Magnesium as an Equine Dietary Supplement

Prevalence: Very rare under normal conditions.
Risk Factors for Deficiency:
- Long-distance transportation of unfed lactating mares.
- Prolonged administration of Mg-deficient enteral or parenteral fluids/nutrition.
- Grazing on deficient winter pastures (low herbage, high potash/nitrogen fertilization).

Maintenance Requirement: ~13 mg/kg body weight/day.
High-Demand Groups: Growing, lactating, and exercising animals may require double the maintenance amount.
Absorption Rate: 30%–60% (significantly higher than in ruminants).

Equine Metabolic Syndrome (EMS): While EMS horses may have low intracellular Mg, a controlled study found no improvement in insulin sensitivity or morphometrics with supplementation.
Headshaking (Trigeminal-mediated): Behavioral improvement noted with both IV and oral Mg supplementation (sometimes combined with boron).
Calming Agent: Frequently used by laypeople; however, low-level doses (3–10 g) are unlikely to influence behavior.
Sedation: Occurs only at high doses (>500 g of Mg sulfate via stomach tube).

General Safety: Considered safe for animals with normal renal function as excess is readily excreted by kidneys.
Palatability: Large amounts are often unpalatable.
Long-term Risk: Excessive supplementation may increase the risk of enterolith (intestinal stone) formation.
Regulatory Note: Some jurisdictions perform drug tests for excessive Mg supplementation.

Fractional Clearance (Preferred):
- Measures renal Mg clearance relative to creatinine clearance using a single sample of urine and serum.
- Normal: 15%–35%; Deficient: <6%.
24-Hour Urine Collection: Measures total renal excretion in mg/kg/day.
Mg Retention Test: Requires 48 hours of volumetric urine collection; currently offers no benefit over fractional clearance testing.

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