Efficient nutrient utilization by livestock is a function of adequate intake of a given nutrient, its unimpaired release from the ingested feed and optimal availability to, absorption by, and retention in animals. Optimization of these functions to fulfil livestock physiological requirements is not often achieved under normal feeding conditions. A nutrient imbalance, especially of minerals, can initiate the onset of clinical disease such as parturient paresis and hypomagnesaemia. An additional difficulty of improper nutrient balance arises in diet formulation, since the addition of one nutrient almost invariably affects the concentration and utilization of another. The antagonism between sodium (Na) and potassium (K) and their specific involvement at the cellular level in water metabolism, nutrient uptake and transmission of nerve impulses is a firmly established example. High levels of Na ingestion such as often occurs in saline environments or when loose forms of Na are included in conserved feeds can be detrimental to animal performance. However, at levels more typical of practical livestock feeding, Na nutrition is beneficial to livestock productivity. Sodium increases the divalent cation contents of forage and depresses K (Moseley, 1980; Chiy and Phillips, 1993), improves in vivo DM and fibre digestion and the release of inorganic nutrient from fresh forage grazed by ruminants (see recent review by Chiy and Phillips, 1995).