A popular riddle about Milk Duds candy is:
 
Q: What do you call a cow that doesn’t give milk?
A: A milk dud.
 
The joke was cited in print in 1972. A slightly different version was posted on Twitter on August 29, 2009:
 
“What do you call a cow that can’t moo? A Milk Dud.”
 
 
Wikipedia: Milk Duds
Milk Duds are a caramel candy, historically enrobed with milk chocolate and currently enrobed with a confectionery coating made from cocoa and vegetable oil. They are manufactured by The Hershey Company, and sold in a yellow box.
 
18 May 1972, The Oregonian (Portland, OR), “Dairywives crack jokes, make pitch to Clark County businessmen” by Sally Allen Gear, sec. 2, pg. 1, col. 2:
“There are more than 10,500 cows in Clark County…and by the way, what do they call a cow that doesn’t give milk” she asks.
 
“A bum steer?” “An udder failure?” are the responses. Stock answer is “a milk dud,” but she waits for other puns before spieling it.
     
27 August 1990, Detroit (MI) Free Press, Bob Talbert column, pg. 7C, col. 6:
What would you call a cow that doesn’t give a milk dud?
   
16 January 1996, Chicago (IL) Tribune, “Playing Around,” pg. 9:
What do you call a cow that doesn’t give milk?
A milk dud.
 
Google Groups: rec.humor
2 jokes for kids
1st Mate
6/3/97
What do you call a cow that doesn’t give milk?
A Milk Dud.
 
Google Books
March 2003, Boys’ Life, “Think & Grin,” pg. 57, col. 2:
Biff: What do you call a cow that doesn’t give milk?
Cliff: What?
Biff: A milk dud.
—Matt Vandeberg, Vancouver, Wash.
 
Google Groups: alt.humor.puns
Puns of the Weak 10-10-03: Part 2 The Puns
Stan Kegel
10/10/03
(...)
What do you call a cow that doesn’t give milk?
A milk dud (or an udder failure) (Lacey, 6)
 
Google Groups: alt.humor.puns
Kids Puns of the Weak 06-01-04
Stan Kegel
6/2/04
(...)
What do you call a cow that doesn’t give milk?
A Milk Dud   (Daily Groaner)
 
Google Books
The Orange Mama Laid:
The guide for aunties, uncles, parents, grandparents, sisters and anyone esle who likes to play with kids
By Kristen Denney
Norwalk, CT: Whalerockbooks.com
2007
Pg. 156:
Q: What do you call a cow that does not give milk?
A: A milk-
 
Twitter
JeremyWilburn
‏@JeremyWilburn
One joke before lunch, What do you call a cow with now milk?  A Milk Dud! #cornyjokes
12:59 PM - 29 Jan 2009
 
Twitter
spadoll
‏@spadoll
What do you call a cow that produces no milk?  Answer:  A Milk Dud
11:47 PM - 2 Apr 2009
 
Twitter
Jean L.
‏@JeanL58
Good morning people!
Q: what do u call a cow that doesn’t give milk?
A: a milk dud !
5:46 AM - 14 Apr 2009
 
Twitter
Compa_ jose
‏@thereisawayjose
RT @GuitarDragon: What do you call a cow that can’t moo? A Milk Dud
6:06 PM - 29 Aug 2009
 
Twitter
Pop Rocks Australia
‏@PopRocksAussie
What do you call a cow that can’t moo?  A MILK DUD.  Bahahahaha
10:04 PM - 10 Sep 2009
 
Google Books
Zoolarious Animal Jokes for Kids
By Rob Elliott
Grand Rapids, MI: Revell
2012
Pg. 34:
Q: What do you call a cow that can’t give milk?
A: A milk dud.
   
Google Books
Halloween Jokes 4 Kids Part 2
By Peter Crumpton
PeteyRF Creative
2016
Pg. ?:
What do you call a cow that can’t moo?
A Milk Dud!