Is your horse a hard, easy, or average keeper?
For many easy to average keepers, just grass hay fed in sufficient quantities (1.5-2% of the ideal body weight) is enough to meet their energy needs. However, it's also a good idea to feed a ration balancer because dried grass doesn't contain all the nutrients of fresh grass, and even if it did, it can still be mineral deficient.
For harder keepers, you can feed the same grass hay and ration balancer, but you will probably need to add some beet pulp and/or rice bran to supply additional calories.
Ask your stable if they've had their hay analyzed, and if they haven't ask them to do so. Many local extension offices will do hay analysis for very cheap, so it might even be worth doing it yourself if they won't.
If figuring out whether or not your horse is getting enough minerals, Ca:Ph balance, etc. is a little daunting, look into FeedXL.com, which is incredibly helpful in balancing your horse's diet.