction coefficient, however, its effect on agronomic traits such as root yield was not significant. However, different levels of defoliation affected both quality and quantitative traits of sugar beet. At the 100% defoliation level, root yield reduced by 36% as compared to no defoliation (47.49 t.ha-1). In the meantime the least root yield (23.90 t.ha-1) was recorded at S2 with 100% defoliation (S2I4). The interactions between defoliation intensity × growth stages were significant (P<0.01) for sugar content and white sugar content. At the 25% defoliation (I1) at final growth stage(S4), sugar content decreased by 4.65 units whic was 22% of no defoliation (sugar content of 21.50%). In conclusion, any defoliation during the growing season may increase sugar losses in the sugar factory.