I think that breeding birds need a slightly higher protein and there are some vitamins that are different in a breeding feed, for optimal hatchability. I have used Game Bird Breeder and I've used Southern States Super Breeder Crumble, though the SS brand breeder has veg protein only. I prefer the Game Bird breeder feeds with animal protein for my Blue Orpingtons and on occasion, I'll get a 22% layer pellet containing animal protein. I dont have them on it constantly, but alternate a bag of regular layer pellets with one of the breeder feeds, just to save money.
Forgot to mention that they get yogurt and on occasion oatmeal with olive oil, buttermilk and pumpkin seed. When I remember, I get them some greens like mustard greens or kale.