## 3 Results and Discussion
According to many authors, milk from cows suffering from mastitisti has lower Ca content than milk of the highest cytological quality [20; 22]. [PERSON] and [PERSON] [25] compared milk from different animal species and found that Ca concentration in cow's milk was generally low, at 801 mg dm\({}^{-3}\) on average, ranging widely from 684 to 1095 mg dm\({}^{-3}\). The lower Ca content of milk in dairy cattle herds is probably related to the intensive production system based on conserved grass (silage, hay) and high quantities of concentrates [10; 26]. On the other hand, [PERSON] et al. [27] demonstrated that the Ca content of milk from cows raised on an organic farm was only 637.4 mg/dm\({}^{3}\). In the current study, the Na content of milk was higher (\(p<0.05\)) in the first month of lactation than in the third month. Considering udder health, the Na content of milk ranged from 401.3 mg dm\({}^{-3}\) in cows with clinical mastitisti to 322.5 mg dm\({}^{-3}\) in cows with an increased risk of developing mastitisti, and the observed differences were highly significant (Table 2). The Na content of milk from cows with subclinical and clinical mastitis was at the lower limit of the normal range, i.e., 350-600 mg dm\({}^{-3}\)[24]. In the remaining animals, i.e., cows with healthy udders and with an increased risk of developing mastitisti, Na concentration in milk was below the normal values. In a study by [PERSON] and [PERSON] [22], the Na content of milk increased significantly with increasing SCC. Sodium chloride and lactose are responsible for maintaining the osmotic pressure of milk equal to the osmotic pressure of blood. During mammary gland inflammation, the lactose content of milk decreases, which increases the rate of NaCl diffusion from the bloodstream [20; 24]. Since selected minerals and vitamins directly affect immune system function, they can also play a role in mastitis prevention. Similarly to Ca concentration, the Mg content of milk from high-producing cows also often falls below the lower reference limits [10; 23; 24; 28; 25]. In the present experiment, the average Mg content of milk reached 98.5 mg dm\({}^{-3}\) (Table 2), and it was slightly below the lower reference limits (100-150 mg dm\({}^{-3}\)) [24]. The highest concentration of this macronutrient (103.4 mg Mg dm\({}^{-3}\)) was observed in milk from cows in the first month of lactation. The difference relative to the third month of lactation reached 8.9 mg Mg dm\({}^{-3}\), and it was statistically significant. Considering udder health, the highest concentration of this macronutrient (104.8 mg Mg dm\({}^{-3}\)) was observed in milk from cows with subclinical mastitisti, and it was significantly higher than in milk from healthy cows and cows with clinical mastitisti. [PERSON] and [PERSON] [22] found that SCC had no significant influence on the content of Ca, K and Mg in milk, but their levels decreased with increasing SCC. Zinc is the most important micronutrient in cow's milk. A glass of cow's milk covers around 10% of the recommended daily intake of Zn in adults. The average Zn content of milk from the analyzed cows was high, at 4.76 mg dm\({}^{-3}\) (Table 2). The concentration of Zn was highest in the first month of lactation (5.42 mg Zn dm\({}^{-3}\)), and it decreased in subsequent months. The difference between the first month of lactation vs. the fourth and fifth month was statistically significant. Zinc concentration in milk tended to increase steadily with increasing SCC. A significant increase in the Zn content of milk was noted in cows with subclinical and clinical mastitisti, compared with cows with healthy udders and with an increased risk of developing mastitisti. The highest Zn concentration was observed in milk containing more than 1 million somatic cells per ml (5.60 mg Zn dm\({}^{-3}\)). An interaction (\(p<0.01\)) between the month of lactation and udder health was found for the Zn content of milk (Figure 1).