This is a very interesting thread. Where, I live, I am severely limited on what I feed my chickens. I'm currently giving them 50:50 Country Feeds (Nutrena) Gamebird/Turkey Grower 21% : Pruina Layena Sunfresh Recipe. I mix them because the Purina only provides 16% protein. I supplement their diet with greens, fruit and insects from my garden along with oyster shells.
I did some research and compared Nutrena and Purina with the following brands: 'King's Organic Layer Pellets', "Buckeye Nutrition", and "Blue Seal OrganicLife Layer Pellets."
This is what I found out:
Protein, Lysine, Methionine, crude fat, crude fiber, calcium, ash, phosphorus and sodium percentages were pretty much identical for all of the brands listed above.
Really, their is no difference in the nutritional value, but a difference in whether or not it is organic. I'd much rather feed my chickens organic food because it is more environmentally friendly, pesticide/herbicide/GMO/GE free, but Nutrena and Purina aren't bad choices if you don't care about organic.