More information would be necessary to offer any sort of advice -
What is the horse's current body weight and condition?
What is the goal weight and condition?
When was the horse last vetted, teeth floated and de-wormed?
What is the horse currently being fed (weight, not scoops, flakes, etc - be specific as to type of hay, grains, etc)
What is the horse's work schedule?
Are there any underlying issues (ie ulcers) to be considered?