Abstract

In the course of adapting an existing sheep model of magnesium metabolism for use with lactating dairy cattle it was found that the model failed to predict plasma magnesium concentrations in agreement with experimental data after scaling the model for animal size dependent parameters. An inspection of the model indicated that faecal endogenous Mg loss and Mg absorption from the hindgut were the model components requiring further development. Pooled data from Mg balance experiments were used to investigate the relationship between plasma Mg concentration, net faecal endogenous Mg loss and the apparent availability of magnesium. A mathematical relationship between Mg intake and faecal Mg output is presented. It was derived from the generalised non-linear case to allow for variability in both apparent absorption of Mg and faecal endogenous Mg loss. A balance trial simulation was carried out in which net faecal endogenous Mg was implemented as a linear function of plasma Mg concentration. The results of this simulation were in close agreement with the analysis of pooled balance trial data and suggest that net faecal endogenous Mg loss should be applied to the model as a function of plasma Mg concentration. In fact net faecal endogenous Mg loss is not useful in modelling since it is the net result of a true Mg secretion function being subject to reabsorption in distal regions of the tract. Post rumen magnesium exchanges are examined in the model by implementing net Mg exchanges in the small intestine and hindgut as separate secretion and absorption processes. The difficulties in isolating the true secretion and absorption functions in the hindgut are discussed in conjunction with an initial estimate of these functions. In the modified model the incorporation of a variable endogenous Mg secretion dependent on plasma magnesium concentration provides an additional mechanism for magnesium homeostasis which is sufficient to allow the model to scale between sheep and cattle.

ST, Bell, JA Laporte, AB Robson, and AR Sykes

Proceedings of the New Zealand Society of Animal Production, Volume 61, Christchurch, 160-163, 2001