Studies on magnesium in ruminant nutrition

Abstract

1. The excretion of magnesium in urine, milk and faeces and of faecal dry matter (DM) by grazing monozygotic twin cows has been determined at intervals throughout four grazing seasons.2. In Expt 1 three sets of twins were used to study the within-pair variance in the excretion of Mg and faecal DM. It was found that the variation within pairs was non-significant and small, less than 5% of the overall mean for all but one of the measurements made. The variation in urinary excretion of Mg within pairs was significant (P <0.05) in 1962.3. In Expt 2 the effects of lactation on the excretion of Mg and faecal DM were studied. Only one of each of the monozygotic twins was in milk at a time, each twin being in milk for one season.4. There were no significant differences in faecal nitrogen within and between pairs. On average, lactation increased faecal DM by 30% in 1964 and 24% in 1965.5. The effect of lactation on urinary Mg was not consistent. For faecal Mg the effect of lactation was a simple reflection of the increased faecal DM since lactation had no effect on the concentration of Mg in the faeces. There was no correlation between the Mg excretion in urine and milk, either singly or together, with faecal Mg excretion.6. The concentration of Mg in milk differed between twins and increased with length of lactation.7. Mean intakes of Mg by the cows have been calculated from the values for the intake of DM and the Mg content of cut herbage and compared with total excretion of Mg in urine, milk and faeces. There was a significant correlation (P <0.05) between the sets of values but estimated intakes were lower (P <0.05) than excretions. The differences between intake and excretion differed between periods (P <0.001), being greatest between July and September. Selective grazing was considered to be the main factor responsible for these differences.