They are grazing animals so they will eat grasses. Usually alpaca farmers will have suppliment pellets of sorts for them and feed them alfalfa. They like to chew on hay. most like carrots, others have particular veggie/fruit tastes. Some really enjoy sliced apples.

BUT SOME PLANTS ARE POISONOUS TO THEM! So you have to be careful about what kind of plants are growing in your field.

I don't own alpacas but my brain abosrbs all kinds of information about cute animals. :B