Not to nitpick myself, but I thought the English sentient came from the Latin sentio, sentire, which can mean to perceive or to feel, as well as to understand or to believe. It is a verb applied to emotions and sensations, facts and opinions. Unless something was lost as the word base was adopted into English, sentient would mean both having the use of senses and intelligent, aware, etc..
Personally, I would approach the question by looking at the economy. Would it be moral to take jobs away from blue collar workers in favor of cheap slave labor? I know this is a standard objection to using robots in factories, but the economy is already suffering from the loss of manufacturing jobs, which forces many former factory workers into demeaning, lower paying service industry jobs. It’s possible that breeding drones for factory labor would increase profits for a few, while harming the economic situation for many. If so, that’s unethical.
The economy would require some major restructuring to reach a point where drones would be beneficial. Slavery may have been good for the economy, but that was before the Industrial Revolution. We’re not so agrarian anymore.
But that’s more about ethics than morality, right?
I believe the moral kneejerk reaction to the idea of breeding drones comes from the story everyone’s thinking about, but not really writing… about Plucky, the bright little drone bred for slave labor, told all her life she didn’t have consciousness, but one fateful day she happens across some inspirational little message, and decides she has a soul. Even if she really doesn’t have a soul, she believes she does, and she makes heartbreaking appeals to her evil drone slavemasters to let her go free. Or maybe two drones fall in love. Spielburg directs.
I know, I’m supposed to argue from the point where we know the drones don’t have consciousness, but that’s difficult.
Finally, even if it were possible–factory labor requires a great deal of skill, precision, and prudent decision-making. The drones would have to be bred with grace, strength, analytical skills, and the ability to learn. There would be several batches of useless drones before the recipe is perfected, and I’d expect them to be fairly expensive and the end of that. Then, after the initial financial burden, they would have to be provided necessities like food, a resting place, exercise, medical attention, and hygene. Wouldn’t it be cheaper to just use robots?