"stubborn as a Missouri mule" -- Is the "Missouri" reference here another put-down on residents of my fair state?

[Gerald Cohen]

I received the following query from a student at the Missouri School
of Journalism:


>"...I'm working on a story about the phrase "As stubborn as a
>Missouri mule." I'd like to know  where it came from and what it
>symbolizes in terms of the character of Missourians or the state of
>Missouri.
I'd like to get your comment on it.  ...


   In particular, the student would like to know whether "stubborn as
a Missouri mule" is somehow based on a popular perception of
Missourians as stubborn.
Or does the phrase simply refer to the popularity of the mule among
Missouri farmers and the fact that Missouri has been the leading
producer of mules.
The mule, of course, is also the official state animal.

Gerald Cohen