Help me refute this vegetarian argument if possible

[Guest guest]

I'd love solid back-up, if possible, to refute the following (quoted

from a vegetarian I know):

'most of the agricultural output goes to feed animals so if animals

were not in the picture A LOT less agriculture would be needed to

support humans and the planet could support a lot more people

environmentally. Look up how much water/grain/land is necessary for a

single hamburger patty and you'll be surprised.'

This person is aware that grass-feeding is better than grain yet still

stated this. And of course they feel that vegetarianism is healthy for

humans.

Anything solid to refute this is welcome - good links and articles

that address exactly these points.