|Waters, W - IOWA STATE UNIV., AMES|
|Dubeski, Paula - AGRIFOOD RES CTR, CANADA|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 13, 1999
Publication Date: N/A
Interpretive Summary: In the U.S., newborn calves are fed milk replacers containing vitamin A at levels that exceed current recommendations of the National Research Council by more than 10-fold. Although the justification for this practice is based on the desire of the producer to provide a formulated diet which ensures optimal growth and health of the calf, scientific evidence supporting this practice is limited. The present study evaluated the effects of dietary vit A on vitamin status and the immune system of calves from birth to 7 wk of age. Comparing calves on a "low" vit A diet with those on a "high" vit A diet, we found that an excess of vit A in the diet was associated with a pronounced decrease in the amount of vit E present in the blood. The apparent interaction between vitamins A and E is of concern because a reduction in vit E, an essential antioxidant in biological systems, may impact negatively the growth and health of the calf. Analysis sof populations of immune cells in the blood of calves indicated that the dietary vit A also influences the composition of immune cell populations. Because these cells are essential for assuring the optimal function of the immune system, it is conceivable that vit A induced alterations in these critical cell populations. Substituting beta-carotene for vit A did not affect concentrations of vitamins A and E or influence the populations of immune cells in the blood of the calves. Additional research is necessary to determine if the effects of vit A on the availability of vit E and composition of immune cell populations affect negatively the growth and health of young calves. These results, which will be beneficial to dairy producers, also suggest that the amount of vit A presently added to commercial milk replacers should be reevaluated in controlled studies.
Technical Abstract: Dairy calves (n=18), separated from the dam at birth, were fed 1 L of pooled colostrum. For the remaining 7 wk of the study, they were fed one of three diets consisting of either a custom-formulated milk replacer without vitamin A (controls), or supplemented with retinyl palmitate (equivalent to 32,000 IU of vitamin A/d) or with beta-carotene (equivalent to 20,000 IU of vitamin A/d). Plasma retinol, beta-carotene, and RRR-alpha-tocopherol concentrations were lowest at birth, and increased substantially from birth to 1 wk postpartum in all groups, a probable consequence of ingestion of colostrum. From 1-7 wk of age, retinol concentrations were greatest in retinyl palmitate-supplemented calves, intermediate in beta-carotene-supplemented calves and lowest in control calves. At 2, 3, 5, 6 and 7 wk, RRR-alpha-tocopherol concentrations were lower in retinyl palmitate-supplemented calves than in control calves. A negative correlation between plasma retinol and vitamin E concentrations existed from wk 2 to 7, suggesting vitamin A influences the absorption and/or distribution of RRR-alpha-tocopherol. Supplemental retinyl palmitate, but not beta-carotene, was associated with a reduction in the percentage of blood mononuclear leukocytes expressing CD2, CD4, and CD8 T cell antigens and interleukin-2 receptors. By wk 7, leukocyte populations from retinyl palmitate-supplemented calves were more similar to those from adults than those from control calves, suggesting that supplemental vitamin A, as retinyl palmitate, affects the maturation of the neonatal immune system. Differences in the composition of blood mononuclear leukocyte populations may represent changes in immune competency.