Not sure what country your in or what feed you have but here in AUS a lot of our feed companies have a questionnaire you can fill out online about your horse and they give you a feed plan for everything you need to feed
If your horse isn't over/under weight I would recommend a ration balance to give them the vitamins and minerals they don't always get in hay plus some of them even have protein for the horse. We use Essential K at our barn for a ration balancer.
Also depends on if your horse is under of ooverweight
Some horses also get grain. The type and amount depends on there workload and body type. If your not 100 percent sure try talking to your vet
Depends on what you do with your horse. If you ride regularly you'll need an energy supplement mixed with chaff.