THE EFFECT OF DIETARY 1,25-DIHYDROXYCHOLECALCIFEROL IN PREVENTING TIBIAL DYSCHONDROPLASIA IN BROILERS FED ON DIETS IMBALANCED IN CALCIUM AND PHOSPHORUS

Three experiments were carried out to investigate the effects of supplemental dietary 1,25-dihydroxycholecalciferol (1,25(OH)2cholecalciferol) and a low dietary Ca: P ratio on the occurrence of tibial dyschondroplasia (TD) in 3-week-old broilers. Histopathology was used to diagnose TD. In the first experiment, feeding a diet containing 7.5 g Ca and 7.6 g P/kg gave a higher incidence of TD than a control diet containing normal amounts of Ca and P (12 and 6 g/kg respectively). Increasing the dietary supplement of cholecalciferol in the imbalanced diet prevented rickets but did not decrease the incidence of TD. In the second experiment, supplementing the imbalanced diet with 10 mug 1,25(OH)2cholecalciferol/kg prevented TD completely but also gave a slight growth depression. In the third experiment the imbalanced diet was supplemented with 0, 2.5, 5 or 10 mug 1,25(OH)2 cholecalciferol/kg. The supplement of 2.5 mug/kg depressed and the higher supplements prevented the occurrence of TD, this time without a growth depression. Feeding the 10 mug/kg supplement for the first week only did not prevent TD. Plasma total Ca, inorganic P and alkaline phosphatase (EC 3. 1. 3. 1) were unaffected by diet but 1,25(OH)2cholecalciferol was higher on the imbalanced than on the control diet. Supplementation of the imbalanced diet with 1,25(OH)2cholecalciferol did not increase plasma levels. It is concluded that 1,25(OH)2cholecalciferol is exerting a powerful biological effect in this model of TD, but the mechanism is unclear.