As far as grains go it is far better to feed organic wholegrain feed, rather than synthetic pelleted diets, which are unnatural and highly processed. Synthetic vitamins added to commercial feeds are not well utilized by the body and natural enzymes are destroyed in the manufacturing process. Wholegrains are healthier, natural and more easily digested than pelleted food. The main grains to consider feeding are barley, oats and corn. The exact ratio's depend on the type of horse, the work that they are expected to do and individual preferences.
Some horses just do better on one particular type of grain.
As with all dietary changes for horses it is best to do it gradually. Horses are so susceptible to colic if the food is suddenly changed so just start adding a little of the new food daily, and slowly increase the amounts, and decrease the old food over several days.