Many years ago, a horse belonging to a friend of mine died of colic. When the autopsy was done, the vet found a huge ball of minerals in his stomach. This ball of unabsorbed and probably unabsorbable minerals was what had caused his death. Since then I have been very wary indeed of what I put into my horse’s feed.
There are a couple of things you need to watch for:
The best example of this is Selenium. An average horse needs about 20 mg of Selenium per day. Most horses will get enough from their normal diet. For these horses, supplementing even 5 mg extra can cause signs of toxicity.
This is because Selenium has an extremely low therapeutic index. This is the ratio of how much of a substance kills a horse to how much is needed. The difference, in the case of Selenium, is minute. It is a fine line to walk and I would warn anyone wanting to supplement Selenium in amounts larger than 5mg to be very aware of the status of their horse – blood tests as well as grazing, water and soil analysis will need to be done. Also be very sure of the quality of mixing done by the supplement company you are using.
A lot of supplements use the cheapest form of mineral, inorganics salts – eg. zinc oxide or copper sulphate. It is almost impossible for the horse’s digestive tract to absorb these – ie. they are not bioavailable. At best, they will pass out unused. Or they could dissacociate in the acid environment of the stomach then reassociate with other minerals, forming complexes that will never be absorbed and could potentially cause big problems.
The best way to fix this problem is to use chelated forms of minerals. This is where the mineral is bound to an organic carbon containing molecule, like an amino acid. These make sure that the mineral is bioavailable to the horse. But, of course, they are much more expensive, so are not widely used.
If your horse has problem feet, the odds are he or she will benefit from added zinc and copper (the chelated form, of course ;-)). A lot of nervous horses need extra magnesium. If your horse shows clinical signs of a deficiency of a mineral, then get tests done and supplement.
But, if your horse is healthy and happy, anything other than trace mineral supplementation is probably unnecessary and might even be dangerous.