Agree with the others. But why??
Firstly why do you want to feed grain(or whatever)? If your horse needs more energy, for high performance work, there are other low starch options, but if grain is the choice, oats are a pretty good one for horses. Far more digestible than others & (relatively) low starch. But it's still important to feed little & often, rather than infrequent &/or large meals daily. So if you can only manage once daily feeds, I'd opt for low starch, easily digestible options, such as alfalfa, rice bran, copra, etc. If you need to put weigh on a horse, there are also other options, such as the above & grain isn't generally the best for putting weight on anyway.
Horses are built for eating small amounts of 'low grade'(compared to 'improved' pastures & rich feeds) forage near constantly. They don't cope well with long periods of hunger, too rich/large/infrequent feed & need to develop different gut enzymes & flora for some feeds, such as starch, fats & oils, etc. They develop them with regular small amounts & they die off without use or if overloaded.
So... basically Spirit said it above - hay/grass & a nutritional supp to 'fill the gaps' is fine for maintenance. If you find your horse is losing (too much) weight, then you can add as necessary. Oh & google 'magnesium for horses'