Quantitative secretion and maximal secretion capacity of retinol, beta-carotene and alpha-tocopherol into cows' milk

Changes in the contents of retinol, alpha-tocopherol and beta-carotene in p lasma, milk and milk fat from 38 Holstein-Friesian cows were followed durin g their first lactation, and the quantitative and kinetic relationships for secretion of alpha-tocopherol and beta-carotene from blood into milli were determined. The cows were assigned to three groups such that all cows in t he same group had the same sire, Milk yield and milk fat content differed w ith stage of lactation, but not according to sire. The plasma concentration s of retinol, alpha-tocopherol and beta-carotene differed with stage of lac tation; in addition, retinol and beta-carotene concentration also differed according to sire. The concentrations of all three vitamins in milk and mil k fat differed according to sire and stage of lactation. Furthermore, the t otal secretion of retinol, alpha-tocopherol and beta-carotene into milk (ex pressed as mg/d) differed with sire and stage of lactation. The quantitativ e secretion of alpha-tocopherol and beta-carotene from blood into milk foll owed Michaelis-Menten kinetics for active transport across membranes. Value s of maximum secretory capacity V-max and the half-rate constant K-m for bo th alpha-tocopherol and beta-carotene varied according to sire. Overall mea ns for V-max for alpha-tocopherol and beta-carotene were 32.4 and 2.5 mg/d. Thus, the daily secretion of alpha-tocopherol and beta-carotene is limited in quantity, and is independent of the yields of milk and milk fat. It fol lows that continuing breeding and management systems that focus solely on i ncreasing milk and milk fat yield will result in a steady dilution in the m ilk fat of these vitamins and antioxidants important for the immune defence of the cows and oxidative stability of milli, products. The genetic variat ion found offers the possibility of utilizing these variations in breeding systems.