Unless your horse is in really hard work and/or being heavily schooled every day for some sort of eventing, it doesn't need grain. Just a vit/min supplement to make up for whatever shortage there is in your pasture and hay; which the hay should be grass hay:)
EquiPride is a soy-free vit/min supplement witha pre-probiotic in it. It is in meal form and smells deliciously like wine. EquiPride - Pure Horse Products
Depending where you live, you may have to do some driving to get a bag and it isn't cheap. Their website has a dealer locator.
I have been driving 92 miles one way for the last three years to buy it in bulk and they give me a price break. Sadly, I just can't make that drive anymore so good bye EquiPride and I'm going to Plan B which is Triple Crown's 30%. It has soy it in and I will be holding my breath to see how my oat/corn/soy intolerant horse behaves on it.
I bought enough EquiPride for a year and stored it in a spare bedroom so it wouldn't go bad.
You use 10 ounces daily per horse so a 50 lb bag lasts me around 22 days for four horses.
Your horse may not like the taste of it so you could mix in some equine Rice Bran if your horse isn't way over weight. Most horses love the taste of rice bran. Tractor Supply sells Manna Pro's MaxiGlo in either the meal form or pellet form.
Hope this helps