On the subject of nutrients, one that has had a great deal of attention in recent years in horse nutrition is magnesium. Magnesium is an essential macro-mineral, which is involved in bone structure, muscle and nerve function, and enzyme activation. The horse’s body consists of about 0.05% of magnesium, 60% of which is found in the bones and 30% in the muscle.
Magnesium depletion is associated with abnormal muscle and heart function, neurological dysfunction and brain malfunction. Overdosing on magnesium can induce an anaesthetic effect. A 500 kg horse requires a minimum of 7.5 g of magnesium daily, rising to 15 g for a hard working horse. Magnesium is lost in sweat but in low quantities, and the bone and muscle adaptations that result from exercise raises requirements. Growing youngsters require elevated levels, especially if they are also in work. Lactating mares require 11.5 g during month 1, dropping to 10.5 g at month 4. These levels are minimum requirements and horses are often fed a little more to ensure their individual needs are met. Magnesium is absorbed from both the small and large intestine, although most is taken up via the small intestine.
Most horse feeds supply between 0.1 and 0.3% magnesium, with leafy forages being a good source. Magnesium absorption from natural feeds is about 46-60%, whereas inorganic sources such as magnesium oxide, carbonate and sulphate have higher absorption rates, of about 70%. These absorption rates are taken into account when requirements are calculated.
Magnesium oxide is commonly used to supplement diets, and at 56% magnesium, it is a good source compared to others. Magnesium sulphate (Epsom salts) is a poorer source at just 9.8% magnesium, and causes gut irritation and a laxative effect if fed in high quantities. Magnesium chloride, lactate, and aspartate have higher absorption rates than the oxide or carbonate compounds in humans, but they contain much less magnesium per gram so much more needs to be fed, and any absorption benefit in the horse is probably negated. Excess dietary phosphorus or potassium inhibits magnesium absorption, but excess calcium and oxalates does not seem to affect it.
Magnesium is widely supplemented for a calming effect, and although there is currently no evidence that increased magnesium intake over and above requirements improves brain function or reduces excitability in horses, it is possible that some individuals could have higher requirements than published values. Those individuals may be responding to normal, balanced intakes when supplemented, rather than responding to any therapeutic effect. The same might apply to horses who seem to respond to magnesium supplementation with healthier hoof growth and function.
Magnesium intakes can safely be doubled compared to requirements, with 10 g of magnesium (equivalent to 20 g of magnesium oxide) for a 500 kg horse commonly supplemented to support normal behaviour general metabolism.