Don't know if this would help your horse, but since not all minerals are alike there is another choice of minerals that you could try as a supplement. From all of my research this is the best I have ever found. Only recently become available. Identified after a farmer saw animals eating the dirt in one location, and no where else. Turned out was 15 million year old deposit, that came from the Colorado River picking up pieces of mountains along its 1450 mile length. They were laid of rest over the San Andreas Fault so they were cooked at high temperatures. The 57 minerals are micron size. May be worth a try in some cases because of the high cost of severe cases that could have been prevented.
Type in Ion Charged Horses to find on Amazon
Have saw do great things. Could try and experiment.
You need to determine for yourself.