## 3 Results
The milk of mb cows was characterized by a greater variability of minerals in the subsequent months of lactation (Figures 1-5). There was noted an increase in the potassium content of the milk of phf cows with the time since parturition, with the highest content of this element in the milk of phf cows at the sixth and ninth months of lactation (Figure 1). Differences in the potassium content of milk between the sixth and ninth months of lactation in comparison to the first month after the parturition of phf cows were confirmed statistically (\(p\leq 0.01\)). A descending tendency was determined in the potassium content of the milk of mb cows in the ninth month of lactation (Figure 1), and differences in its content were confirmed statistically (\(p\leq 0.01\)). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Specification**} & \multicolumn{3}{c}{**Stage of Lactation**} \\ \cline{2-4}  & **Early** & **Middle** & **Late** & \(p\)**-Value** \\ \hline K (mg/L) & 1472.27 \({}^{\text{B}}\)\(\pm\) 146.19 & 1674.82 \({}^{\text{A}}\)\(\pm\) 128.37 & 1657.15 \({}^{\text{A}}\)\(\pm\) 143.89 & 0.001 \\ Na (mg/L) & 415.70 \(\pm\) 48.93 & 426.28 \(\pm\) 48.71 & 417.85 \(\pm\) 51.96 & 0.847 \\ Mg (mg/L) & 95.87 \({}^{\text{Aa}}\)\(\pm\) 8.55 & 85.98 \({}^{\text{ABb}}\)\(\pm\) 10.89 & 85.39 \({}^{\text{BBb}}\)\(\pm\) 9.79 & 0.008 \\ Ca (mg/L) & 870.90 \({}^{\text{a}}\)\(\pm\) 222.41 & 742.36 \({}^{\text{ab}}\)\(\pm\) 155.09 & 731.04 \({}^{\text{b}}\)\(\pm\) 145.62 & 0.020 \\ Zn (mg/L) & 3.97 \({}^{\text{ab}}\)\(\pm\) 0.85 & 4.59 \({}^{\text{a}}\)\(\pm\) 1.18 & 3.84 \({}^{\text{b}}\)\(\pm\) 0.85 & 0.071 \\ \hline \hline \end{tabular} \({}^{\text{A,B}}\)–\(p\leq 0.01\); \({}^{\text{a,b}}\)–\(p\leq 0.05\); phf–Polish Holstein-Friesian breed.  
\end{table} 
Table 5: Mineral composition of milk from phf cows at different stages of lactation (mean \(\pm\) standard deviation). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specification**} & \multicolumn{3}{c}{**Stage of Lactation**} \\ \cline{2-4}  & **Early** & **Middle** & **Late** & \(p\)**-Value** \\ \hline
**Number of Samples** & **30** & **30** & **30** & \\ \hline K (mg/L) & 1472.27 \({}^{\text{B}}\)\(\pm\) 146.19 & 1674.82 \({}^{\text{A}}\)\(\pm\) 128.37 & 1657.15 \({}^{\text{A}}\)\(\pm\) 143.89 & 0.001 \\ Na (mg/L) & 415.70 \(\pm\) 48.93 & 426.28 \(\pm\) 48.71 & 417.85 \(\pm\) 51.96 & 0.847 \\ Mg (mg/L) & 95.87 \({}^{\text{Aa}}\)\(\pm\) 8.55 & 85.98 \({}^{\text{ABb}}\)\(\pm\) 10.89 & 85.39 \({}^{\text{BBb}}\)\(\pm\) 9.79 & 0.008 \\ Ca (mg/L) & 870.90 \({}^{\text{a}}\)\(\pm\) 222.41 & 742.36 \({}^{\text{ab}}\)\(\pm\) 155.09 & 731.04 \({}^{\text{b}}\)\(\pm\) 145.62 & 0.020 \\ Zn (mg/L) & 3.97 \({}^{\text{ab}}\)\(\pm\) 0.85 & 4.59 \({}^{\text{a}}\)\(\pm\) 1.18 & 3.84 \({}^{\text{b}}\)\(\pm\) 0.85 & 0.071 \\ \hline \hline \end{tabular} \({}^{\text{A,B}}\)–\(p\leq 0.01\); \({}^{\text{a,b}}\)–\(p\leq 0.05\); phf–Polish Holstein-Friesian breed.  
\end{table} 
Table 5: Mineral composition of milk from phf cows at different stages of lactation (mean \(\pm\) standard deviation). In turn, the sodium content in the milk of phf cows in the subsequent months of lactation was almost unchanged (Figure 2). However, its content in the milk of the mb cows was significantly higher (\(p\leq 0.01\)) than in the milk of phf cows in the first, ninth and tenth months after parturition. Irrespective of the breed, the highest content of calcium was determined in milk from the tenth month of lactation compared to the previous months (Figure 3). The milk of mb cows, irrespective of the month of lactation, contained more calcium than the milk of the Polish Holstein-Friesian breed. A higher content of magnesium, irrespective of the breed, was determined in milk from the tenth month of lactation compared to the previous months (Figure 4). A descending tendency was determined in the zinc content of the milk of phf cows (Figure 5), while its content in the milk of mb cows was at a similar level.