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-dihydroxycholecalciferol [1,25-(OH)2D3] can prevent tibial dyschondroplasia in broiler chickens throughout the growing period when withdrawn from the grower diet. The birds were reared in floor pens with pine shavings 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)2D3 was fed at 0 or 5 micrograms/kg to 3 wk of age. Half the birds consuming 1,25-(OH)2D3 were then fed 0 microgram/kg until the end of the experiments. The higher level of calcium decreased the incidences of tibial dyschondroplasia and severe lesions and increased bone ash. Dietary 1,25-(OH)2D3 increased bone ash at both levels of calcium at 3 wk and the end of the experiments when supplemented for the duration of the studies. When 1,25-(OH)2D3 was 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)2D3 from 3 to 5 wk of age. There were no treatment effects on plasma calcium, dialyzable phosphorus, or 25-hydroxycholecalciferol. Plasma 1,25-(OH)2D3 was decreased at 3 and 5 wk in Experiment 2 when 1.00% calcium was fed. The results of Experiment 2 suggest that 1,25-(OH)2D3 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 that obtained when 5 micrograms/kg 1,25-(OH)2D3 is fed with .65% calcium for the entire growout period.