As you begin working your horse more, he will need more calories. At some
point he could stuff himself silly with hay and still not get enough energy.
Adding more hay might not be the answer, adding something with higher
calorie content might be. Nutritional balance is important too. Oh yes (as
Bob might say) lots of good, clean water!
I look forward to Truman's (who knows way more than me) and others' educated
and experienced comments to this question.
Melody