It's very hard doing it without water. With water it washes right out after 10-20 seconds from a high pressure showerhead. Works wonders. And yeah, what everyone else said about dog food. Try some fish-based, grain free dog food. We have a friend's German Shepard who is swapping to this in order to make his stool look less like soft serve:
https://www.amazon.com/CANIDAE-Grain-Formula-Fresh-Salmon/dp/B00ECOLCUM/