Purina Equine Senior, beet pulp based, my 25yo stud has been on a 10# a day ration for 10 years. I buy it by the ton twice a year. He gets ridden alot, most horses wouldn't need that much.

You get the idea, spend your money on a good chow that has the basics built in, baseline the ration to the chart on the bag, adjust it up or down gradually as needed using body fat as a guide, and then figure out if you need supplements for specific problems. Always change a horses diet as gradually as possible. Consider custom mixing Senior with a 10-12% oat based sweet feed at 3 to 1 or so to economize. Make sure whatever you might mix does not have corn, corn is tough for any horse to digest, much less an older one.

How old is this horse and what is his current diet?