You don't need to feed grain at all. If you do, it puts even more Phosphorus into the horse's unbalanced diet.
You only need a loose mineral that is designed for livestock that is on lush grass. In our area, it is labeled as an 'un-medicated wheat pasture mineral'.
It solves several problems:
1) It supplies the additional Calcium that horses or other livestock need on grass.
2) It supplies the Magnesium needed to utilize the calcium and also helps horses that are nervous and anxious.
3) Any good loose mineral will also have Vitamin A added so it prevents rain rot, goopy eyes, bad skin, etc that is seen in the winter and spring months.
4) Any good loose mineral is palatable and horses readily eat it without any feed mixed in. As a matter of fact, when we get in a new horse that is mineral starved, they eat it like grain until they catch up with their mineral needs. I have had horses eat 10 or 15 pounds in a week or two until they catch up. Then, they eat 2 to 6 ounces a day with 4 being about average.
In the summer months, we also keep out a white salt block but they get most of their salt from their loose mineral.
Horses will stop eating trees, wood and dirt almost immediately when they are given a loose mineral like this one.