“What’s the difference between an orchestra and a bull?” is an old joke told by many musicians. The answer is that the bull has horns in front and an asshole in the back.
 
The joke has been cited in print since at least the late 1980s.
 
     
Google Books
The Horn Call (International Horn Society)
Volumes 19-20
1988
Pg. 114:
(What’s the difference between an orchestra and a bull????)
 
Google Books
Reflections on Toscanini
By Harvey Sachs
London: Robson Books
1992
Pg. 7:
“What’s the difference between a bull and an orchestra?” goes a riddle that recently made the rounds of players’ dressing rooms the world over. Answer: “On a bull the horns are at the front, and the arse is at the back.”
 
Google Groups: rec.music.classical
Musical jokes
“Vinay K. Chaudhri”
7/31/92
(...)
Q. What’s the difference between a bull and a symphony orchestra?
A. A bull has its horns at the front and its asshole at the back.
(...)
What’s the difference between a moose and a blues band?
The moose has the horns up front and the asshole behind.
 
Google Groups: rec.humor
Canonical List Of Music Humor (part 1/1)
Her Jangle Is Wide Angle
10/14/93
(...)
What’s the difference between a bull and an orchestra?
The bull has the horns in front and the asshole in the back.
 
Google Books
Grave Music:
A Bill Slider Mystery
BY Cynthia Harrod-Eagles
Thorndike, ME: Thorndike Press
1996, ©1994
Pg. 96:
‘You know the old saying, don’t you: what’s the difference between a bull and an orchestra?’
 
Google Books
The Musician’s Joke Book:
Knowing the Score
By Nancy Groce
New York, NY: Schirmer Books; London: Prentice Hall International
1996
Pg. ?:
What’s the difference between an orchestra and a bull? On a bull, you have the horns up front and the asshole in the back, while in the orchestra…
 
Google Books
Getting Opera:
A Guide for the Cultured But Confused
By Matt Dobkin
New York, NY: Pocket Books (Simon & Schuster)
2000
Pg. 11:
What’s the difference between an orchestra and a bull? The bull has the horns in the front and the asshole in the back.
 
Google Books
The Best Musicians’ Jokes
By Bruno Kassel and Carlo May
Pacific, MO: Mel Bay Publications, Inc.
2005
Pg. 73:
What’s the difference between a bull and a symphony orchestra?
The bull has its horns in front and its ass in the back.
 
Twitter
Roger von Oech
‏@RogervonOech
Music humor:
What’s the difference between an orchestra and a bull?
On a bull, the horns are in the front!
11:03 PM - 10 Nov 2013