Animal husbandry is a great way of converting inputs unfit for human consumption (animal feed) into something that is (meat) but it’s never very efficient.
Ideally, yes. In practice, we feed animals mostly things that we ourselves could eat ourselves - like corn, soy, and grains. Sure, you can find cute examples of cattle eating grass on rocky soil... but around 99% of animals are not grown that way... they are grown in factory farms where they're fed soy, corn, and grains.
Meat is still a useful medium for converting what humans can't consume very well to something they can. Especially pigs, which in much of the world are used as recyclers (not in the west anymore, due to safety issues, and probably not for long in the undeveloped world, but they have bigger problems to worry about).
The undeveloped world or the pigs?