Studies on magnesium in ruminant nutrition. 11. Effect of lactation on the excretion of calcium and potassium by grazing monozygotic twin cows.

Abstract

1. The excretion of calcium and potassium in urine, milk and faeces by grazing monozygotic twin cows has been determined at intervals throughout four grazing seasons.2. In Expt I three sets of twins were used to study the variation within pairs in the excretion of Ca and K. 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 factors measured. The variation in urinary excretion of Ca within pairs was significant (P <0.05) in 1962.3. In Expt 2 the effects of lactation on the excretion of Ca and K by three sets of twins were studied over two grazing seasons. Only one of each of the twins was in milk at a time, each twin being in milk for one season.4. Lactation increased (P <0001) urinary and faecal Ca in 1964 but not in 1965. The concentrations of Cain the faeceswere lower (P <0.01) for lactatingthanfornon-lactating cows.5. Lactation increased (P < 0.001) the excretion of K in urine and faeces. Differences within pairs in faecal concentration were significant in 1964 (P <0001) and 1965 (P < 0.01), but the effect was not attributable to lactation.6. Significant differences between pairs were found for urinary Ca and K in 1964 (P < 0.05) and for the concentration of K in faeces in both years ( P < 001 or P < 0.001).7. In 1964 greatest total excretions of Ca were found in June and July and of K in May, whereas in 1965, although the differences between periods for Ca and K were significant (P < 0.001), there were no obvious seasonal trends.8. Mean intakes of Ca and K by the cows have been calculated from the estimated drymatter intake of herbage and the Ca and K content of the cut herbage and compared with total excretion in urine, milk and faeces. The differences between intake and excretion differed between periods (P < 0.001); for Ca, negative values were greatest in July and August in 1964 but not in 1965 and for K the values were small in July and large and positive in May, September and October in both years. The differences between lactating and non-lactating cows were significant for Ca(P < 0.001) and K (P < 0.01) in 1964. Selective grazing was considered to be the main factor responsible for these differences.