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]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "herbivorous":
   cannibal, cannibalistic, carnivorous, commensal, dietetic, dining,
   eating, feeding, flesh-eating, fruitarian, gastronomic, gluttonous,
   grain-eating, graminivorous, granivorous, grass-eating,
   insect-eating, insectivorous, lactovegetarian, man-eating,
   meat-eating, mensal, nourishing, nutritious, omnivorous,
   omophagous, pantophagous, phytivorous, phytophagous, plant-eating,
   postprandial, prandial, predacious, preprandial, vegetable-eating,
   vegetarian