Serum total calcium, phosphorus, 1,25-dihydroxycholecalciferol, and endochondral ossification defects in commercial broiler chickens

The research described in this paper relates the changes in serum concentration of calcium, phosphorus, and 1,25-dihydroxycholecalciferol [1,25(OH)2D3] to changes in tibial ash percentage and the incidence of endochondral ossification defects (EOD) in flocks of commercially reared broiler chickens at 14 d of age. Sequential studies of six Australian broiler flocks representing three major genetic lines were undertaken at weekly intervals from 1 to 28 d of age. Serum collected from birds was analyzed for total calcium, inorganic phosphorus, and 1,25(OH)2D3. Tibial ash percentage was also determined at weekly intervals, and the incidence of EOD was determined at 14 d of age by examining sagittal sections of the proximal tibiotarsus. The EOD observed in the 14-d-old broiler chickens were characterized by enlarged zones of proliferating chondrocytes, similar to that which occurs during calcium- or vitamin D-dependent rickets. Three flocks had a 50% incidence of EOD at 14 d of age and were classified as severely affected. The other three flocks had incidences ranging from 12 to 16% and were classified as mildly affected. Broiler flocks severely affected with EOD (50% incidence at Day 14) had lower (P less than or equal to.05) concentrations of 1,25(OH)2D3 than flocks mildly affected (12 to 16% incidence). Tibial ash percentages were lower (P less than or equal to 0.05) in the severely affected flocks between Days 14 to 28, and it is likely that a lower rate of ash accretion between Days 7 to 14 precedes the development of the EOD. From these studies it is evident that flocks with a high incidence of EOD have significantly lower bone ash and 1,25(OH)2D3 compared with mildly affected flocks, and it seems probable that higher systemic concentrations of 1,25(OH)2D3 between 7 to 14 d of age will enhance the ability of broiler chickens to effectively mineralize the cartilaginous growth plates in the appendicular skeleton during early bone maturation