Studies on magnesium in ruminant nutrition

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Abstract
1. In expt 1 daily supplements of 0 or 27–28 g potassium, with 0 or 7·5 1. water, were given to each of eight fistulated wether sheep on a hay and concentrate diet in two 4 × 4 latin square experiments. Faeces were collected for the last 4 days of, and urine throughout, 10-day treatment periods.2. Adding K to the diet decreased the urinary output of magnesium by 33% (P< 0·001) and significantly increased those of phosphorus, sodium and calcium by 98, 76 and 150%, respectively. Faecal outputs of Mg and K were increased, whereas that of P was decreased. The retentions of P and Na tended to be decreased, whereas that of K was increased (P< 0·001). Mg in serum was decreased by 0·4 mg/100 ml (P< 0·05) and K increased by 4·9 mg/100 ml (P< 0·001).3. Increasing the water intake increased the urinary outputs of Mg, P, Na and Ca by 33, 165, 47 and 200%. The faecal output of Ca was increased and the retentions of Mg, P and Ca were decreased (P< 0·01).4. The effects of water and K were generally independent, but interactions affected the urinary outputs of P and K and the retention of K.5. The increases in urinary Na output were three- and eight-fold greater during the first 3 days of increased water and K intakes than during the balance study.6. In expt 2, mineral balance studies were conducted before and after supplementing the diet with potassium acetate, using five wethers from Expt 1. K intakes were similar to those of Expt 1. The effects of potassium acetate and KCl were generally similar qualitatively but the acetate produced greater decreases in urinary Mg and faecal P outputs and greater increases in urinary Na and K outputs than KCl. K in serum was increased by 28 mg/100 ml but Mg was not affected.7. The nature of these responses in discussed with particular reference to the aetiology of hypomagnesaemic tetany.