Fifty-four lactating dairy cows randomly allocated to one of three groups. Ttreatment groups received the basal diet with no supplemental Zn (control), basal diet plus 500 mg of Zn/kg of DM as ZnSO4.H2O (ZnS) and basal diet plus 500 mg of Zn/kg of DM as Zinc methionine (ZnM). This experiment was started in the first phase of lactation (35±3 days after parturition). The data were analysed using the mixed procedure of SAS (9.1) for a block randomized design with repeated measures. Plasma and serum were taken from samples of blood on 0, 2, 4, 6, 8 and 10 weeks of experiment. The number of red blood cell, haemoglobin, MCHC, number of platelet and serum zinc in the ZnM and ZnS groups were significantly higher than control group (P<0.05). Mentioned factors were also higher (P<0.05) in dairy cattle that received ZnM than ZnS. There were no differences for WBC, PCV, MCH and PLT. There were not significant differences among different sources of Zn for ALP, but LDH and SOD in ZnM group were significantly higher than ZnS. In the current study, lactating dairy cattles fed ZnM had higher concentration of Zn in blood serum, some of haematological parameters and enzyme activity of LDH and SOD which is consisted with previous studies. In this study, despite the diet was fortified at levels exceeding the NRC requirements, improvements in blood parameters and enzyme activities of LDH and SOD were observed when organic Zn sources were supplemented.

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