This is an excerpt from an article by Dr. Kathleen Crandell in EQUINEWS, Dec 29, 2011.
Magnesium is a vital macromineral, and it is becoming increasingly recommended by veterinarians for various treatments in the horse. So why do horses
require magnesium, and how does it fit for therapeutic use?
Magnesium is one of the major minerals in bone. Up to 60% of the body’s magnesium is found in the skeleton, with only 30% of that available for mobilization during times when it is needed elsewhere in the body. Magnesium is involved in over 300 enzyme reactions, including the generation of cellular energy and decoding of genetic information. It works in concert with calcium in nerve transmission and muscle contraction. The role of magnesium in muscle relaxation is the key to understanding subclinical symptoms.
Magnesium is found in varying quantities in forages and grains. Concentrate feeds may or may not have additional magnesium, depending on the amount in
the other ingredients. The amount of indigestible fiber and presence of oxalates will affect availability of magnesium in the forages. Supplemental magnesium
can be either inorganic (magnesium sulfate or magnesium oxide) or organic (
chelated magnesium). Magnesium oxide is perhaps the most commonly used source and has an approximate 50% absorption rate. The advantage of magnesium oxide is that the body will not absorb it if there is no deficiency, so it is difficult to overdose a horse on it. Magnesium sulfate has an effect of drawing water into the bowel and causing diarrhea, so it is not recommended for daily use.
Horses with colic that results in endotoxin release or laminitis are known to often have low blood magnesium levels, and there is hope that treatment with magnesium during these critical times may decrease the amount of damage that occurs.
Within the muscle, calcium and magnesium work antagonistically, calcium causing muscle contraction and magnesium inducing relaxation. If there is not
enough magnesium, muscles tend to spasm. Although the presence of low magnesium in the muscle tissue may stem from a genetic disorder rather than
dietary quantities, there are reports of horses that have responded to magnesium supplementation for treatment of chronic tying-up. However, the role of magnesium in nerve excitability has been established as a problem when synchronous diaphragmatic flutter occurs. Synchronous diaphragmatic flutter involves spasmodic contractions of the diaphragm and is usually seen in
endurance horses with electrolyte imbalances. The condition is also known as thumps. Treatment with calcium and magnesium has been found to speed recovery.
Severe magnesium deficiency effects such as seen in grass tetany in cows are rare in horses but have been documented. Just as calcium and magnesium have a balancing act within the body, so should it be in the diet. The two minerals are ideally kept within a ratio of 2.5:1 to 3:1, calcium to magnesium. The typical diet of the horse will usually fall into a suitable range of calcium and magnesium; if supplementation with magnesium is warranted, the amount of calcium in the diet should be considered.