My mare and 9 month old giant foal (he's still feeding off mum) are on fibre beet, chop (chopped grass) and ready mash (not sure what the equivalent is in the US - will have to look) and a vit supplement and salt - along with the odd treat of apples and carrots. They are both grazing during the day although there's not much grass growing during winter and have free access to hay 24/7. (My mare stiffens up if kept in so she's out to graze every day no matter what the weather - we had gale force winds last week and on one of my checks, she was grazing as though it was a beautiful summer day - nothing fazes my girl.
The weather in the UK is now starting to get very cold but both are looking good (if not muddy haha) and are still unrugged as yet.
I'd keep it plain and as simple as you can - if you look at the "additives" in your local fees store you'll find you could use every one - when in fact you probably don't need the majority.