EFFECTS OF DIETARY CALCIUM AND 1,25-DIHYDROXYCHOLECALCIFEROL ON THE DEVELOPMENT OF TIBIAL DYSCHONDROPLASIA IN BROILERS DURING THE STARTER AND GROWER PERIODS

Two experiments were conducted to determine whether dietary 1,25-dihyd roxycholecalciferol [1,25-(OH)(2)D-3] can prevent tibial dyschondropla sia in broiler chickens throughout the growing period when withdrawn f rom the grower diet. The birds were reared in floor pens with pine sha vings to 6 wk in Experiment 1 and 5 wk of age in Experiment 2. Calcium was fed at .65 or 1.00% and 1,25-(OH)(2)D-3 was fed at 0 or 5 mu g/kg to 3 wk of age. Half the birds consuming 1,25-(OH)(2)D-3 were then fe d 0 mu g/kg until the end of the experiments. The higher level of calc ium decreased the incidences of tibial dyschondroplasia and severe les ions and increased bone ash. Dietary 1,25-(OH)(2)D-3 increased bone as h at both levels of calcium at 3 wk and the end of the experiments whe n supplemented for the duration of the studies. When 1,25-(OH)(2)D-3 w as fed, tibial dyschondroplasia was reduced in Experiment 2 only at 3 wk. Tibial dyschondroplasia was decreased at 5 wk in Experiment 2 when .65% calcium was fed with or without 1,25-(OH)(2)D-3 hom 3 to 5 wk of age. There were no treatment effects on plasma calcium, dialyzable ph osphorus, or 25-hydroxycholecalciferol. Plasma 1,25-(OH)(2)D-3 was dec reased at 3 and 5 wk in Experiment 2 when 1.00% calcium was fed. The r esults of Experiment 2 suggest that 1,25-(OH)(2)D-3 can prevent tibial dyschondroplasia caused by inadequate calcium when fed for only 3 wk. The bone ash observed when 1.00% dietary calcium is fed is equal to t hat obtained when 5 mu g/kg 1,25-(OH)(2)D-3 is fed with .65% calcium f or the entire growout period.