Diarrhea - is it serious?

I noticed today that Berdi has diarrhea. She hasn't had it before. It is almost like a cow pie but a little more solid. I can't think of any stresses or changes. She's had a grazing muzzle on but I've had that on her for 2-3 weeks already. Do you think it may be serious or should I just watch her and see if it clears up?

As long as it does not have any blood or anything in the poo your horse should be fine. If it keeps it up though I would go to the vet.

Ok - thanks for the advice!