Effectiveness of twenty-five-hydroxycholecalciferol in the prevention of tibial dyschondroplasia in Ross cockerels depends on dietary calcium level.

Five experiments were conducted to evaluate the efficacy of 25-hydroxycholecalciferol [25-(OH)D3] to minimize the development of tibial dyschondroplasia (TD) and improve phytate phosphorus retention in Ross cockerels during the starter period. In experiment 1, chicks were fed a TD-inducing (0.67% calcium) diet with or without exposure to ultraviolet light and no supplemental cholecalciferol. Dietary 25-(OH)D3 was added at 0, 10, or 70 microg/kg for both light treatments. In experiment 2, 25-(OH)D3 was added at 0, 10, 40, or 70 microg/kg to a TD-inducing diet containing 27.5 microg/kg added cholecalciferol. Experiment 3 was similar to experiment 2 except a diet marginal (0.85%) in calcium was fed, and cholecalciferol was added at 55 microg/kg. In experiments 4 and 5, 25-(OH)D3 was added at 0, 18, 36, 54, 72, or 90 microg/kg to a diet marginal in calcium. Dietary 25-(OH)D3 decreased the incidence of TD similarly at 40 and 70 microg/kg 25-(OH)D3 and improved phytate phosphorus retention when the TD-inducing diet was fed. The incidence of TD was decreased when 70 microg/kg 25-(OH)D3 was added to a diet marginal in calcium in experiment 3 only. Phytate phosphorus retention was generally not affected by dietary 25-(OH)D3 when a diet containing marginal calcium, adequate phosphorus, and high cholecalciferol was fed. The effectiveness of 25-(OH)D3 to reduce the incidence of TD in young broilers was higher when the dietary calcium level was below 0.85%. The incidence of TD in Ross cockerels was low (< 25%) when dietary calcium was greater than 0.85%.