Calcium carbonate calculi formed in digestive tract of horses fed a diet of 35% wheat and 25%rice with other ingredients

We used a commercial product that caused massive amounts of calcium carbonate calculi in the horses' small and large intestines, and cecum. On necropsy, the gut looked like the horse had eaten a sack of Quikcrete. Eight horses died, 2 went to surgery and survived, but had copious amounts of these gravel looking stones that had to be laved out . Everyone we have talked to is totally stumped on how this occurs in the horses digestive tract. Our water and gay has been analyzed, no problems there. We changed over to oats after losing the 8th horse last April. We utilized apple cider vinegar in attempts to clear the intestines and cecum of residual material. So far no reoccurances, but have been told by vets there is no way to know if the plaqueing agents are gone. So it's wait and see for 41 other horses that were fed this product. Horses that were fed more of the feed succumbed earlier, rodents in the feed room would not eat this feed, nor would local bird flocks ( grackles, blackbirds, sparrows, doves) eat what the horses left. Horses never seemed satiated by feed, either. Anyone out there have any ideas??? It's got to be some radical chemical event, a perfect storm so to speak.