Alpacas are a type of semi-ruminant, with three stomachs, and a very efficient digestive system, converting food into energy with little wastage. They enjoy short moist grass, plus hay when the grass is limited. In the winter you could allow a small (square) bale of hay per month per animal (some of which of course will be trampled on) and some calf nuts. They are browsers as well as grazers and will make short work of the young leaves on many trees and hedges if permitted to.
They enjoy twigs but they rarely ring bark trees unless extremely short of food. During lactation and the last weeks of pregnancy they may benefit from a light protein rich feed such as is given to young calves. Lengths of roof guttering make good and hygienic feed troughs. Allow 18" per animal - even if they don't use it! This will prevent the more timid animals being pushed aside. Plants that are toxic include such common ones such as ragwort, rhododendron, yew, laurel and laburnum. Alpacas need ready access to clean drinking water and can consume up to two pints a day. They will generally refuse brackish water even if they are dehydrated.