I have a mare that poops like a cow. She's been scoped and had all the vet work. The conclusion is this is just how she poops. She's on pasture in the summer and 24/7 hay in the winter plus triple crown senior. We tried 2 jugs of Gut X and it did nothing. I treated 7 days with Sand Clear and nothing. Using a probiotic seems to be helping a little off and on so I think getting one that is high quality with better ingredients could be more helpful. I'm trying to decide between 2 probiotics. Has anyone had any experience with immune-biome lean muscle or Forco? Anyone had a horse that sh**s like a cow and found something that helped? 

https://www.immubiome.com/collections/horse-supplements/products/immubiome-lean-muscle

https://forco.com/feed-supplement-digestive-fortifier/

Forco has worked on horses like this. It's a prebiotic, not a probiotic. Prebiotics are needed so the horse can actually utilize probiotics. 
I would try a double dose for 2-4 weeks and then go to a single dose when things get better. It's very affordable. On a normal dose you would use about 5# per horse per month. 

Yogurt....had one that would splatter the walls.  Old Timer told me to use yogurt and it would either stop or make it not so runny.  Fed it my horse twice a day and stopped it for awhile. When it would come back I'd start feeding him the yogurt again

Thank you both!

If possible you might try feeding her some dry hay ( not alfalfa) 

I have an old gelding who often gets like this, Bio Sponge is the only thing that clears it up.

Soaked beet pulp worked wonders on 2 of mine that had this issue. 

Try giving psyllium for a few days. Thats what cleared my gelding up. 

I'm assuming your vet work included a blood sample and a fecal sample?

I tried Forco for nearly 9 months. Didn't do anything. I'm having great luck with Probios powder. 

My gelding had cow patty poops, vets never pinpointed a reason. He was on Bluebonnet feed/supplements, their 21 day gut program 2x.  Switched to Triple Crown soy free- no real improvement with TC. I then went to forage only diet and removed all the supplements. I saw a big improvement on the forage only diet and eventually added in any necessary supplements. The forage only diet was the answer for him. The vets did not suggest this option, it was something I looked into myself. 

When I work with horses that exibit this problem I take away soy and grain based feeds and add Renew Gold and Forco. Unless there is a underlying serious issue, this usually does the trick. I have found that soy is often the problem.