Ethical/moral objections aside, initiating the practice of human farming wouldn't be a logical or practical choice, as presumably farm-rearing humans would be just as energy-inefficient as farm-rearing livestock:

Animal protein production requires more than eight times as much fossil-fuel energy than production of plant protein while yielding animal protein that is only 1.4 times more nutritious for humans than the comparable amount of plant protein, according to the Cornell ecologist's analysis.

Killing and eating excess humans in the process of reducing the world's population to a sustainable level, on the other hand, might qualify as a logical use of resources.