Vegetarian Dog Food would probably not be a food option considered by many because they cannot imagine a dog just eating vegetables. It does not fit the image of dogs in the wild who we perceive as tucking into a hunted prey!
To some extent that is a valid point HOWEVER (there's always a however!) we need to remember and consider that those "wild hunting dogs" would usually first devour the stomach / intestines of the prey, as can be noted as feeding habits of wild dogs in countries where they still survive.
In most cases the prey would have been a grass or plant eating species, which would mean the dog would have been eating "vegetarian" as they consumed the intestines of the prey.
Another consideration is that before the advent of manufactured dog food most domesticated dogs would have been the "left-over food disposal unit" for scraps off the plate, which it is fair to assume would have included some vegetables.
To my knowledge at least two of the oldest dogs in the world have been fed a vegetarian diet.
"Bramble" (Labrador) lived to be over 27 years old whilst "Tykie" (mixed Terrier/Schnauzer/Husky) made the 25 year mark.
Those life spans are just as impressive as dogs fed diets of mainly meat, that have similiar survival longevity.
Is It a Safe Option?
Yes BUT providing you DO IT PROPERLY!
My own search and research indicated there are plenty of dogs on a vegetarian diet, with the owners praising the health and wellbeing aspects of Vegetarian Dog Food. It was also apparent that those owners are themselves following a vegetarian eating regime.
Two important considerations if you wish to feed only a vegetarian diet are;
1) Dogs nutritional requirements ARE NOT the same as for us humans so ensure what you are feeding includes all nutrients required to maintain your dogs' health and wellbeing. More information on this below.
2) Will your dog be able to tolerate a meat free diet. I have read cases where the dog's health, and or appearance, was affected and the owner had to keep some meat in the diet, for the better health of the dog.
Approach the Vegetarian Dog Food feeding option on a "trial and error" basis monitoring any changes in your dogs' skin, coat, energy levels, brightness of eyes etc. as the wellbeing of the dog must remain paramount, and not the preferences of the owner. Discuss any concerns with your vet or dog health specialist.
There is a huge reference book of over 400 pages available from the American organization National Academies which has a hard bound version currently selling at US$295.00 or you can read it online for free!
You can use this link if you wish to investigate further (new window opens)
Alternatively you may want to consider a book recommended by PETA which is authored by James Peden and titled "Vegetarian Cats and Dogs". This book is a fraction of the above price and available as an e-book or on cd-rom.
You will also find the product "Vegedog" on his website (Harbingers Of A New Age -link below) which is a vegetarian dog food supplement designed to be included with homemade vegetarian dog food recipes. There are recipes included. This link opens a new window.
Maybe the best bet to begin with is to try a "specialist dog food manufacturer brand" and a couple that I like the look of are;
Evanger's Super Premium All Fresh Vegetarian Dinner Canned.
Just looking at the ingredient list and there are plenty of vegetable and fruit inclusions and it is also soy free which is a plus for me.
Dick Van Patten's Natural Balance Vegetarian Formula.
This is a dry kibble and again there is no soy listed amongst the ingredients. This product is also noted as Vegan
If these products are not readily available for you then go to the manufacturers Evanger's and Natural Balance websites to see the ingredients, and nutritional information, and then use that information as a guide for your own selection.
These two products will save you having to worry about getting all of the right balances for a vegetarian meal for your dog.