Magnesium
Magnesium (Mg) is very often deficient in horses, and can be safely supplemented at levels of up to 25g a day (50g Magnesium Oxide). Animal Health Trust research gives a recommended RDA of 25g per day, particularly if the diet is high in calcium. Supplementation of magnesium in a deficient horse will have a huge effect on its wellbeing.
Any horse becoming loose on Mg should be given less until they are normal.
Magnesium Oxide, in the fine pharmaceutical form, is safer and more easily absorbed than most other forms. Magnesium Oxide in the form of Calcined Magnesite or CalMag is also safe, but less easily absorbed and some horses simply do not like the gritty taste.
Magnesium Oxide does not have the side effects associated with other forms (such as magnesium sulphate/Epsom salts found in some leading supplements or magnesium aspartate which is under suspicion of being a potential neurotoxin in sensitive horses).
Magnesium oxide is available from this web site for horse owner in my area - please email me for details.
Magnesium plays an important part in nerve and muscle function, and horses deficient in this important element can show signs of nervousness, wariness, excitability, and muscle tremors. This gives magnesium its reputation for having a calming influence on equines - however be aware that Magnesium in itself is not a "calmer", a horse that is not deficient in magnesium will not be "calmed" by being given more!
A magnesium deficient horse will probably have a poor tollerance to a heavy workload and may be prone to tying up.
Magnesium is also known to play an important part in reducing equine obesity, and can lessen the risk of laminitis in animals prone to it during periods of strong spring grass growth. When a horse with obvious signs of metabolic issues such as a fat pads on rump and shoulders and a cresty neck are fed magnesium, the crest and fat pads may start to dissappear, note that it will take about 3-4 weeks before an effect is noticeable.
One of our ponies had a crest so big and hard it felt like it had been inflated with a bicycle pump! After a few months on magnesium the crest has significantly diminished and is now back to being a normal neck. Also his mane became silky and soft and was no longer wiry and rough, and his coat became softer and actually changed colour in places! Evidence on the outside of changes happening on the inside.
Magnesium Oxide also acts as a buffer and reduces the acidity of the gut. This can be useful in horses with ulcers.
The arithmetic around how much to feed is a bit of a mind bending exercise!
If the average horse needs say 13 mg (milligrammes) of elemental magnesium per Kg of body weight it will need:
31 mg/kg/day of MgO - Magnesium Oxide
64 mg/kg/day of MgCO3 - Magnesium Carbonate
93 mg/kg/day of MgSO4 - Magnesium Sulphate
There are 1000mg to a gram, so the total daily magnesium requirements of a 500kg horse would be:
15.5 grams of magnesium oxide
32 grams of magnesium carbonate
46.5 grams of magnesium sulfate (that's a lot of Epsom Salts! )
Magnesium oxide provides the greatest available amount of magnesium among these three. There is some evidence that feeding salt (Sodium Chloride) at the same time as magnesium increases the horse's uptake of magnesium. Salt is usually very low in compounded feeds becuase it is hydrophilic and will shorten the shelf life of the feed.