Studies on magnesium in ruminant nutrition

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Abstract
1. The excretions of magnesium, calcium, potassium and faecal dry matter (DM) by sheep grazing the same pasture have been determined at intervals throughout two grazing seasons. Two groups of four wethers, 2 and 7 years old, were used. Collections of faeces were total and of urine only partial; creatinine was used as an indicator of urine volume.2.The overall mean outputs of DM in the faeces of the young and old sheep were 509 and 387 g/day respectively.3. The values for the percentage digestibility of herbage DM ranges from 81.1 in spring to 63.2 in winter. The overall means for the estimated intake of DM by the young and old sheep were 1859 and 1405 g/day respectively and the difference was highly significant (P < 0.001).4. The mean values for overall excretion of Mg by the young and old sheep were respectively 0.452 and 0.292 g/day in the urine and 2.70 and 2.21 g/day in the faeces. For all sheep there was a highly significant rectilinear relationship between the amount in urine and in faeces, but there were significant differences between the regression coefficients for the individual sheep.5. The values for the excretion of Ca in urine and faeces were significantly higher for the young than for the old sheep; the respective means were 0.385 and 0.306 g/day for urinary Ca and 13.02 and 10.8 g/day for faecal Ca.6.The mean values for the excretion of K by the young and old sheep were respectively 28.2 and 29.1 g/day in the urine and 7.1 and 3.9 g/day in the faeces, the latter differences being highly significant.7.High values for the total excretion of Mg, Ca and K occurred in June and low values in winter.8. Intakes of Mg, Ca and K by the sheep have been calculated from the information on intake of DM and chemical composition of the cut herbage and compared with total excretion in urine and faeces. Good agreement between the two sets of values was obtained for Mg, but not for either Ca or K. The possible causes of these findings have been discussed and it was concluded that the sheep selected herbage with concentrations of Ca and K different from those in the samples of cut herbage.