Chocolate milk has been a popular product since the 1920s. The old joke is that city children are so unfamiliar about where food comes from that they believe “chocolate milk comes from brown cows” (or. “chocolate milk comes from chocolate cows”). The chocolate milk origin joke has been told since the 1950s and 1960s.
 
     
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Progressive Education: a quarterly review of the newer tendencies in education
By Progressive Education Association (U.S.), American Education Association, American Education Fellowship
Published by The Association, 1952
Item notes: v. 30
Pg. 178:
We all are familiar with, but should not laugh at, the sad lack of information on nature which we find in city children who ask on which trees potatoes and carrots grow, or whether chocolate milk comes from brown cows.
 
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Handbook of Counseling Techniques
By Ernest Harms and Paul Schreiber
New York, NY: Macmillan
1963
Pg. 61:
It is a tragedy of our times, and not to be laughed at, when a child asks if potatoes grow on trees, or if chocolate milk comes from brown cows, etc.
   
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Research in Pesticides:
proceedings
By C. O. Chichester, United States. Public Health Service, University of California, Davis
Published by Academic Press
1965
Pg. 8:
They think that food comes out of the supermarket and that chocolate milk comes from brown cows.
       
25 August 1969, Cumberland (MD) Evening Times, “State Fair,” pg. 13, col. 1:
But most of all, he would probably be surprised by the number of “city people” who travel to the fair, bringing children who think chocolate milk comes from brown cows and only factories can fruits and vegetables.
 
29 June 1974, El Paso (TX) Herald-Post, pg. 14, col. 3:
...secondsacker Jerry Remy, a Sommerset, Mass., resident who still believes chocolate milk comes from chocolate cows,...
 
New York (NY) Times
Places to See How Things Are Made
By SHARON L. BASS
Published: Sunday, February 19, 1989
(...)
’‘People in general appreciate knowing how things are made, that it’s a complex process,’’ said Raymond D. Mathews Jr., a glassblower at Branford Craft Village at Bittersweet Farm in Branford. ‘‘If you ask kids where an egg comes from, they say, ‘Stop & Shop,’ and that chocolate milk comes from a brown cow.’‘
   
8 June 1995, Washington (DC) Post, “Getting a Feel for Barnyard Life” by Peter Pae, weekly-Virginia section, pg. V1:
“There are a lot of kids in Leesburg and Eastern Loudoun who have never been on a farm,” said John McBreen, a park ranger who manages the facility for the Northern Virginia Park Authority. “We’ve had a lot of kids who believe chocolate milk comes from chocolate cows.”   
 
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Mennonites in Texas
By Laura L. Camden and Susan Gaetz Duarte
College Station, TX: Texas A&M University Press
2006
Pg. 67:
(During one of my supper visits, where I acknowledged my limited farm experience, then ten-year-old Maria Yoder tried to convince me that chocolate milk comes from Holstein cows, the black-and-white variety raised on her family’s dairy farm.)