A “milk and crackers” pun is:
 
Q: What do you get when you cross a cow and a duck?
A: Milk and quackers.
   
“Helen Griffith—If you had a cow and some ducks, what would you have?—Milk and quackers!” was printed in The Press-Gazette (Hillsboro, OH) on December 2, 1941. “If you had a cow and some ducks, what would you have? (...) You’d have milk and quackers” was printed in the Albany (MO) Capital on December 4, 1941.
 
         
2 December 1941, The Press-Gazette (Hillsboro, OH), pg. 8, col. 3: 
Helen Griffith—If you had a cow and some ducks, what would you have?
—Milk and quackers!
 
4 December 1941, Albany (MO) Capital, “This Is Terrible!,” pg. 5, col. 1:
Queries he, “If you had a cow and some ducks, what would you have?” Well, right away we gave up,  because we don’t ever anticipate that that particular question will ever come up with us. We don’t want a cow and we don’t like ducks, dead or alive. But the officer, not to be deterred, comes up with the answer, which, as closely as we can figure it out was “You’d have milk and quackers.”
   
19 February 1942, Argos (IN) Reflector, “Argosy” by The Reflector, pg. 1, col. 1:
What would you have if you found a cow and three ducks?
Answer: “Milk and Quackers.”
 
21 March 1945, San Francisco (CA) Chronicle, “Animal Crackers” by Warren Goodrich, pg. 11, col. 2:
Bennett Cerf’s description of a cow followed by a couple of ducks: milk and quackers.
     
Google Books
Saturday Review of Literature
Volume 28, Part 1
1945
Pg. 25:
Description of a cow followed by a couple of ducks: milk and quackers.
 
24 July 1946 Boston (MA) Daily Globe, “Try and Stop Me” by Bennett Cerf, pg. 18, col. 5:
Description of a cow followed by a couple of ducks: milk and quackers.
     
Google Books
Organized Crimes
By Nicholas Von Hoffman
New York, NY: Harper & Row
1984
Pg. 87:
What do you get when you cross a cow with a duck? Milk and quackers.
 
Google Books
A Treasury of Science Jokes
By Morris Goran
Springfield, IL: Lincoln-Herndon Press
1986
Pg. 46:
Q: Cross a duck with a cow and what do you get?
A: Milk and quackers.
 
Twitter
❤️ FizzyDuck ❤️
@FizzyDuck
RT @Sammi_O: @FizzyDuck Whaddya get when you cross a cow with a duck?  Milk and Quackers—
4:45 PM - 24 Apr 2009
   
Twitter
Austin Gragg
@gragg16
What do you get when you cross a cow and a duck? Milk and quackers!
12:55 PM - 15 Jun 2009
   
Twitter
Hourly Facts
@factstorm
What do you have when you have a cow a…-  What do you have when you have a cow and two ducks? Milk and quackers. http://bit.ly/L9xAf
5:52 PM - 19 Jun 2009
   
Google Books
The Ultimate Book of Dad Jokes:
1,001+ Punny Jokes Your Pops Will Love Telling Over and Over and Over…
By Gordon Hideaki Nagai
Berkeley, CA: Ulysses Press
2016
Pg. 10:
Q: What do you get when you cross a cow and a duck?
A: Milk and quackers.
 
Twitter
lorraine Jo King
@LahLahBean
#mondaymirth
If you crossed two cows with a flock of ducks, what would you get?
Milk and quakers!
2:04 AM - 24 Sep 2018