The vet does not recommend that we use a flake type food in the park. She says rabbits are like children and will eat the bits they like and leave the rest. What they leave is usually the pellets which are the good bits they should be eating. She says we should always use a completely pelleted food that contains everything rabbits need. In this way they can't pick and choose the bits they like best.
This site has good advice, except the bit about keeping your rabbit in the house.