Aside from the fact this has already been answered in other posts by this person (do a search).

I will add for those other people reading that eating horse poop is VERY bad for dogs. A couple years ago we had a very sick dog that was heck to try and find out what was wrong with it. A few cultures and blood tests later and turns out it had Ecoli!! Upon discussing it with the owners turns out the dog had been eating horse poo but they didn't think it was important enough to mention it during the visits.