A good quality legume and grass hay or pasture available 24/7 is the ideal diet for most adult horses. Horse under training may need additional energy in the diet in the form of grain. Free choice salt should also be available. If the horse is getting a commercial horse feed they do not need the red mineral block since the feed already has that added. The plain white salt block should be used. Supplements need to be added in careful consideration of everything in the horse’s ration to keep the diet balanced.
Pastures and hay might be lacking in some nutrients. A lot depends on climate and growing conditions. Hay can be tested by your cooperative extension service to evaluate its nutrients, and a vitamin/mineral supplement can be added as needed.
Here is a link to a manual on feeding performance horses published by Washington State University, which I think you’ll find helpful:
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