I prefer feeding a ration balancer, plus hay pellets/beet pulp/rice bran if calories are needed.
A ration balancer will provide the vitamins and minerals, and occasionally other goodies - like probiotics, your horse needs in a much smaller quantity than an "average" grain. You generally won't feed much more than a pound of ration balancer per day. Feeding a ration balancer also often ends up being cheaper than feeding grain which can be reallllly nice! Extra money is always good with horses around. Haha
My favorite ration balancer is one called Triple Crown 30 [it has a fixed formula which means every bag has the same ingredients and the same ratios of ingredients]. There's also Purina's Enrich and various other brands.
Here's a link that'll tell you more about ration balancers. They're really great. I've been using them for years and it's one of the best decisions I ever made. Ration Balancer
[also, I noticed you're new here - welcome to the forum!!]