Magnesium metabolism in the dairy cow. II. Metabolism during the spring grazing season

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Abstract
1. Three experiments are reported in which magnesium metabolism trials were carried out with milking cows that had been changed abruptly from typical winter rations to herbage cut freshly from swards at an early or at a more mature stage of growth and fed in the stall.2. The intake of herbage magnesium by individual animals varied from 9·5 to 15·2 g./day. Much of this variation was, however, associated more with differences in the palatability and dry-matter content of the herbage offered than with individual differences in appetite.3. The mean proportion of the ingested herbage magnesium excreted in the faeces was similar in all three experiments, being 82·3, 83·0 and 82·4% respectively. With any given sward there were, however, wide variations between individual animals in their utilization of herbage magnesium.4. The supply of ‘available’ magnesium to animals fed cut herbage in the stall varied from 0·5 to 4·2 g./day, as compared with values of from 2·6 to 10·5 g./day obtained previously (Rook et al. 1958) with cattle fed typical winter rations. In spite of this lower intake of ‘available’ magnesium, the animals maintained a positive magnesium balance.