Instead of grain, I would add in a vit/min supplement, one for mares in foal if you can find one, and a fat source. For fat, look for something like stabilized rice bran, Omega Horseshine (flax meal), rice bran oil, or fortified wheat germ oil. Start with 1/2 cup of whatever fat source you choose. After a few days, go up to 1 cup. If you're feeding the flax meal, you can go up to 2 cups.
I'm not a fan of feeding grain either. I use ADM's StaySTRONG Mineral Pellets. It is lower in protein than ration balancers, but provides 100% or more of required vitamins and minerals a horse needs. I feed it to all 6 of my horses and they do really well on it. If you don't have a dealer in your area, you can order it online. StaySTRONG low starch mineral feed supplement
I also feed grass hay and alfalfa pellets for horses that need more calories. The StaySTRONG mineral pellets compliment that kind of diet well.