“Cauliflower” sounds like “collie flour.” “Dog biscuits are made out of collie-flour” is a pun that has been cited in print since at least 1928.
 
A riddle was printed in Boys’ Life in May 1975:
 
Holly: What’s the main ingredient in dog biscuits?
Ollie: Collie flour!
 
“When baking dog biscuits, be sure to use collie flour” was posted on Twitter on March 28, 2009.
 
   
Wikipedia: Cauliflower
Cauliflower is one of several vegetables in the species Brassica oleracea in the genus Brassica, which is in the family Brassicaceae. It is an annual plant that reproduces by seed. Typically, only the head is eaten – the edible white flesh sometimes called “curd” (similar appearance to cheese curd). The cauliflower head is composed of a white inflorescence meristem. Cauliflower heads resemble those in broccoli, which differs in having flower buds as the edible portion. Brassica oleracea also includes broccoli, brussels sprouts, cabbage, collard greens, and kale, collectively called “cole” crops, though they are of different cultivar groups.
     
13 March 1928, The Jersey Journal (Jersey City, NJ), “F.L.G. His Column,” pg. 28, col. 7:
Dog biscuits are made out of cauliflower. And because Tom Kittrick is likely to be unduly puzzled, I’ll put it another way. Dog biscuits are made out of collie-flour.
 
Google Books
December 1931, Boys’ Life, “Think and Grin” edited by F. J. Rgney, pg. 56, col. 3:
Intelligence Test
Teacher: What is cauliflower?
Bright Bobby: Dog-biscuits are made from collie flour.
   
Google Books
May 1975, Boys’ Life, “Think & Grin,” pg. 64, col. 2:
Holly: What’s the main ingredient in dog biscuits?
Ollie: Collie flour! — Jim Vinton, New Lenox, Ill.
 
2 June 1975, News Journal (Mansfield, OH), “Junior Journal,” pg. 11, col. 1:
Q. What do they make dog biscuits with?
A. Collie flour.
 
Google Groups: k12.chat.teacher
Dog’s Biscuits
Nathan Loveland
11/9/93
Hello Peter,
  > ~ SLMR 2.1 ~ Are dog biscuits made of collie flour?
 
Google Books
Nutty, Good, Clean Jokes for Kids
By Bob Phillips
Eugene, OR: Harvest House Publishers
1995
Pg. 127:
Ichabod: What is the main ingredient of dog biscuits?
Eutychus: You tell me.
Ichabod: Collie-flour
 
Twitter
Claire‏
@crpitt
@DrowseyMonkey Are dog biscuits made from collie flour?
9:55 PM - 20 Oct 2008
     
Twitter
Sally Conner‏
@SallysFamous
When baking dog biscuits, be sure to use collie flour.
6:09 AM - 28 Mar 2009
 
Google Books
Best Ever Classroom Jokes:
Because some of us never grow up
By Mike Haskins
London, UK: Pavilion Books
2015
Pg. ?:
What’s the main ingredient of dog biscuits?
Collie flour.
 
Reddit—Oneliners
When baking dog biscuits, be sure to use collie flour.
submitted November 18, 2017 by wonliners
(...)
SQLDave
Also good for canine grain bread