THE EFFECTS OF ULTRAVIOLET-LIGHT AND CHOLECALCIFEROL AND ITS METABOLITES ON THE DEVELOPMENT OF LEG ABNORMALITIES IN CHICKENS GENETICALLY SELECTED FOR A HIGH AND LOW INCIDENCE OF TIBIAL DYSCHONDROPLASIA

Four experiments were conducted to investigate the effects of ultravio let (UV) light exposure and several cholecalciferol metabolites on the development of tibial dyschondroplasia (TD) and other parameters asso ciated with vitamin D metabolism in chickens selected for high (HTD) a nd low (LTD) incidence of TD. In Experiment 1, exposure of chickens to UV light reduced the incidence and severity of TD more in LTD chicken s than in HTD chickens, as evident by the significant interactions (P < 0.10 and 0.04). In Experiment 2, the addition of cholecalciferol to diets that were deficient in cholecalciferol linearly decreased the in cidence of vitamin D rickets and increased bone ash, but increased the incidence of severe TD. The LTD chickens had a higher maximal bone as h of 40.0 +/- 0.7% than did the HTD chickens, which had a maximal bone ash of 37.0 +/- 0.7%. In Experiment 3, the addition of 5 mu g/kg of 2 5-hydroxycholecalciferol [25-(OH)D-3], 1-alpha-hydroxycholecalciferol, or 1,25-dihydroxycholecalciferol decreased the incidence and severity of TD in the LTD chickens and had no effect on TD in HTD chickens. In Experiment 4, increasing dietary 25-(OH)D-3 increased plasma 25-(OH)D -3 levels in both lines, hut HTD chickens had higher plasma 25-(OH)D-3 levels at 20 and 40 mu g/kg of dietary 25-(OH)D-3. The incidence and severity of TD were reduced in the LTD chickens by dietary 25-(OH)D-3, but little effect was noted in HTD chickens. The LTD chickens reached a maximal bone ash at 9.7 +/- 1.9 mu g/kg and HTD chickens reached th e same bone ash at 33.0 +/- 7.0 mu g/kg. These results indicate that U V light and vitamin D metabolites are not effective in preventing TD i n HTD chickens, but that altered vitamin D metabolism does exist betwe en HTD and LTD chickens.