Attempts to scale up to dairy cattle from a model of magnesium (Mg) metabolism in sheep has shown more rapid reduction in plasma Mg concentration than has been observed in practice when rates of Mg absorption are reduced. Nutrient requirement tables of agriculture research council (1980) provide an estimate for endogenous loss of 3 mg/kg LW/d and, because of the paucity of data, apply this value to sheep and cattle. Such a value represents over 40-50% of the daily requirement for Mg of a typical New Zealand dairy cow even in early lactation. Moreover, the estimate is represented as a constant function of body weight. Net absorption of Mg in ruminants occurs largely in the rumen, but there is recent evidence suggests significant absorption occurs in the large intestine. Therefore, endogenous secretions into the gastrointestinal tract will be available for re-absorption in the large intestine. Under these circumstances it is important for modelling purposes to have an understanding of the true secretion of Mg from which net endogenous Mg losses and, therefore, maintenance needs can be predicted with accuracy. This paper argues from evidence in the literature that net endogenous losses of Mg are unlikely to be a constant. True secretion could be up to 15 times the net endogenous loss because of potentially large exchanges of Mg in different regions of the alimentary tract, which are probably a function of plasma Mg concentration. The case is made a new approach to estimation of maintenance requirement of Mg.
Proceedings of the New Zealand Society of Animal Production, Volume 61, Christchurch, 156-159, 2001
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