Effect of genetic strain, calcium, and feed withdrawal on growth, tibial dyschondroplasia, plasma 1,25-dihydroxycholecalciferol, and plasma 25-hydroxycholecalciferol in sixteen-day-old chickens.

Four experiments were conducted to study the effects of genetic strain, dietary Ca level, and feed withdrawal on growth, feed efficiency, tibia bone ash, tibial dyschondroplasia (TD), and plasma 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and 25-hydroxycholecalciferol [25(OH)D3]. Experiment 1 used five strains and Experiments 2 to 4 used three broiler strains. A TD-inducing basal diet was used in each experiment. In Experiment 3, the birds were fed .60 or .95% dietary Ca, and in Experiment 4 the birds were fed the basal diet and were fed or deprived of feed for 8 h daily. In Experiment 1, Athens-Canadian Randombred and Single Comb White Leghorn chicks did not develop TD. In birds fed the basal diet alone, Peterson x Hubbard chicks had a significantly higher incidence of the most severe TD lesion than two other broiler strains in each experiment and were significantly lighter in BW in two of the four experiments. Of the three broiler strains, the incidence and average lesion score of TD was significantly higher in Peterson x Hubbard birds in Experiment 2 and was numerically highest in the other three experiments. In birds fed the basal diet, Peterson x Hubbard birds had significantly higher plasma 1,25(OH)2D3 in two of the four experiments. Both feed deprivation and .95% Ca increased bone ash and decreased the incidence and severity of TD. There were no significant differences in plasma 1,25(OH)2D3 among strains of birds deprived of feed or fed .95% Ca. In three of the four experiments, high plasma 1,25(OH)2D3 in Peterson x Hubbard birds was associated with an increase in the incidence and severity of TD.