Ah, the million dollar question! My opinion is if they are on a decent amount of a high quality grain, say 5 pounds of Ultium or Senior or Omolene, you would not need a vitamin/mineral supplement but there are a lot of variables. While my gelding is getting 10 pounds a day of high quality feed, he is in heavy work so gets SmartLytes, DMG and Smart Senior Combo Ultra. Because of his work load, I feel he needs optimal nutrition. There are a lot of things we feed that we have to go on faith and research that they are of use. I would say if he gets no grain, maybe a ration balancer like Enrich32.