Summary
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 < 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 < 0.01).