I was asked why alfalfa hay is only for baby guinea pigs. It is the same with rabbits and most other small pets. The reason is that alfalfa has a very high protein content and the fiber content is actually relatively low. This is fine for babies because they need more protein since they are growing. Adults however need more fiber and less protein so alfalfa hay does not meet their nutritional needs. It also contains a high calcium content which is too much for adult rabbits and guinea pigs.