Dietary Mg deficiency in horses is very rare, unless extreme conditions combine to result in decreased consumption and increased demand, eg, long-distance transportation of unfed lactating mares or prolonged administration of enteral or parenteral fluid or nutrition solutions deficient in Mg. Despite this, Mg supplements have been advocated by laymen as a calming agent or as an adjunctive therapy for equine metabolic syndrome, and even to prevent laminitis. There is no evidence that these uses are beneficial or effective, and such practices should be discouraged. One randomized placebo controlled study showed no difference in either insulin sensitivity or morphometric variables in horses supplemented with Mg (8.8 g/d) and chromium. Although sedation can be seen in horses administered >500 g of MgSO4 as a cathartic via stomach tube, low level Mg supplements are unlikely to influence behavior, and larger amounts would be unpalatable in the feed or even dangerous if administered as repeated dosages via nasogastric tube (see Hypermagnesemia). Recommended doses of 3-–10 g are unlikely to have a clinical benefit. However, low-level supplementation is considered relatively safe even if not efficacious, because excess Mg will be readily excreted by the kidneys in animals with normal renal function.
The maintenance Mg requirement for horses has been estimated at 13 mg/kg body wt/day and can be provided by a diet containing 0.16% Mg (1,600 ppm of feed) or by adding Mg oxide at 31 mg/kg/day, MgCO3 at 64 mg/kg/day, or MgSO4 at 93 mg/kg/day. This may be important when formulating oral replacement fluids for inappetent horses. For a 500-kg horse, this would equate to Mg oxide at ~16 g/day, MgCO3 at 32 g/d, or MgSO4 at 47 g/day. Growing, lactating, and exercising animals may require double these amounts. Horses that are obtaining adequate feed by grazing, with hay or grain, are unlikely to be Mg deficient. Horses are able to absorb 30%–60% of the Mg provided in their feed, which is higher than the absorption rate in ruminants.
If a pasture is considered Mg deficient (winter pastures with little herbage that have been fertilized with potash and /or nitrogen), then renal excretion of Mg may be used to evaluate Mg balance. With low dietary Mg intake, urinary Mg excretion falls to negligible levels. Renal Mg excretion is measured in urine collected throughout 24 hr (mg/kg/day). The fractional clearance of Mg can be determined by expressing the renal Mg clearance relative to the creatinine clearance and requires only a single sample each of urine and serum. Fractional clearance of Mg in healthy horses fed grass hay ranges from 15% to 35%, and values <6% indicate inadequate dietary Mg intake. A Mg retention test to assess total body status has been evaluated in horses receiving Mg-deficient diets, but this test offered no benefit over performing fractional clearances and requires 48 hr of volumetric urine collection.