## 3 Results and Discussion  
The average SCC per mL of milk from high-yielding PHF cows varied throughout lactation (Table 1). The highest SCC was noted in the second month of lactation, and it was significantly (\(p<0.05\)) higher than in the fifth month of lactation. The month of lactation of high-yielding PHF cows had a significant effect on milk production. The highest daily milk yield (peak of lactation) was achieved in the second month after calving (49.1 kg), and it decreased in subsequent months to reach 37.9 kg of milk on average in the fifth month of lactation. A significant (\(p<0.01\)) difference was found between daily yields in the firsts three months of lactating vs. the fourth and fifth month. In a study by [PERSON] et al. [16], milk yield peaked in high-producing cows on day 70 after calving. Other authors who investigated PHF cows also demonstrated that milk yield continued to increase until day 60 of lactation, and it remained stable until day 120 [17]. Unfortunately, high productivity may adversely affect the performance traits of PHF cows, such as fertility and productive lifetime, and contribute to metabolic and locomotor disorders [18]. Mastitis is the most common disease in high-yielding cows [19]. This disease reduces milk production and causes undesirable changes in the chemical composition of milk, thus decreasing its processing suitability. The number of somatic cells per ml of milk (SCC) is the most popular indicator of udder health status during lactation. In the present study, udder health had a significant influence on the average daily milk yield in high-producing PHF cows (Table 1). The highest average daily milk yield (48.4 kg) was noted in cows whose milk contained 201,000 to 400,000 somatic cells per ml (denoting risk of mastitisti), and the difference relative to cows with clinical mastitisti (6.0 kg) was highly significant. According to some authors [20], SCC of 250,000 to 300,000 per mL of milk may mark the beginning of infection. In the current study, cows with an increased risk of developing mastitisti (201,000 to 400,000 somatic cells per ml of milk) produced the largest amounts of milk. [PERSON] et al. [21] found that an increase in SCC impaired milk productivity, profitability and quality, and that very low SCC had a similar or even more detrimental effect on milk yield and quality than high SCC. This implies that the relationship between SCC vs. milk yield and composition is not linear. The month of lactation and udder health had significant effects on the mineral composition of milk (Table 2). The analyzed milk had high K content (1447.6 mg dm\({}^{-3}\) on average), and similar values were reported by other authors [11; 22; 23]. In the current experiment, K concentration increased with the progress of lactation. The difference between the fourth and fifth month of lactation vs. the first three months was statistically significant. The milk of cows with healthy udders had the highest K content (1513.6 mg dm\({}^{-3}\)), which was significantly higher than in cows with an increased risk of developing mastitisti (1385.4 mg dm\({}^{-3}\)) and cows with subclinical mastitisti (1336.3 mg dm\({}^{-3}\)). Only in the subclinical mastitisti group was the K content of milk below the lower reference limits of 1350 to 1550 mg K dm\({}^{-3}\)[24]. A considerable decrease in the K content of milk from mastitisti cows was also reported by other authors [20]. In the present study, Ca concentration in milk was low, the average Ca content reached 842 mg\(\cdot\)dm\({}^{-3}\), and it was approximately 16% below the lower limit of the normal physiological range (1000-1400 mg dm\({}^{-3}\)) [24] (Table 2). The Ca content of milk was highest (\(p<0.01\)) in the first month after calving, compared with the next months. Milk from cows with subclinical mastitisti contained 905.1 mg Ca dm\({}^{3}\), and milk from healthy cows contained 815.8 mg Ca\(\cdot\)dm\({}^{3}\), but the noted difference was not significant.