Some medical conditions can make a horse more likely to founder, such as low thyroid. Another sign is a long, curly coat slow to shed out. The tumor grows slowly and the signs develop gradually. The tests are ACTH and Dex suppression. There is medication available to control it, but it is expensive.
Malnutrition is a subtle thing.
A horse can be 300 lbs. I recently checked with the agricultural lime dealers in my area, who tell me that the only lime available in my area is calcium carbonate. To get dolomite in large quantities for spreading on large fields, you have to go to a quarry in Northern Ohio. So even if you are keeping your fields limed, it is not enough. Add 1 Tbsp.