Generally, mastiffs are allergic to chicken and grains in kibble. If your pup did ok on Merrick with the grains, then you may be able to continue to feed grains. However, it's been proven that grains fed to dogs is not a good thing.
Personally, I would choose either TOTW Grain Free or Earthborn Holistics Grain Free.
Personally, I would choose either TOTW Grain Free or Earthborn Holistics Grain Free.