Singer's book _Animal Liberation_ was recommended to me about 25 years ago. In it, he argues that concern for animal welfare is based upon fact that, as sentient beings, they are capable of suffering. No problem, humans should try, whenever possible to minimize animal suffering. But S goes on to posit 'speciesism' as arbitrary and irrational prejudice similar to racism and sexism. As such, he condemns attempts to place interests of humans above those of animals.
Irony of animal rights position is that it is derived directly from *human* rights theories. Concern for animal welfare, however, does not logically lead to animals right position. Former is altruistic concern that does not imply equal treatment. One can oppose corporate-factory farming because it is cruel to animals but not go so far as to insist upon vegetarianism.
About 15 years ago, an acquaintance suggested I read Tom Regan's _The Case for Animal Rights_. TR argues 'right to life' position in which killing an animal, however painless, is as indefensible as killing a human (or a fetus?). While acknowledging that free speech & worship, educational & employment opportunity seem absurd if invested in animals, he proceeds to posit so-called 'marginal cases' as humans beings who have limited capacity to experience autonomy or exercise reason. Then Regan point outs that some animals possess mental capacities similar to 'normal' humans (citing dolphin communication research). Why couldn't, he asks, 'marginal' humans be treated as animals traditionally have been: used for clothing, food, scientific experimentation, etc (fwiw, he doesn't actually advocate these things, it's a 'thought exercise'). Logically pursued, this argument conceivably granting 'rights' to some animals while denying them to certain humans. And why confine such 'rights' to animals since as biologist Lyall Watson suggested about 30 years ago, plant life may possess capacity to experience physical pain.
This stuff is light years from Marx's notion (appearing in both young writings such as _Economic & Philosophic Manuscripts_ & mature writings such as _Capital_) of humans rationally regulating their exchange with nature. And attempts to suggests that such ideas are restating belief in interconnectedness of all forms of life expressed by non-western philosophies/religions or pre- Christian pagans don't help matters.
Re. veggie dogs, dog food companies will have to more vigorously market to elderly poor who I read some years ago purchase a substantial percentage of dog food for their own consumption. Michael Hoover (who was veggie for years and still eats lots of soy dishes)