ASSESSING ADEQUACY OF CHOLECALCIFEROL SUPPLEMENTATION IN CHICKS USINGPLASMA CHOLECALCIFEROL METABOLITE CONCENTRATIONS AS AN INDICATOR

Cholecalciferol (vitamin D) deficiency rickets remains an occasional p roblem in poultry. Diagnosis currently relies on analysis of feed and histopathological examination of bone. These experiments were designed to provide data that might allow diagnosis of cholecalciferol deficie ncy on the basis of plasma concentrations of 25-hydroxycholecalciferol , a circulating metabolite of cholecalciferol. Day-old broiler chicks were fed corn-soybean meal or purified ingredient cholecalciferol-defi cient diets supplemented with 0, 5, 10, 15, 20, 25, 37.5 or 75 mu g ch olecalciferol/kg diet. Plasma and bone samples were collected 21 d lat er. Chicks fed the unsupplemented purified ingredient diet became trul y deficient, having no detectable plasma concentrations of the choleca lciferol metabolites 25-hydroxycholecalciferol, 1,25-dihydroxycholecal ciferol, or 24,25-dihydroxycholecalciferol. Chicks fed the corn-soybea n meal diet without supplementation had low but detectable concentrati ons of both 25-hydroxycholecalciferol and 1,25-dihydroxycholecalcifero l in plasma. Body weight, bone calcium and bone phosphorus concentrati ons of chicks fed the corn-soybean meal diet suggest that the cholecal ciferol requirement of broiler chicks is at least 10 mu g/kg diet. At this dietary level of cholecalciferol, plasma 25-dihydroxycholecalcife rol concentration was 12.5 nmol/L. One hundred percent of the theoreti cal maximal response in body weight and bone calcium content was seen at 20 mu g cholecalciferol/kg diet, which increased plasma 25-hydroxyc holecalciferol concentration to 25 nmol/L in the chicks fed the corn-s oybean meal diet. These data provide a nomogram of plasma 25-dihydroxy cholecalciferol concentration that can be expected from including diff erent concentrations of cholecalciferol in the diet, and also offer a means of diagnosing cholecalciferol deficiency in field cases of ricke ts.