I know precisely what is being pumped into farm animals: Deliciousness.
Human beings are opportunistic omnivores (like dogs, who evolved to become a companion species because of our dietary similarities) adapted to eating meat, to hunting, killing, and scavenging. Both the complex social behaviors which most efficiently obtain food in this manner, and the high density of proteins, carbohydrates, vitamins, and minerals made available by the consumption of flesh were a significant factor in the development of man's superior intelligence. The development of the human brain requires a great deal of protein, animal protein, which is why women who do not consume meat during pregnancy tend to produce children with stunted mental development. Retarded? Not necessarily, but significantly short of their full potential.
You want to live off of wheat grass and soy beans*? Be my guest.
But do not presume to tell me it's better for the environment when you consume foodstuffs which typically have only a fraction of the nutritional density of meats and eggs, which means the shipping, processing, packaging, and reshipping of several times the mass.
Do not presume to tell me it is "cruelty free" or "more humane" when the thrashers, combines, plows, and other equipment required to grow and harvest your feed thrashes, combines, and plows countless fuzzy little bunnies, field mice, baby birds, stray Mexican children, etc. to bring your little feel-good farts to the table.
* You want to worry about growth hormones? Really? Try looking into the effects of a soy-rich diet on male physiology.
PS- Lions sleep heavily after a successful hunt for the same reason all cats do: They are ambush predators who gorge themselves on as much of a kill as they can cram down, and then drop off their activity level to conserve energy for the next hunt. It is a common survival strategy, and it is common because it works. Humans have a similar, albeit much less pronounced reaction to consuming a high-carbohydrate meal for much the same reason. It has nothing to do with the energy requirements of digesting meat, which is a much simpler process than digesting most flora (hence why most species which remain dedicated herbivores over a long enough period evolve multi-chambered stomachs... or a habit of eating their own dung) and returns much more potential chemical energy to the body. This is why it isn't just eating high-fat meats (like hamburger) that will act as a soporific; eating bread will make you just as drowsy.