“I had to give up my vegetarian diet. Turns out they’re a lot harder to catch than cows”

There’s a difference between “eating vegetarian” and “eating a vegetarian.” A joke was posted on the newsgroup alt.humor.puns on January 31, 2003, taken from the comic strip Zits by cartoonist Jerry Scott and illustrator Jim Borgman:
 
“‘I’ve been eating an all-vegetarian diet this week.’ ‘I’m impressed. How is it working out?’ ‘Not so good, I’m afraid. It turns out they are harder to catch than cows are. Plus the meat is a lot stringier.’ (Zits: Scott & Borgman)”
         
   
Wikipedia: Zits
Zits is a comic strip written by cartoonist Jerry Scott and illustrated by Jim Borgman about the life of Jeremy Duncan, a 16-year-old high school sophomore (previously a 15-year-old for the life of the comic). The comic debuted in July 1997 in over 200 newspapers and has since become popular worldwide and received multiple awards.
 
Google Groups: alt.support.sleep-disorder
New People
Lee Babcock
1/28/03
(...)
Makes me think of the Zits cartoon in the Star today.
He; I’ve been eating vegatarian for a week now.
She: assorted congrats on this direction
He:  Yeh, but they are harder to catch than cows and their meat is very stringy!
 
Google Groups: alt.humor.puns
Puns of the Weak 1-31-03
Stan Kegel
1/31/03
(...)
“I’ve been eating an all-vegetarian diet this week.” ”I’m impressed. How is it working out?” “Not so good, I’m afraid. It turns out they are harder to catch than cows are. Plus the meat is a lot stringier.” (Zits: Scott & Borgman)
 
Twitter 
Katherine Cederberg‏
@kathyceed
I tried an all vegetarian diet for a little while, it didn’t work out very well. Turns out they’re harder to catch than cows. evan willford
11:15 PM - 9 Aug 2010
 
Twitter
Michael Kearney‏
@Mikekearn
“I tried a vegan diet once. Turns out vegans are harder to catch than cows.”
7:20 PM - 10 Apr 2011
 
Twitter 
Alex Walter‏
@walter2824
I’ve been trying to eat an all vegetarian diet this week, it’s not going so well. It turns out they are a lot harder to catch than cows.
11:25 AM - 19 Aug 2011
 
Reddit—Jokes
I had to quit my vegetarian diet
submitted July 23, 2017 by coderotten
Turns out they’re a lot harder to catch than cows.
   
Twitter
LaffGaff‏
@LaffGaff
I had to give up my vegetarian diet.
Turns out they’re a lot harder to catch than cows.
#funny #jokes #jokeoftheday
4:12 PM - 29 Jul 2017