EFFECT OF HIGH POTASSIUM AND LOW MAGNESIUM INTAKES ON THE MINERAL METABOLISM OF MONOZYGOTIC TWIN COWS

Balance trials were carried out with three sets of monozygotic (MZ) twin cows, each of which received 1 kg oat straw and 4·5 kg of a concentrate providing 63·8 or 184·6 g K per d and 2·28, 5·02 or 7·83 g Mg per d. The six diets were allocated to the cows according to a 6 × 6 Latin square design balanced for residual effects. The effect of K intake on plasma concentration and urinary excretion of Mg was dependent on the Mg intake, whereas that on faecal excretion and retention of Mg was not. The reduction in plasma concentration of Mg was greatest (4·2 mg per 1) at the lowest Mg intake. For urinary Mg the reduction was, in order of increasing Mg intake, 0·09, 0·45 and 0·66 g per d. Faecal Mg was increased by 0·62 g per d and retention reduced by 0·46 g per d at the higher K intake. The slopes of the regression between urinary excretion and intake of Mg were significantly different (P < 0·05) between sets of MZ twins; the regression coefficients were 0·07, 0·16 and 0·23 g per g indicating a three-fold variation in the efficiency of absorption. Significant differences between sets of twins were seen for urinary excretion of calcium (P < 0·05) and P (P < 0·001) and for faecal excretion of P (P < 0·01). There was a negative correlation between urinary and faecal P within the sets of twins. It was inferred that the high K diet had reduced the absorption of Mg from the gut and that this effect was of importance in the aetiology of hypomagnesaemia only where the dietary intake of Mg was also low. © 1979.