Does anybody happen to know where the 10% rule actually comes from?
Being a rather jaded and cynical person, I would tend to suppose that it comes directly from the manufacturers of chicken feed - but, of course, I could be quite wrong about that, and would love to know for sure.
Since our girls free range all day and eat bugs and leaves and weeds, it's hard to measure how much would be 10% of their diet. But I haven't noticed a difference in egg production when I spoil them from time to time with mealworms and fruit.