Even as a new breeder, they should be educated and know the lines. They would have a mentor and be able to talk to the breeders of the sire and dam, etc. If it's a good breeder, they did not go in blindly.
FWIW, ProPlan Sport 30/20 is what several local breeders feed and what I was recommended. But, there are different lines and they do better on different foods. It's not really a one size fits all situation. My boys are big dogs with lots of bone.