The foal body mass is about 0.05 percent magnesium, about 60 percent within the skeleton and about 30 percent within muscle.
Besides its critical importance in bone composition, magnesium is an important ion in blood, necessary for muscle contraction
and as a component of several enzymes.
Inorganic magnesium sources for foals are as beneficial, as is magnesium found in natural sources. Unlike calcium and phosphorus,
magnesium absorption is not affected by phytate or oxalate, though excess phosphorus has been shown to decrease magnesium
Clinical signs of magnesium deficiency include nervousness, muscle tremors and ataxia, with a potential for collapse, hyperpnea
and death. Hypomagnesemia also induces mineralization (calcium and phosphorus) in the aorta.
According to the NRC 2007, a 200-kg foal gaining 1 kg daily would need 1.25 g of magnesium for growth, plus 3g for endogenous
fecal losses, a total of 4.25 g/d.
The major intracellular cation, potassium, found mostly in skeletal muscle, is critical for acid-base balance, osmotic pressure,
and is the most quantitatively important ion involved in neuromuscular excitability.
According to the NRC 2007, "foals fed potassium-deficient, pelleted, purified diets gradually refused to eat and, therefore,
lost weight, became unthrifty in appearance and experienced moderate hypokalemia."
NRC 2007 also states that "for growth of foals with an anticipated mature body weight of 600 kg," the daily potassium requirement
is 11 g from 3 to 6 months of age, and 14 g from 7 to 12 months. A 215-kg foal gaining 0.85 kg bw/d, with a true potassium
retention efficiency of 50 percent, requires 2.6 g dietary potassium/d for skeletal growth in addition to 10.8 g/d for maintenance.
Sodium, with a growth requirement of 0.85g/kg/bw, is the primary extracellular cation, the primary electrolyte involved in
acid-base balance, and is necessary for osmotic regulation of body fluids. Sodium is essential for normal function of the
central nervous system, generation of action potentials and transport of substances, such as glucose across cellular membranes.
Sodium depletion results in decreased skin turgor, and causes horses to eat dirt and lick other objects to find it. In addition,
sodium-deficient horses show a slowed rate of eating, decreased water intake and ultimately cessation of eating. In acute
sodium deficiency, muscle contractions and chewing are uncoordinated and horses have an unsteady gait, decreased serum sodium
and chlorine concentrations and increased serum potassium. Thus, it is important that foals be introduced to a white salt
block or loose salt lick before weaning. The licking behavior may need to be learned by their dam, which is why pre-weaning
blocks would be necessary.
Chloride is an extracellular anion engaged in acid-base balance and osmotic regulation. Chlorine is an essential component
of bile and a constituent of hydrochloric acid, necessary for stomach digestion. The daily equine requirement for growth is
the maintenance requirement of 20 mg per kg/bw plus 13mg chlorine/kg/d up to 6 months of age (or 0.093 g chlorine x kg/bw),
and maintenance plus 5mg/kg/bw/d at six to 12 months of age (or 0.085 g chlorine x kg/bw).
Sulfur is found in the sulfur-containing amino acids (cysteine, methionine), B-vitamins (thiamin, biotin), heparin, insulin
and chondroitin sulfate.