Use of the relative dose response (RDR) assay to determine vitamin A status of calves at birth and four weeks of age

An accurate assessment of vitamin A status can be determined by analysis of liver biopsy samples; however, liver biopsies are not always feasible. Pla sma concentrations of vitamin A do not provide an accurate indication of vi tamin A status. The objective of this study, therefore, was to determine th e ability of the relative dose response assay to indicate the vitamin A sta tus of Holstein calves. Calves were obtained at birth and assigned to vitam in A treatments (0, 1700, 34,000, or 68,000 IU/d) added to milk replacer. L iver biopsies and relative dose response assays were performed at birth and 4 wk. Calves supplemented with 1700, 34,000, or 68,000 IU of vitamin Aid h ad adequate (greater than 20 mu g/g) liver concentrations of vitamin A at 4 wk of age. The relative dose response assay at 4 wk was correlated with li ver concentrations of vitamin A. Both the relative dose response assay and liver concentrations of vitamin A indicated that calves not supplemented wi th vitamin A had low vitamin A status, whereas other treatment groups had a dequate vitamin A status. Plasma concentrations of retinol increased by 4 w k Of age in calves receiving supplemental vitamin A at 34,000 IU and 68,000 IU/d and decreased in unsupplemented calves; however, all calves had conce ntrations of <20 mu g of retinol/dl of plasma. The relative dose response a ssay agreed with liver biopsies as an indication of vitamin A status, where as plasma concentrations of retinol incorrectly indicated all treatment gro ups were deficient in vitamin A.