Magnesium (Mg) is an essential mineral that horses require for proper nerve and muscle function. Magnesium functions as an electrolyte, plays a role in protein synthesis and is involved in over 300 metabolic processes in the horse’s body. It is especially important for growing and heavily exercised horses. 
Horses that are not getting enough magnesium in their diet may be excessively irritable, nervous, or they may chronically tie-up. If your horse experiences thumps (synchronous diaphragmatic flutter) they likely are low in magnesium.
Magnesium deficiency is more likely to occur in the springtime when fast-growing grasses do not accumulate much of this macromineral. Although severe deficiency is rare in horses, suboptimal levels can affect their mood and performance.
Magnesium toxicity is unlikely in horses receiving supplementation because excess amounts are efficiently excreted in the urine. Providing too much, particularly in the form of magnesium sulfate, might cause temporary diarrhea. Magnesium oxide is the recommended form for supplementation.
Why Horses Need Magnesium
Magnesium is a macromineral that horses need for optimal muscle function and nerve transmission. It is involved in energy metabolism, enzyme activity and muscle tissue recovery following exercise.
In horses, 60% of magnesium can be found in the bones and the other 40% can be found in extracellular fluids and soft tissues. It is especially important to maintain optimal levels of this mineral in horses that are growing or being heavily exercised.
Horses may not get enough magnesium when their feed levels are low. This is most likely to occur in the spring but can also happen at other times of year, for example if pastures are fertilized to encourage fast growth.
Horses that exercise frequently may have a greater need for magnesium because it is lost in sweat. Young foals also have a higher need to support growth.
Horses with colic might benefit from supplementation. Low magnesium levels are seen in 78% of horses with enterocolitis.
Approximately 40% of horses that have colic will have endotoxemia (presence of bacteria from inside the intestine in the blood) which triggers an immune response and causes magnesium levels to drop. 
More studies need to be done to assess whether administering magnesium would benefit these cases.
Health Benefits of Magnesium for Horses
Below are the top 10 reasons why horses need adequate magnesium in their diet:
- It works closely with calcium to maintain proper muscle function. When muscles receive nerve signals telling them to contract, calcium is released from special compartments within cells and moves onto muscle fibres causing them to contract. Magnesium stops the contraction by pushing calcium back into these compartments.
- By enabling muscle relaxation, it can support recovery and help to ease muscle pain and cramping in animals following heavy work.
- This macromineral can support athletic performance by enhancing oxygen delivery to muscles, and promoting muscle strength and metabolic processes that result in protein synthesis.
- Magnesium helps nerve cells transmit signals to each other and to muscles by regulating ion balance across cell membranes.
- It is thought to have a calming effect on horses, helping to ease muscle tremors and nervousness. It is frequently fed as a mood supplement to horses showing signs of excitability or abnormal behaviour. 
- It can improve tolerance to stress and resistance to illness and injury. Stress causes magnesium to be depleted in the body faster. Ensuring optimal magnesium levels can help speed up return to homeostasis following exposure to an external stressor. 
- It can contribute to normal fat distri