Specific reference intervals are needed for each animal species for appropriate interpretation of hematological and serum biochemical results. The aim of the present study was to investigate the blood composition of growing calves in order to evaluate the need for defining reference values for different age groups. Thirty two Holstein calves (18 male and 14 female) were blood sampled. A blood sample was taken within 24–48 h following birth and at 14, 28, 42, 56, 70 and 84 days of age. CBC determination and the measurements of some blood serum metabolites, enzymes, electrolytes and minerals were performed. There were significant age related changes for most hematological and biochemical parameters (p < 0.05) except for the numbers of band neutrophils and monocytes and the amounts of sodium, potassium, chloride and BUN. The results of the present study showed that for some hematological and biochemical parameters such as hemoglobin, MCV, MCH, MCHC, inorganic phosphorus, serum total protein, globulin, AST and ALP at the first three months of life and also, neutrophil numbers and glucose levels at the 24–48 h of life, the age specific reference values must be considered for precise interpretation of laboratory results.

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