Two square meals of Alfalfa (1 flake each). Maybe 1/2 flake extra for an active horse. 1/2 pound O&B with an electrolyte. Keep it simple. There are 250 healthy happy horses at my stable. That's what they eat.
If the horse is competing once a week and is worked every day for more than 2 hours.... go see your vet. Be careful. Remember these anmals are designed to carry on at a slow pace and graze.