VITAMIN-A REPLETION IN THOROUGHBRED MARES WITH RETINYL PALMITATE OR BETA-CAROTENE

Forty-five Thoroughbred mares used in an 8-mo depletion study were kep t for an additional 20 mo on the same three forage diets (15 mares eac h): 2-yr-old orchardgrass hay and vitamin A-free concentrate on a dryl ot (HC); pasture, orchardgrass/alfalfa hay,and vitamin A-free concentr ate (PHC); or pasture and orchardgrass/alfalfa hay only (PH). Each die t group was divided into three subgroups, and mares (n = 5) in each gr oup were given either retinyl palmitate (A) at twice the NRC (1989) re commended 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 1 8.65 +/- .84 mu g/dL (mean +/- SE) and the RDR, was 16.26 +/- 1.72% ov er the 20 mo. Retinol and RDR fluctuated seasonally regardless of supp lementation. Vitamin A status, based on serum retinol (P = .001) and R DR (P < .001) values, was lower in the HC than in the PII and PHC. Vit amin A status, based on retinol(P = .05) and RDR (P = .013) values, wa s improved by retinyl palmitate supplementation in all diet groups, bu t not by water-dispersible beta-carotene supplementation. Supplementat ion 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 supplemen tation with twice the currently recommended daily intake of vitamin A.