Your alien might have some 3rd property that you do not, and thus may farm you.
A future AI that can produce and consume the sum total of all recorded human knowledge within the amount of time that you have a single thought will likely have many emergent properties that you do not, and thus may farm you as well.
> Indeed, it usually rests on sentiment or convention rather than a sound and rationally grounded objective ethics.
Your whole argument rests on sentiment and convention, and would have been summarily rejected by the slave owner based on his own.
I'd argue it's much baser than that. Animals have feelings and often feel very bad when kept in enslaved conditions. Since humans can understand the pain they inflict on enslaved animals, then it's wrong of us to continue enslaving them when we have alternatives that are just as healthy for us, if not more healthy.
I would also say your assumption that pigs do not comprehend their actions and cannot choose between alternatives is false.
Chattel slavery was first and foremost morally objectionable, because human beings have rights that conflict with its practice. Rights are rooted in two properties human beings have, namely, the ability to comprehend one's actions and one's situation, and the ability to freely choose between alternatives. If I can understand my actions and I can freely choose to act one way or another, then I am, in principle [1], a moral agent and thus morally responsible for my actions. But for me to be able to fulfill those responsibilities as a moral agent, certain conditions must be met and this claim on others to supply me with those conditions we call rights. Without those conditions, I cannot do what I have a responsibility to do. Non-human animals [2] lack these properties, which is why we do not hold them morally accountable, and because they don't have responsibilities, they do not have rights. (I realize that it has become customary to pull rights out of thin air without the slightest moral scruple or justification about doing so.)
Of course, it would be morally objectionable for us to torment animals, but we are free to make use of animals in ways that do not contract the human good, rightly understood.
[0] The only sound, objective basis for morality is human nature, which determines what actions accord with it and which contradict it. So, it is morally objectionable to torment animals, even though they have no rights, because - in short - it contradicts human nature and thus my good as a human being. Sadism is a serious defect.
[1] I say "in principle", because in practice, as you'll recall, mens rea has legal significance for a reason. If I kill someone by accident, then I did not choose freely to kill him, and so I have not committed murder, only involuntary manslaughter or whatever. If I kill someone, because I believed he was a monster from the 7th dimension trying to kill me, then I did not comprehend my situation and thus the nature of my action. So, in practice, I may fail to exercise what in principle I have the power to do by virtue of my nature as a human being. But other animals do not have this power by nature.
[2] To preempt the inevitable petty drive-by pedant, I define "human" as any animal with these two properties, so according to this view, an intelligent alien from another planet would also be human, despite occupying a place in a separate phylogenetic tree or whatever.