The Collaborative International Dictionary of English v.0.48:

Herbivorous \Her*biv"o*rous\, a. (Zool.)
   Eating plants; of or pertaining to the Herbivora.
   [1913 Webster]

WordNet (r) 3.0 (2006):

herbivorous
    adj 1: feeding only on plants [ant: carnivorous,
           insectivorous, omnivorous]