Newfoundlands deserve the best.
Having been around newfoundlands for 11+ years, My eldest is 11 years old. she is fed on Royal Canin.... Still able! and can keep up with the puppies and keep them in line.
I recomend Royal Canin. I Have "show" Newfies and Landseers. You only get out what you put in.... It may seem expensive to feed your giant dogs on this food, the quantities will become less as your dogs get older.
Giant Puppy at such a discounted price! knocking almost £10 off each bag here at zooplus and free delivery!.