I have been dealing with Tying Up horses for the past 25 years and in 95% or more of these horses the problem relates to feeding. In my research there are three nutrients that are related to Tying Up; Electrolytes, Vitamin E and Selenium. All horses should have access to plain white salt at all times; in pasture, in pens, in stalls, and when traveling.
If your horse does not use salt blocks then one tablespoon of salt should be added to the feed daily. When adding loose salt only use the salt that does not contain Iodine. The Iodine in the salt at the groceries store is for humans and the level is too high for horses. Stamina Plus has designed its Equine Supplement with specific research so that it includes electrolytes and Vitamin E and Selenium at the correct levels to prevent Tying Up.
If your horse is on high grain diets (over 5 pounds per day) there is a shift in the anion/cation balance which in turn calls for specific electrolyte supplementation.