Due to their high rates of metabolism, dairy cows experience considerable oxidative stress, resulting in susceptibility to disease pathogens. Vitamin A (VA) is an important factor in improving immune function and attenuating oxidative stress. The effects of dietary VA concentration on milk production, antioxidant status and immune function in dairy cows were assessed. Twenty-eight Holstein cows with similar parity, body weight (BW, 550 +/- 50 kg), and milk production (20.75 +/- 0.50 kg) and at similar stages of lactation (150 10 d in milk) were divided into two groups with 14 replicates in each group. The only independent factor in this experiment was the concentration of supplemental VA provided in the cows' diet (control: 110 IU/kg BW or high dose VA supplementation: 220 IU/kg BW). This experiment consisted of a 15-d adaptation period and a 60-d collection period. The ratio of concentrate to forage in the diets was 42:58. The results indicated that milk yield and milk composition were not changed upon supplementation with a higher dose of VA compared with the control, but VA concentration within the milk was increased. The results also indicated that supplementation with 220 IU of VA/kg of BW significantly increased the concentrations of immunoglobulin A, immunoglobulin M, immunoglobulin G, interleukin-1, tumour necrosis factor-alpha and soluble CD4; selenoprotein P content; the activities of glutathione peroxidase, superoxide dismutase and catalase; total antioxidant capacity; the hydroxyl radical inhibition capability; and soluble CD4/soluble CD8 in the serum. Feeding a high dose of VA (220IU/kg BW) resulted in a significant reduction in the somatic cell count of the milk, the concentrations of soluble CD8 and malonaldehyde, as well as reactive oxygen species in the serum of dairy cows. These results suggested that supplementation of the diet with 220IU of VA/kg of BW may enhance the antioxidant and immune functions of dairy cows and implied that the VA requirement necessary to ensure beneficial immune and antioxidant functions of dairy cows is higher than the current recommended dose that ensures optimum production performance. (c) 2014 Elsevier B.V. All rights reserved.
