The study was conducted on 15 Holstein-Friesian cows aged between 3 and 5 years kept in the free grazing conditions in summer. The samples of milk were taken twice a year for monitoring subclinical mastitis. In addition to milk samples, the blood samples were taken to determine the concentration of zinc in serum. The occurrence of subclinical mastitis in high yielding cows was estimated based on the values of zinc concentration in blood serum and somatic cells of milk. By analyzing the samples of blood serum, a variation in the concentration of zinc in blood serum was noted depending on the season. Somatic cell count in the spring and autumn period in average was over 400.000/ml milk. Based on statistical analysis, the correlation test, it was concluded that the concentration of zinc in blood serum of cows in the experimental group had no significant effect on the somatic cells count in milk.
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