DIETARY 1,25-DIHYDROXYCHOLECALCIFEROL HAS VARIABLE EFFECTS ON THE INCIDENCES OF LEG ABNORMALITIES, PLASMA VITAMIN-D METABOLITES, AND VITAMIN-D RECEPTORS IN CHICKENS DIVERGENTLY SELECTED FOR TIBIAL DYSCHONDROPLASIA

Three experiments were conducted to examine the efficacy of dietary 1, 25-dihydroxycholecalciferol [(1,25-(OH)(2)D-3)] on the development of tibial dyschondroplasia (TD) in chickens divergently selected for high (HTD) and low (LTD) incidences of TD. In Experiment 1, chickens from the two lines were fed two calcium levels (0.75 and 1.0%), with and wi thout 5 mu g/kg dietary 1,25-(OH)(2)D-3. In Experiment 2, both lines w ere fed diets containing 1.0% calcium and 0, 5, 10, or 15 mu g/kg 1,25 -(OH)(2)D-3. The addition of 1,25-(OH)(2)D-3 did not reduce the overal l incidence of TD in Experiment 1, but did reduce the incidence of sev ere TD from 69 to 48% in the chickens receiving the 0.75% calcium diet . In this experiment, LTD chickens had higher plasma phosphorus and bo ne ash. No line differences were noted between plasma vitamin D metabo lites or intestinal vitamin D receptors. In Experiment 2, 5 mu g/kg of 1,25-(OH)(2)D-3 decreased the incidence of TD from 94 to 76% and numb er three scores from 69 to 44% (P less than or equal to 0.001). Higher amounts of 1,25-(OH)(2)D-3 further decreased TD, but there was a redu ction in body weight above 5 mu g/kg. Plasma 25-hydroxycholecalciferol [25-(OH)D-3] and 1,25-(OH)(2)D-3 were higher and intestinal vitamin D receptors were lower in HTD chickens than in LTD chickens. Plasma 1,2 5-(OH)(2)D-3 was not affected by dietary treatment, but 25-(OH)D-3 was reduced by dietary 1,25-(OH)(2)D-3. Experiment 3 was conducted to exa mine effects of line and dietary 1,25-(OH)(2)D-3 on plasma vitamin D m etabolites and intestinal and growth plate receptors. No effect of gen etic line or dietary 1,25-(OH)(2)D-3 was observed for vitamin D recept ors concentration or plasma 1,25-(OH)(2)D-3 levels. Plasma 25-(OH)D-3 was reduced when 1,25-(OH)(2)D-3 was fed. These results indicate that HTD chickens are somewhat responsive to dietary 1,25-(OH)(2)D-3, but t l-Lis treatment failed to prevent the lesion in a large portion of the population.