Unfortunately, because large breed chickens have been bred to reach slaughter weight at such a young age, not very much is known about their nutritional needs, and there is currently no commercial food created with their long term needs in mind. Because of this, it can be difficult to know what type of food is best.
Regardless of the type of food you use, large breed chickens, unlike other breeds of chickens, must have their diet restricted. Because of the way they have been bred, they grow quite quickly. Even on a restricted diet, a large breed chicken will grow quite large and be considered “overweight” by someone comparing them to their non-large breed cousins.
Allowing large breeds chickens to free feed (have unlimited food throughout the day) will lead to severe obesity and obesity related health issues such as joint and heart issues and will greatly reduce the length of their life. Unrestricted access to food, even temporarily, can also lead to crop issues such as an impacted crop and or a damaged crop that can no longer work properly. Additionally, large breed chickens are ravenous eaters and are not good at self-regulating. If left to their own devices, they will often eat until the food runs out or they physically cannot eat more.