A natural thing happened the other day. I was out walking dogs, and my dog was sniffing lots and lots of grass. He acts like a vacuum cleaner most of the time, but this was even more than normal, because its spring.
In spring there are lot of animals mating and lots of tasty small animals for dogs to try and hunt. Most of our domestic dogs are very bad at hunting by selective breeding this instinct out of them, but mine does fine.
The whole point of this story is that he came across a birds nest on the ground and proceeded to eat the baby bird. While this sounds bad, Its what happens in nature. You see dogs dont run through fields sniffing and hunting vegetables.
They dont get that excited when they eat grass, if in fact they do. They dont crave rice grains or wheat fields, they crave live animal prey and the proteins that are included after the hunt.
If you have any doubts about what a dog should eat (ie meat) take them somewhere that introduced (non protected) species are, and let them satisfy their instincts. If you feel squeamish abou this, perhaps then its a good time to start feeding them meat in their diet and in their treats, they will love you back for it!