I understand....they are possibly using more calories to grind the grain themselves, therefore losing the total benefit, than if the grains were already partially ground, thus not expending energy to grind them during digestion. Makes sense!
Also a question of how much the gizzard can process, which is about 6 ounces. But the pre-processed (ground) food doesn't get bottle-necked there.
One thing with poultry feed is enough density of nutrients so birds are nourished, and enough bulk so birds are satisfied and don't just keep putting more in. It's a balance.
I read that birds like to have some food in crop, gizzard, and digestive tract or they "feel" hungry.

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I've never killed a chicken with a completely empty crop or gizzard in all my days, even up to 17 hrs past the last meal. I'm thinking folks don't realize how long it takes for food to be processed through a chicken. 
Haha!
