Sound Advice Calm Mix may assist horses who show symptoms of magnesium deficiency.
 
Symptoms of magnesium deficiency can include: irritability, muscle twitching or spasm, gait disturbances, hypersensitivity, nervousness, and fatigue.

Magnesium performs a broad range of purposes in the body. It has a role in the nervous system, heart, skeletal, muscle, intestinal, uterus, urinary tract and in the blood. 

Contains: magnesium oxide, distillers dried grains with solubles, saccharomyces cerevisiae yeast culture.
 
 
Dosage:
 
Magnesium requirements vary from horse to horse. Dosage will vary depending on other sources of magnesium in the diet. As a rule of thumb, 30 – 90grams of Calm Mix per day. (1 level scoop to 2 heaped scoops).

 
Some more information about magnesium:
 
http://lpi.oregonstate.edu/mic/minerals/magnesium
http://www.thehorse.com/articles/35764/study-magnesium-could-have-calming-effects
https://www.foxdenequine.com/About/Articles/Magnesium-and-Behavior.aspx