Do you remember anything about the plot? Maybe one of our literature detectives can find it. :)
I just found it. I typed the following words into google -- "William Faulkner" "short story" manure -- and it popped right up: Barn Burning.
Problem was, once Faulkner got famous the temptation to publish the contents of his wastepaper basket was just too great. And IIRC he was occasionally not entirely sober.
Still, he wrote some good stuff, you just have to pick and choose. I read "As I Lay Dying" in high school - scary, gross in spots, very effective. I like "The Bear" (taken from Go Down, Moses and "Spotted Horses" - taken from The Hamlet and of course Absalom, Absalom!