I would go to the various websites of grain sellers, check out what is in each type of grain, and then make a decision. Supplements are not meant to take the place of grain, but to supplement it. The nutritional needs of each horse can vary widely, depending on age, type of work and quality of feed available. Pasture alone may not meet the needs of any horse, again depending on what the horse is eating out there.
It is never the horse's fault