Vitamin A repletion in thoroughbred mares with retinyl palmitate or beta-carotene.

Forty-five Thoroughbred mares used in an 8-mo depletion study were kept for an additional 20 mo on the same three forage diets (15 mares each): 2-yr-old orchardgrass hay and vitamin A-free concentrate on a drylot (HC); pasture, orchardgrass/alfalfa hay, and vitamin A-free concentrate (PHC); or pasture and orchardgrass/alfalfa hay only (PH). Each diet group was divided into three subgroups, and mares (n = 5) in each group were given either retinyl palmitate (A) at twice the NRC (1989) recommended daily intake, the equivalent amount of vitamin A in the form of water-dispersible beta-carotene (B), or the vehicle (C). Vitamin A status was monitored with serum retinol and a relative dose response (RDR) test every 60 d. In the C subgroups, retinol concentration was 18.65 +/- .84 micrograms/dL (mean +/- SE) and the RDR was 16.26 +/- 1.72% over the 20 mo. Retinol and RDR fluctuated seasonally regardless of supplementation. Vitamin A status, based on serum retinol (P = .001) and RDR (P < .001) values, was lower in the HC than in the PH and PHC. Vitamin A status, based on retinol (P = .05) and RDR (P = .013) values, was improved by retinyl palmitate supplementation in all diet groups, but not by water-dispersible beta-carotene supplementation. Supplementation of the HC mares with vitamin A matched the serum retinol, but not the RDR, of the two pasture, control subgroups. Thus, replete vitamin A status in previously depleted mares was barely obtained by supplementation with twice the currently recommended daily intake of vitamin A.