EFFECT OF GENETIC STRAIN, CALCIUM, AND FEED WITHDRAWAL ON GROWTH, TIBIAL DYSCHONDROPLASIA, PLASMA 1,25-DIHYDROXYCHOLECALCIFEROL, AND PLASMA25-HYDROXYCHOLECALCIFEROL IN 16-DAY-OLD CHICKENS

Four experiments were conducted to study the effects of genetic strain , dietary Ca level, and feed withdrawal on growth, feed efficiency, ti bia bone ash, tibial dyschondroplasia (TD), and plasma 1,25-dihydroxyc holecalciferol [1,25(OH)2D3] and 25-hydroxycholecalciferol 125(OH)D3]. Experiment 1 used five strains and Experiments 2 to 4 used three broi ler 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 Experime nt 4 the birds were fed the basal diet and were fed or deprived of fee d for 8 h daily. In Experiment 1, Athens-Canadian Randombred and Singl e Comb White Leghorn chicks did not develop TD. In birds fed the basal diet alone, Peterson x Hubbard chicks had a significantly higher inci dence of the most severe TD lesion than two other broiler strains in e ach 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 bird s in Experiment 2 and was numerically highest in the other three exper iments. In birds fed the basal diet, Peterson x Hubbard birds had sign ificantly higher plasma 1,25(OH)2D3 in two of the four experiments. Bo th feed deprivation and .95% Ca increased bone ash and decreased the i ncidence 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 Pete rson x Hubbard birds was associated with an increase in the incidence and severity of TD.