Grain doesn't build body weight or mass, it's hay, and other forage\roughage that does. I would suggest that you switch the current free choice option to hay...unless what she is getting free choice is a mineral, which is fine, but I would still give her access to hay at all times. You may want to add some kind of feed that is designed for babies too; such as strategy, or a feed specifically for 'growth'. These will have a better balance of what she needs to grow properly.
Again, though, I would get her on a free choice hay as that is what will help her gain weight and body mass the best. A horse can live without grain, really, but roughage, free choice, or spread out into several feedings (for the adult horse)is what is necessary.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."