Hogwash! by Karma Wilson, illustrated by Jim McMullan

From chapters.ca: He washed the horses, ducks and cows; The goats, the cats and dogs. Everything went dandy... Until the farmer reached his hogs. "No hogwash for us today. Pigs love dirt - so go away!" Farmer's stubborn pigs refuse to be hogwashed, and no matter what he tries, poor Farmer cannot trick them into getting clean! But when several failed attempts leave him stuck in the mud, our determined farmer realizes his hogs may have been on to something all along... sometimes a mud bath is even better!