Can you mix diatomaceous earth, MSM and glucosamine in the feed?

It is sensible to think about any side effects of giving these supplements together. Unfortunately there is very little research on how individual supplements work, and none at all on how they interact with each other. The studies that do exist on glucosamine show that horses absorb almost none of it from the gut. MSM is a sulphur supplement. Sulphur occurs in several amino acids, which are the building blocks of protein, so a horse getting enough protein takes in sulphur that way. I hope this answers your question a little, even if there is a great deal we still do not know.