Here is an excellent guide to rabbit nutrition. There is a great variety of foods that rabbits require for optimal health.
http://www.peteducation.com/article.cfm?c=18+1803&aid=1638
Regarding hay:
"Hay: Rabbits should have fresh hay available 24 hours a day. Rabbits less than 7 months old may have alfalfa hay, but older rabbits should have grass hays such as timothy or oat hay. Hay is essential to a rabbit's good health, providing the roughage that helps reduce the danger of hairballs and other blockages."