Let's agree that hunters hunt ducks, and that ducks, even ducks that are hunted by hunters, eat. So:
"Ducks that hunters hunt eat."
Let's also agree that alligators will eat ducks if they have the chance. So:
"Ducks that alligators eat eat."
Now, what if there were cannibalistic ducks? Well, even the ducks that those ducks eat eat:
"Ducks that ducks eat eat."
In sentences with restrictive relative clauses like these, "that" can be omitted:
"Ducks ducks eat eat."
Now, suppose that ducks not only eat, but they also duck. Then one can also say:
"Ducks that ducks eat duck."
"Ducks ducks eat duck."
Finally, suppose that ducks not only eat and duck, but they also duck other ducks. Then:
"Ducks that ducks duck duck."
"Ducks ducks duck duck."
Wikipedia has an article on this sort of thing, involving buffalo from Buffalo that buffalo other buffalo from Buffalo: https://en.wikipedia.org/wiki/Buffalo_buffalo_Buffalo_buffalo_buffalo_buffalo_Buffalo_buffalo
Your attempt with "moo" doesn't work because the concept of cows mooing ducks doesn't work--"moo" doesn't take a direct object. It might have been a bit clearer if you'd gone with "Ducks cows milk duck" if cows were in the habit of milking ducks.