In the book Watership Down the rabbits are often "eating pellets". My pet rabbit eats food pellets I buy from the pet store, but I don't see how that would apply to the rabbits in Watership Down.
What pellets are rabbits eating in Watership Down?
A rabbit's digestive system is not as efficient as that of a larger herbivore. There is a lot of nutrition left after good has made one pass through, and it's been concentrated and pretreated by digestive enzymes... and rabbits may send it through for a second pass.
They do distinguish between cecotropes and feces, so the gutter insult near the end of the book is still appropriately rude.