Studies on magnesium in ruminant nutrition

In Expt 1, five groups of four dry non-pregant ewes were given 1 kg/day of a semi-purified diet containing either 0.05, 0.10 or 0.15% magnesium with 0 or 4.44% added potassium. Plasma Mg Concentrations were reduced by increasing the K intake (P < 0.001) and by reducing the Mg intake (P < 0.1). The effects were additive and, at the lowest Mg intake, supplementary K induced two cases of hypomagnesaemic tetany. 2. Expt 2 was similar to Expt 1 except that the ewes were suckling single lambs, they were given 2 kg food/day and the lowest dietary Mg concentration was 0.075%. K again lowered plasma Mg values and induced two cases of tetany but reductions in Mg intake had no effect. 3. After repletion on the basal diet, sixteen lactating ewes from Expt 2 were transferred to a diet containing 0.05% Mg with 0, 2.22 or 4.44% added K. Plasma Mg concentrations fell rapidly on each treatment; the effect was greatest in K-supplemented groups ( P < 0.05) and two cases of hypomagnesaemic tetany were induced at the highest K intake. 4. In each experiment the K supplements caused a rapid increase in plasma K concentrations to a relatively constant level some 5 mg/100 ml above the resting value of 18.0 mg/100 ml. 5. Some sheep refused K-supplemented diets at first and there was a tendency for refusals to be greatest at the lowest Mg intakes. 6. The results are discussed in relation to the aetiology of hypomagnesaemic tetany.