EVIDENCE OF INCREASED CHOLECALCIFEROL REQUIREMENT IN CHICKS WITH TIBIAL DYSCHONDROPLASIA

A series of experiments was conducted to test the hypothesis that vita min D utilization may not be as efficient in chicks with tibial dyscho ndroplasia (TD). The basal diet contained 1.0% Ca and 0.45% available P with no supplemental cholecalciferol (D-3) Chicks from low TD (LTD) and high TD (HTD) selected lines were fed diets supplemented with vari ous levels of vitamin D compounds and examined for rickets and TD. Whe n chicks were fed a D-3-deficient diet containing only 1.25 mu g/kg ad ded D-3, HTD chicks had a greater incidence of severe rickets than LTD chicks (P < 0.05). The LTD chicks did not exhibit TD when fed a diet containing adequate (20 mu g/kg) Dg. The LTD chicks fed a diet supplem ented with 5 mu g/kg D-3, however, had 22% incidence of TD. When HTD c hicks were fed diets supplemented with 5 mu g/kg Dg [control diet that meets NRC (1994) requirement for D-3], 20 mu g/kg D-3, 5 mu g/kg 1,25 -dihydroxycholecalciferol [1,25-(OH)(2)D-3] or the combination of both D-3 (20 mu g/kg) and 1,25-(OH)(2)D-3 (5 mu g/kg), TD incidence was hi ghest in HTD chicks fed the control diet. When HTD chicks were fed die ts with an increased dietary level of 1,25-(OH)(2)D-3 (10 mu g/kg) fur ther reduction of TD incidence (P < 0.05) occurred. A potentially toxi c level (Soares et al., 1983) of 1,25-(OH)(2)D-3 (15 mu g/kg) fed to H TD chicks resulted in still greater suppression of incidence of TD eve n though growth and feed intake in HTD chicks was greater than those o f LTD chicks. It is concluded that the development of TD in HTD chicks is associated with subnormal ability to metabolize vitamin D.