Kiarie. Medullary bone attributes in aged Lohmann LSL-lite layers fed different levels of calcium and top-dressed 25-hydroxy vitamin D3. Can. J. Anim. Sci. 98: XXX-XXX. Structural bone depletion over the course of lay cycle predisposes hens to skeletal problems. We investigated the effects of dietary Ca and top-dressed 25-hydroxy vitamin D3 (25OHD3) on attributes (relative weight, ash content (AC) and concentration (ACN)) in whole ulna, femur, tibia and sub-parts of femur and tibia (epiphysis,

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... edullary and cortical) in 74-wk old Lohmann LSL-lite layers. Four levels of Ca (3.0, 3.5, 4.0 and 4.5%) and three levels of 25OHD3 (0, 69 and 138 µg/kg) were tested. All diets had basal level of 3,300 IU of vitamin D3/kg. Eighty-four, 74wk old hens were placed in individual cages and 13 spare hens sacrificed for baseline samples. Diets (n=7) were fed to 81-wks of age and hens sacrificed for bone samples. There was no (P > 0.05) diet effects on whole bone attributes. Interaction (P < 0.05) between Ca and 25OHD3 on femur sub-parts was such that 25OHD3 linearly increased medullary ACN and concomitantly decreased cortical ACN at all Ca levels. In tibia, 25OHD3 (P < 0.05) increased AC and ACN in medullary and reduced these parameters in cortical. The results suggested that sub-parts and not whole medullary bone attributes are more amenable to dietary interventions in aged hens.