You've heard of cattle? But why a duck?

In looking for info on the Marx Brothers, and Is that why Groucho had the duck?, I came across this line in their movie "Duck Soup":
"I could dance with you till the cows come home. On second thought, I'd rather dance with the cows when you came home.'

http://www.filmsite.org/duck.html

Are we goin round and round again, now back to cows?

Questioningly,

Angel