The objectives of the present study were to investigate effects of over-supplementation of ascorbic acid on hematology, serum biochemistry and weight gain in Holstein dairy calves. A total of 90 Holstein dairy calves were randomly assigned to receive one of two treatments. Ascorbic acid was administered orally to treatment group (n = 44) as follows: three times 1000 mg a day during first week; two times 1000 mg a day during second week; one time 1000 mg a day during third week. No ascorbic acid was administered to control group (n = 46). The number of lymphocytes (at day 14 after birth) and monocytes (at day 30 after birth) were significantly lower in ascorbic acid fed calves than control ones. The calves in treatment group had a significantly lower concentration of fibrinogen at day 30 after birth. Ascorbic acid fed calves showed significantly increased concentrations of albumin at day 60 after birth. No significant differences were seen for beta and gamma globulin levels and albumin:globulin (A:G) between groups. Total weight gains were not different between groups. However, the female calves which were received ascorbic acid had significantly increased weight gain in comparison with female calves of control group.

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