A cow riddle is:
   
Q: Why don’t cows ever have any money?
A: Because the farmers milk them dry.
 
The joke has been cited in print since at least 1993.
 
       
5 February 1993, Statesman Journal (Salem, OR), “Cheese is town’s consuming passion” by Ron Cowan, pg. 10, col. 3:
“Why don’t cows ever have any money? Answer—Because farmers milk them dry.”
     
Google Groups: alt.religion.rabbet
Humor from the Net
Bill McCray
12/11/03
(...)
Why don’t cows have any money?
Because the farmers milk them dry.  (Lederer & Ertner)
   
Google Books
Penguin Pocket Jokes
By David Pickering
London, UK” Penguin Books
2006
Pg. ?:
Why do cows never have any money?
The farmers milk them dry.
 
Twitter
Reid
@MooneyQuest
Q: Why don’t cows have any money?
A: Because farmers milk them dry
http://www.johncow.com/cowroast/
9:15 PM - 21 Jul 2008
   
Google Books
The Mammoth Book of Really Silly Jokes:
Humour for the whole family
Edited by Geoff Tibballs  
London, UK: Constable & Robinson Ltd
2011
Pg. ?:
Why don’t cows have any money?
Because farmers milk them dry.
 
Columbia (MO) Daily Tribune
Milk Man: Craftsman builds a cow for a cause.
By Karen Worley / .(JavaScript must be enabled to view this email address) | 815-1704
Posted Oct 30, 2015 at 12:01 AM
Updated Oct 30, 2015 at 1:00 PM
(...)
And one about money: Why don’t cows ever have any money? Because the farmers milk them dry!
   
21 February 2016, Sunday Telegraph (Surry Hills, New South Wales), “Laugh till your face hurts,” pg. 65:
Q: Why don’t cows have any money?
A: Because farmers milk them dry.
 
Twitter
Lucky Score
@LuckyScoreNet
Why don’t cows ever have any
money?
Because the farmers milk them dry!
#Joke from #CowJokes in #Jokes, #Humor and #Satire
1:15 AM - 15 Mar 2018