If your horse will hold weight with just hay and a ration balancer, that is the best way to go. But if you need a little extra energy, you can simply add a bit of fat to that. Otherwise you can use any of the balanced feeds available on the market. I would choose a pelleted one rather than a grain based feed.
Be sure and choose something that is easily available in your area. And if you know what brands of feed are available at your local feedstore you can do a bit of research online before you go in to the feedstore.