I use a mixture of mane and tail conditioner and baby oil. Mix about 1/3 mane and tail and 2/3 baby oil. Squirt on, and let set until dirt balls appear dry (15 min or so). Repeat. Repeat. Then take a skip tooth comb and scratch at them. Don't pull.
Once you have all the loose stuff off, apply another coat of mixture and leave on. Work on it the next day and repeat above.
To sum it up: The mixture will soak into the balls, and will crumble them without damaging the hair. Depending on how bad the poo balls are, I can usually get one complete clean in a few days without harming the hair.
Good luck and remember, the calf didn't get caked with manure and dirt over night, so you probably won't get him clean over night!!!
Good Luck