The effect of dietary 1,25-dihydroxycholecalciferol in preventing tibial dyschondroplasia in broilers fed on diets imbalanced in calcium and phosphorus

Abstract

Three experiments were carried out to investigate the effects of supplemental dietary 1·25-dihydroxycholecalciferol (1·25(OH) 2 cholecalciferol) 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 μ g 1·25(OH) 2 cholecalciferol/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 μg 1·25(OH) 2 cholecalciferol/kg. The supplement of 2·5 μ g/kg depressed and the higher supplements prevented the occurrence of TD, this time without a growth depression. Feeding the 10 μ g/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) 2 cholecalciferol was higher on the imbalanced than on the control diet. Supplementation of the imbalanced diet with 1·25(OH) 2 cholecalciferol did not increase plasma levels. It is concluded that 1·25(OH) 2 cholecalciferol is exerting a powerful biological effect in this model of TD, but the mechanism is unclear.