A cow riddle has been printed on several images:
 
Q: What do you get from a forgetful cow?
A: Milk of amnesia.
 
“Absent-minded cows give milk of amnesia.—J. C. Salak” was cited in print in 1973.
 
         
Wikipedia: Magnesium hydroxide
Magnesium hydroxide is the inorganic compound with the chemical formula Mg(OH)2. It occurs in nature as the mineral brucite. It is a white solid with low solubility in water (Ksp = 5.61×10−12). Magnesium hydroxide is a common component of antacids, such as milk of magnesia, as well as laxatives.
     
Google Books
The American Legion Magazine
1973
Pg. 48:
FORGETFUL BOSSIE
Absent-minded cows give milk of amnesia.
J. C. SALAK
 
20 December 1973, Centre Daily Times (State College, PA), “Good Morning!,” pg. 1, col. 1:
CURDLING: Absent-minded cows, J. C. Salak claims, give milk of amnesia.
 
31 January 1974, Bismarck (ND) Tribune, “Browsing Around” with Jack E. Case, pg. 1, col. 1:
PUN FUN
Does an absent-minded cow give milk of amnesia?
   
Google Books
August 1974, Boys’ Life, “Think and Grin,” pg. 66, col. 2:
Then there was the absent- minded cow who gave milk of amnesia. — Joey Woleszyn, Dunkirk, N.Y.
   
Google Books
Wild Bill Hiccup’s Riddle Book
By Ann Bishop and Jerry Warshaw
Chicago, IL: A. Whitman
1975
Pg. ?:
What did the absent-minded cow give?
Milk of amnesia.
 
Google Groups: net.jokes
Holy Cow
Mirek
3/7/86
(...)
What do you get from a cow that forgets her own name?
Milk of amnesia
 
Google Groups: rec.humor
More Cow Jokes
yikester
5/7/96
(...)
What do you get from a forgetful cow?
Milk of Amnesia.
 
28 March 1998, The Patriot Ledger (Quincy, MA), “Looking at language A good pun’s like a steak: a rare medium, well done” by Richard Lederer, pg. 26:
Forgetful cows give milk of amnesia.  
 
Google Groups: alt.humor.puns
COW PUNS WANTED
Stan Kegel
7/30/98
(...)
Forgetful cows give . . . milk of amnesia. Pampered cows give . . . spoiled milk. Nervous cows give . . .  milk shakes..
   
Google Groups: alt.humor.puns
“Puns of the Weak” 8/25/00
Stan Kegel
8/26/00
(...)
What kind of milk do you get from forgetful cows? Milk of Amnesia.
(James D, Ertner)
   
Google Books
Extremely Embarrassing Dad Jokes:
Because Dads don’t know when to stop
By Ian Allen
London, UK: Pavilion Books
2015
Pg. ?:
What do you give a forgetful cow?
Milk of amnesia.
 
Twitter
lorraine Jo King
@LahLahBean
#badjokefriday What do you get from a forgetful cow? Milk of amnesia! ☺
11:46 AM - 9 Mar 2018