Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/10/1999
Publication Date: N/A
Citation: Interpretive Summary: Because vitamin A plays an essential role in growth and development, as well as in assuring optimal immune function, it is essential that the determination of vitamin A status be performed using the most accurate procedure. Historically, vitamin A status has been most accurately determined by evaluating the amount of vitamin A in liver tissue, a somewhat invasive method. An alternative approach is to determine the amount of vitamin A in blood; however, this method frequently does not provide a valid indication of vitamin A status. In the present study, a procedure, referred to as the relative dose response assay (RDR), was evaluated as a means of determining vitamin A status of milk replacer-fed calves supplemented with 4 different levels of vitamin A [0, 1700 (level of supplementation recommended by National Research Council), 34,000 (typical level of supplementation in commercial milk replacers), and 68,000 International Units (IU) of vitamin A/d]. Calves were evaluated at birth and at 4 wk of age. Results from this assay were compared with results from analysis of vitamin A in liver tissue and blood plasma collected at the same times. Although results from analysis of blood samples indicated all animals were deficient (relative to standard applied to adult cattle), results from RDR assay (and those from liver biopsy) indicated that calves receiving vitamin A had adequate reserves of vitamin A. These data, which will greatly benefit the dairy producer, suggest that the RDR assay may be an accurate, noninvasive method for determining vitamin A status in dairy calves. The data also indicate that current guidelines for estimating vitamin A status in cattle may need not be applicable for evaluating vitamin A status in young calves.
Technical Abstract: The vitamin A status of an animal can be accurately measured by a liver biopsy; however, liver biopsies are not always feasible. Plasma concentrations of vitamin A are not an accurate indication of vitamin A status. The relative dose response assay has been used to determine the vitamin A status of individuals. Therefore, the objectives of this study were to investigate the relative dose response assay as a method for identifying vitamin A deficiency in calves and to determine the optimal sampling time. Calves were obtained at birth and assigned to 1 of 4 vitamin A treatments (0, 1700, 34,000 or 68,000 IU/d) added to milk replacer. Liver biopsies and the relative dose response assay were performed at birth and again after 4 wk. Liver concentrations of vitamin A were greater at 4 wk compared to birth samples for calves supplemented with 34,000 IU or 68,000 IU/d of vitamin A. Plasma concentrations increased by 4 wk in the calves receiving supplemental vitamin A at 34,000 IU and 68,000 IU/d and decreased in unsupplemented calves; however, all calves had concentrations of vitamin A considered to be deficient by standard guidelines (less than 20 ug/dl). The relative dose response assay at 4 wk indicated that calves not supplemented with vitamin A had low vitamin A status with 6-h samples containing peak concentrations of vitamin A. Therefore, the RDR assay would be a useful diagnostic tool to determine the vitamin A status of groups of young calves.