Interaction between dietary 1,25-dihydroxycholecalciferol and calcium and effects of management on the occurrence of tibial dyschondroplasia, leg abnormalities and performance in broiler chickens.

1. Two experiments were performed to compare the relative effectiveness of feeding 1,25-dihydroxycholecalciferol (1,25-DHCC) in minimising leg abnormalities in broilers with other methods and to investigate interactions between dietary 1,25-DHCC and calcium. 2. Adding 5 micrograms 1,25-DHCC/kg to a diet containing 12 g calcium/kg was more effective than early food restriction or meal feeding in preventing leg abnormalities but was found to cause a growth depression. 3. The second experiment, which had a factorial design, with diets containing 7.5, 10.0 and 12.5 g calcium and 0, 2.0, 3.5 and 5.0 micrograms 1,25-DHCC/kg, showed linear and quadratic interactions between these dietary factors. Diets with higher concentrations of both 1,25-DHCC and calcium resulted in growth depression associated with hypercalcaemia. 4. The incidence of tibial dyschondroplasia (TD) at 3 weeks of age was highest with the basal diet containing 7.5 g calcium/kg and was markedly reduced by addition of 1,25-DHCC and/or calcium. The incidence was very low or non-existent when 1,25-DHCC was fed at 3.5 micrograms/kg or greater. 5. Feeding 5 micrograms/kg 1,25-DHCC had no effect on plasma 1,25-DHCC concentrations, although at the higher dietary calcium contents plasma concentrations of 25-hydroxy- and 24,25-dihydroxy-cholecalciferol were lower in those birds fed 1,25-DHCC. 6. It is concluded that 1,25-DHCC is most effective in preventing TD without accompanying growth depression when it is fed in conjunction with diets containing less than 10 g calcium/kg.