Seems like a carrot-and-stick approach would be most effective.
First, the stick: dehumanise your feedee. They're a vessel for fat and all this "person" nonsense is getting in the way. If your feedee is just an object, it can't think, so call it "it" like it deserves. You'll want to discourage any behaviour that deviates from this role, such as excessive motion or trying to communicate. There'll be certain necessities such as bladder relief but this should be kept to a minimum. A particularly bratty feedee will need to be punished, and as food should be the only thing it values, food should be what is sanctioned.
The carrot: reward good behaviour. Blind, obedient consumption should be rewarded with more food. Your feedee will crave mental stimulation, so reward good behaviour with empty-calorie entertainment such as children's cartoons (ideally in a different language - you don't want it practising communication skills). You should try to wean them off this eventually, though, as in the long run you want it to be able to derive all the entertainment it needs from consumption. Eventually the only behaviour you should be rewarding is no behaviour at all.
Or if you're looking to make your feedee a fat cow, you probably want to keep it in the same place as animals. This would be an entirely different path of dumbing down, as instead of becoming an unthinking shell your feedee will deteriorate into a wild animal for you to ranch. Keep it in a barn with cows, dress it to match (either in cowprint or not at all), and feed it at the same time and in the same amounts.
Either way, the most critical stick you have is its food addiction. You need to make its life revolve around what you put into its mouth. At all times it should be craving more and that effects the perfect opportunity to punish unwanted behaviour by withholding that. Following that one rule should allow to assert unequivocable control over your unthinking, food-addicted, ever-growing feedee.
6 years
