MODULATORY EFFECTS OF DIETARY BETA-CAROTENE ON BLOOD AND MAMMARY LEUKOCYTE FUNCTION IN PERIPARTURIENT DAIRY-COWS

Beginning 4 wk prior to predicted calving, 14 Holstein cows per treatm ent were fed diets 1) unsupplemented (control) or supplemented daily w ith 2) 300 mg of beta-carotene, 3) 600 mg of beta-carotene, or 4) 120, 000 IU of vitamin A. Blood was collected around calving on wk -4, -2, -1, 0 (within 24 h postcalving), 1, 2, and 4 for isolation of lymphocy tes and neutrophils and for the analysis of plasma vitamins. Lacteal s ecretions were collected on wk 0, 1, 2, and 4 for the isolation of pha gocytes. Cows supplemented with 600 mg of beta-carotene had higher con centrations of plasma beta-carotene and retinol than did unsupplemente d cows. Supplemental vitamin A increased plasma retinol on wk 4 and de creased plasma beta-carotene on wk -1 and 0. Treatment did not affect concentrations of plasma alpha-tocopherol. Blood lymphocyte proliferat ion in response to concanavalin A, phytohemagglutinin, and pokeweed mi togen during the peripartum period was higher in cows supplemented wit h beta-carotene than in unsupplemented controls. Phagocytic activity o f blood neutrophils was enhanced on wk 1 in cows fed 300 mg of beta-ca rotene. Intracellular killing by blood neutrophils was enhanced in cow s supplemented with beta-carotene (wk 0) and vitamin A (wk 0 and 1). I odine uptake and nitroblue tetrazolium reduction by blood neutrophils was stimulated in cows supplemented with beta-carotene. Phagocytic act ivity, iodine uptake, and nitroblue tetrazolium reduction by mammary p hagocytes from all cows generally were lower postpartum than on the da y of calving. The incidence of retained placenta and metritis was high er for unsupplemented cows than for cows supplemented with beta-carote ne. Therefore, dietary beta-carotene can elevate peripartum concentrat ions of blood beta-carotene, enhance host defense mechanisms by potent iating lymphocyte and phagocyte function, and decrease the incidence o f certain reproductive disorders.