why would a horse do this?  they get feed Tribute Calm n Ez, alfalfa hay, THE MM, salt block and mineral block outside and they are turned out for 10-12 hrs. fed AM & PM.  3x a day is not an option. and they get used. they are not pasture ornaments.

Bump

How much poop do they eat?? Like a whole pile or just a little bit here and there? If it were mine, I'd start with the feed. Try changing or removing it and see if that helps. Process of elimination.... I think they typically eat their own poop if somethings off I  their diet. 

I had one that also did this. Consulted with my vet and told me it typically means they are lacking something. Switched to a complete feed and he stopped almost immediately.

I've heard it a sign for ulcers. 

I am currently treating one of them for ulcers..so hopefully that one stops :)  

Lacking salt or mineral

Mine would do that along with eating on the bark of the trees......I started putting out loose mineral and I haven't had an issue since