A dog's breakfast used to be scraps thrown into a bowl in a heap. And if something was referred to as a dog's breakfast, it was an insult. Messy or poorly done.
Mirriam Webster's example is "It was a dog's breakfast of a match, and our coach was understandably upset."
Millie's breakfast is something pleasant to me - rice, cheese, dental dog crunchies and a spoonful of "turkey stew for sensitive stomachs." That's a big difference from what we fed our dogs in the 1960s. Smelly canned dog food and table scraps. Our dog at the time was partial to dessert scraps.
The science of veterinary nutrition emerged in the late 1800s. It was in the 1890s that dog biscuits and dry kibble came about, and in the 1920s that canned food was available.
In the 1980s nutritional requirements were published for dogs and cats. That got the pet food producers kick-started and here we are with aisles and aisles of dog and cat food of all sorts. There are vegan products now available and as edible and palatable as animal-based diets for dogs, so they say.
Dogs seem more like humans than we imagine. Here are some things that some dogs like:
ice cubes
pickles
dried bananas
raw spinach
watermelon
Things your dog shouldn't eat?
candy, gum toothpaste - things sweetened with xylitol
avocado
onions and garlic
grapes and raisins
persimmons, peaches and plums
And then a cartoon to demonstrate that what dogs chew can be quite different than what dogs eat.
How did a dog's breakfast get such a bad reputation? in the 1937 Dictionary of Slang and Unconventional English, the expression is listed as "a mess." It is suggested that this dates from before the time of canned dog food when a dog's breakfast consisted of dinner leftovers from the night before.
This should not be confused with "a dog's dinner" which means the opposite and is normally expressed as "all dressed up like a dog's dinner" and sarcastically means over-dressed or showy. This comes from makingheadsortailsofidioms.com where the list of expressions is quite fun.
More on the dog's dinner from a New York Times article ON LANGUAGE: Dog's Breakfast: "Why have you got those roses in your hair?" asked a character in "Touch Wood," a 1934 novel by C. L. Anthony. "You look like the dog's dinner ." This expression was defined by the Oxford English Dictionary Supplement as "dressed or arranged in an ostentatiously smart or flashy manner," probably derived from the 1871 usage "to put on the dog ." And lest we forget, the New York Times examines the well-worn expression - 'dog-eared.'
I found this picture of Dezi on a spring search under the wisteria. Is it for a dog's breakfast or a dog's dinner?