Many horse owners are confused about what supplements to give their horse and what exactly supplements a horse needs. Horse supplements are little like vitamins and good nutrition for humans. If a human eats a good, balanced diet and has average nutritional needs they usually don't need to take vitamins or supplements at all. If the human eats exactly what they should in exactly the correct portions the human needs no extra supplements or vitamins. Horses are very similar in this regard. When analyzing what supplements to give a horse, look first at the horses diet. Problems a horse is having should first be addressed through horses diet, not supplements or artificial aids.
In other words, a horse on that eats a grain-byproducts feed and is fed low-quality hay will frequently be lacking in certain vitamins and minerals. This horse could be given supplement to ensure it is getting the nutrients it needs, however, a better way to help this horse would be to put the horse on a higher-quality balanced feed and high quality, nutritionally rich hay. Deciding what supplements to give your horses should be secondary to what you feed your horses. Talk to your county extension agent about your horses feeding program and whether you should get supplements. Supplements should only be added to a horse's diet when feed and hay cannot supply a horse's need. For example, horses in regular and strenuous training often need a supplement that provides the building blocks needed to repair damage incurred by joints during training.
You should keep supplements for your horses to a minimum, as too many supplements can counteract and cause health problems for horses.