Horse manure is every nutricious and rich, that is most likely why they eat it. There is also a lot of undigested food items in manure, which the dogs may be attracted to also. Foals eat their mothers fresh manure to get nutrients from it too.

I would make sure they stop eating it ASAP, since manure can harbor eggs/larva of worms such as whip, hook, and tape- play it safe....