Need advice for mare with loose stools please

You've tried a probiotic, have you tried a prebiotic? Try Diamond V at 1 cup per day. It's cheap at $35/50#.
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You might try a psyllium product such as Sand Clear. If your horse has been getting a bunch of dirt in when she eats her grass, she could have sand/dirt built up in her intestine. Diarrhea is one of the first signs of sand colic.