I like grass or grass alfalfa hay, but mainly grass....
The grains have energy in them. In the feedlot I worked, occasionally a new horse would start getting tired and some of the older guys would add Red Cell which had a lot of Iron and some minerals/vitamins. After a week they'd get on and at coffee, exclaim, "Holy Smokes, he's ready to be taken off of that Red Cell!" I am not here as a proponent of Red Cell, but to say that using horses need to have good mineral and vitamin package.....
We fed oats in the lot, but I know corn has more energy....Some of the horses may have been able to use a tiny bit of corn, but I emphasize, feed way less than you would oats....
We feed our idle horses Purina Ultium. It is high in fat. We feed less than two cups a day of it....Reason????? Makes them dang easy to catch, LOL....I feed so little as it doesn't make them fat, but I will say, give them a high fat grain and the flavor will be something they can't resist....They are on free choice clean bluegrass straw ( they get thier fill, can't founder and don't chew on stuff) with a small flake of grass/Alfalfa hay, the ultium and Horse guard mineral pellets. If they weren't idle, I would replace grass straw with Alfalfa/grass and adjust the grain in accordance to how hard I was using them. You adjust by condition and attitude....
I will say one reason I like Ultium is also if I have to mix a powdered medicine or pain killer for an injured horse, I want that horse to be familiar with molasses flavor and like it. I have had horses on oats that never got a molasses feed and they will turn thier noses up for awhile.
I must be bored, LOL, I talked bunches, but really never addressed the original question in full.....I'd start with FH's recommendation, but also a good vitamin mineral pellet for your area to mix in. Then go up or down according to attitude and condition,
PPRM