@conference {41274,
	title = {The importance of an accurate estimate of net endogenous loss of magnesium in ruminants},
	booktitle = {Proceedings of the New Zealand Society of Animal Production},
	volume = {61},
	year = {2001},
	month = {Jan},
	pages = {156-159},
	publisher = {New Zealand Society of Animal Production},
	organization = {New Zealand Society of Animal Production},
	address = {Christchurch},
	abstract = {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. },
	issn = {1176-5283},
	author = {Laporte JA and Bell ST and Sykes AR}
}
@conference {41275,
	title = {Magnesium endogenous loss as a function of plasma magnesium concentration rather than a constant allows better development of magnesium models in ruminants},
	booktitle = {Proceedings of the New Zealand Society of Animal Production},
	volume = {61},
	year = {2001},
	month = {Jan},
	pages = {160-163},
	publisher = {New Zealand Society of Animal Production},
	organization = {New Zealand Society of Animal Production},
	address = {Christchurch},
	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. },
	issn = {1176-5283},
	author = {Bell ST and Laporte JA and Robson AB and Sykes AR}
}
