Vitamin A profiles of equine serum and milk

Serum and milk samples from mares and serum samples from their foals were taken at parturition and on d 1, 2, 4, 7, 14 and 21 postpartum. The samples were assayed for retinyl (r.) palmitate, r. acetate and retinol by high performance liquid chromatography. Peak vitamin A activity in milk occurred 1 d postpartum and preceded by 3 d the maximum vitamin A activity in foal serum and the lowest vitamin A activity in the mare serum. Mare serum contained approximately a 65:35 ratio of retinol:r. palmitate and less than 1% r. acetate. Retinyl palmitate was the predominant form of vitamin A in milk until 2 to 3 d postpartum, when r. acetate became and remained the predominant form. Retinol represented less than 1% of the milk vitamin A. A significant quantity of r. acetate was present in the foal sera at 4 d of age, but thereafter serum r. acetate appeared unaffected by the increasing r. acetate levels in the mare milk.