Sequential studies of endochondral ossification and serum 1,25-dihydroxycholecalciferol in broiler chickens between one and 21 days of age.

Two commercial broiler flocks of two distinct strains (A and B) were studied at weekly intervals from day old to 21 days, to assess the progressive endochondral ossification of the proximal tibiotarsus and the serum concentration of 1,25-dihydroxycholecalciferol (1,25(OH)2D3). The incidence of defects of endochondral ossification was different in the two strains, strain B having an incidence of tibial dyschondroplasia (TD) of 10 to 70 per cent and strain A an incidence of 10 to 20 per cent. In strain B, 40 per cent of the bone samples collected at 14 days of age also had lesions of calcium deficiency rickets. The concentration of 1,25(OH)2D3 in the two flocks was similar in the day-old chicks, but was 40 to 50 per cent lower at seven, 14 and 21 days of age in strain B, during the development of the rachitic and dyschondroplastic lesions. These results suggest that TD in some broiler strains is related to an inherent predisposition to rickets and to lower serum concentrations of 1,25(OH)2D3.