Griffiths claimed that “9% of land animals are killed” to provide dog food. This refers to a study led by Knight. Whether or not that is an accurate figure, it ignores the fact that most animal derivatives in dog food are by-products. That is to say, they are the bits that humans don’t eat. The animals would have been slaughtered anyway. If none of those by-products were used for animal feed, they would then have to be incinerated or go into landfill.
A peer-reviewed systematic review of the effects of feeding a “vegan” diet to dogs and cats is promoted on Knight and Griffith’s web site; perhaps in the hope or knowledge that most people won’t click on it or read it in full. The summary is worth quoting in some detail: