Because the back teeth are continually growing, cavies need to be grinding there teeth to keep them from over growing causeing pain and illness.
Alfalfa hay is much higher in protein, calcium and carbohydrates and is primarily fed to pups (usually till 6 Months of age)and pregnant or nursing cavies. Most piggie pelleted foods are made of alfapha and the addition of alfalpha hay is too much.
Timothy hay is the best for adult piggies (After 6 Months of age). It should be the main hay in there diet.
Other hays I have found availible are Sweet Meadow Hay (Pet Supplies Plus), Bermuda Grass, Orchard Grass, Oat Wheat and Bareley Mix. (Availble at PetSmart and Petco)
Be sure your guinea pigs have hay at all times!