If a human ate a vegan diet.....?

Well, first and foremost, the Human digestive system works in a totally different manner than that of cows, goats, horses, etc. We do not process whole wheats, corn, oats or other harsh, startchy plants as well as other animals. I believe it would be possible to use the feces for your garden, but it wouldn't be as good as cow manure because they process most of the startches out of the plants they eat.