“Camper: Is it easy to milk a cow? Farmer: Sure it is. Any jerk can do it.”

A joke about milking a cow is:
 
CAMPER: Is it easy to milk a cow?
FARMER: Sure it is. Any jerk can do it.
 
The “jerk” pun has been cited in print since at least 1970.
 
     
26 June 1970, Morning News (Wilmington, DE), “Scram-lets answer” puzzle, pg. 50, col. 1:
It’s a simple matter to milk a cow. ANY JERK can do it.
   
Google Books
The Gigantic Joke Book
By Joseph Rosenbloom
Illustrations by Joyce Behr
New York, NY: Sterling Publishing Company, Inc.
1978
Pg. 16:
City Boy: Is it easy to milk a cow?
Farm Boy: Any jerk can do it.
 
Google Books 
Nothing But Winners:
Over 6,000 one-liners, alphabetized and categorized for easy reference
By Pat Williams and Ken Hussar
Wilmington, DE: TriMark Publishing Company
1984
Pg. 140:
It’s really easy to milk a cow. Any jerk can do it.
 
15 June 1986, Washington (DC) Post, “Mighty Funny’s Mini Jokes,” The Mini Page, pg. 3, col. 2:
Q: Is it hard to milk a cow?
Mighty Funny: No, any jerk can do it.
 
1 June 1988, Philadelphia (PA) Inquirer, “Jumble” puzzle, pg. 17-G, col. 8:
Answer: Why it’s so easy to milk a cow—
ANY “JERK” CAN DO IT
 
Google Groups: alt.tasteless.jokes
Blonde Humor
Robert Nykvist
11/5/94
Canonical Collection of BLONDE Jokes
==============================================
Last Updated: 10/29/94
(...)
280.  Blonde:    “Is it easy to milk a cow?”
Country boy:  “Sure, any jerk could do it.”
 
Google Groups: alt.humor.puns
Puns of the Weak 11/16/01
Stan Kegel
11/17/01
(...)
It’s really easy to milk a cow. Any jerk can do it.  (Syman Hirsch) 
 
Google Groups: alt.humor.puns
Puns of the Weak 09-19-03 Part II The Rest of the Puns
Stan Kegel
9/19/03
(...)
Is it easy to milk a cow?
Sure. Any jerk can do it. (Sam, 8)
       
Twitter
Test My Code‏
@TestMyCode
This is kind of a cool joke it’s great - It’s so easy to milk a cow…: It’s so easy to milk a cow. Any jerk ca… http://76k.com/j/4md5ju
11:00 PM - 14 Sep 2009
 
Twitter
Jeffrey White‏
@WhiteJeffrey
I asked a farmer friend if it’s easy to milk a cow. He said “Sure it is. Any jerk can do it.” #fb
11:20 AM - 15 Oct 2009
 
Google Books
Teacher’s Joke and Story Book
Collected by Stanley Graham
Bloomington, IN: AuthorHouse
2016
Pg. ?:
Q. Is it hard to milk a cow?
A. No, any jerk can do it.
 
Twitter
Lucky Score‏
@LuckyScoreNet
Camper: Is it easy to milk
a
cow?
Farmer: Sure it is. Any jerk can do it.
#Joke from #FarmerJokes in #Jokes,  #Humor and #Satire
6:53 AM - 13 Jul 2017