Magnesium Absorption in horses - Hintz & Schryver
1) If diets are high in calcium, magnesium absorption is impaired leading to a risk of deficiency.
Experiments carried out by the Americans Hintz & Schryver way back in 1972 and 1973 make it very clear that the balance between calcium and magnesium in the diet of horses does not affect the absorption of magnesium. In fact the diet they tested that had the most calcium actually resulted in the highest amount of magnesium retained in the horse. In this work the ponies were fed the different magnesium diets for a month each. Other researchers have done similar work over different periods. Some Dutchmen working with foals had a 16 week time frame, an Australian, Jess Dodd, looked at a few hours or days and in our own work we have horses entering our trials having been on high magnesium diets for months or years.
2) Excess magnesium ingested is simply excreted in the droppings or the urine.
Hintz & Schryver looked at this too and it is clear from their work that whilst excretion increases as the diet level increases, it doesn’t do so enough. The more magnesium you feed, the more is retained in the body and specifically in the blood. Ponies fed not much more than double the RDA of magnesium had blood magnesium levels that we would these days regard as “above the normal range”.
This graph from the Hintz & Schryver 1972 paper clearly shows how the more magnesium you feed (bottom axis) the more is retained (left axis).
There are two Hintz & Schryver papers that deal with magnesium. Google them - they are both available free online:
Magnesium Metabolism in the Horse, Journal of Animal Science, Vol 35, no. 4, 1972 and
Magnesium, Calcium and Phosphorus Metabolism in Ponies Fed Varying Levels of Magnesium, Journal of Animal Science, Vol 37, No 4, 1973