Bar, A., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Striem, S., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Rosenberg, J., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Hurwitz, S., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel

Calcium-binding protein D(28k) (calbindin) synthesis, vitamin D metabolism and shell quality were investigated in young and aged laying hens fed diets containing either cholecalciferol (CC) or its 1-hydroxylated derivatives. Duodenal calbindin concentration was similar in the young and in the aged laying hens. Exogenous 1-hydroxylated CC derivatives increased duodenal calbindin concentration, regardless of age. Shell weight and shell density (mg/cm2) were significantly lower (P < 0.01) in the aged than in the young hens. Egg shell weight and density tended to decrease along the clutch. The rate of decline was higher in aged than in young hens. Feeding aged hens a diet containing 5 μg 1,25-dihydroxycholecalciferol [1,25(OH)2CC] or 1 α-hydroxycholecalciferol per kilogram improved shell quality, slowed down the progressive reduction in shell quality during the clutch and increased culling and mortality. The results indicate a) that the capacity for expression of 1,25(OH)2D3 in the intestine is not altered by age and b) that prolonged feeding of 1-hydroxylated derivatives of vitamin D3 improves shell quality in aged laying hens and increases culling and mortality.