Monthly variation and distribution of erythrocyte Na, K and Mg concentrations in normal dairy cows

Blood samples were taken from the caudal vein of 64 normal dairy cows monthly for 12 months and the concentrations of erythrocyte Na (ENa), erythrocyte K (EK), and erythrocyte (EMg) were determined. The overall range of ENa concentrations in the cows was 16 to 176 mmol/l red cells with a grand mean (+/- SD) of 96 +/- 15.4 mmol/l, and a positively skewed distribution with a degree of skewness of 4.4. The overall range of EK concentrations was 13 to 99 mmol/l, with a grand mean (+/- SD) of 32.8 +/- 13.12 mmol/l red cells, and a distribution which suggested bimodality with modes at 26 and 70 mmol/l red cells. The overall range of EMg concentrations was 0.1 to 2.6 mmol/l red cells with a grand mean (+/- SD) of 0.99 +/- 0.12 and mode and median of 0.97 mmol/l red cells; the distribution was that of a normal Gaussian curve. Analysis of variance showed significant effects with respect to cows and calendar months but not lactation months on all cations (all P less than 0.01). The calendar months effects were similar for cows with mean erythrocyte K concentrations below 35 mmol/l for Friesians and Jerseys but not for Australian Illawarra Shorthorn cows. The repeatabilities were 0.4, 0.9 and 0.2 respectively for ENa, EK and EMg. It was concluded that ENa had a moderate genetic component, EK a high, and EMg a low genetic component. There was no significant correlation between plasma Na and ENA or plasma K and EK, but there was between plasma Mg and EMg (r = 0.21, df = 307, P less than 0.01).