Dogs are obligate carnivores.
Obligate or true carnivores depend solely on the nutrients found in animal flesh for their survival. While they may consume small amounts of plant material, they lack the physiology required for the efficient digestion of vegetable matter and, in fact, some carnivorous mammals eat vegetation specifically as an emetic. The domestic cat is a prime example of an obligate carnivore, as are all of the other felids. The ability to produce synthetic forms of nutrients such as taurine in the lab has allowed feed manufacturers to formulate foods for carnivores (zoo animals and pets) with varying amounts of plant material.

What does this mean in laymans terms?
Dogs are meat eaters, and it doesn't have to be live meat. This is a nice way of saying, dogs are scavangers that thrive on meat, but can eat other things in a temporary pinch. They aren't picky when hungry, but do much better on whole raw meat.

Dogs (and cats) ingest plant matter when they want to vomit!