Originaly used as a value added marketing ploy, “Grain Fed” is supposed to mean that the chickens that produced the eggs were fed on a diet consisting exclusively of a mixture of various grains and pulses. Between the lines one should read that no protein from animal origin were added to their diet. Although free roaming chickens will add worms, insects etc. to their diet, to do so on a commercial scale will be impossible, except if fishmeal and/or other similar sources of animal protein are added to the diet. These are in any case too expensive so, in practice all eggs that you will buy are probably “grain fed”. Why ” supposed to”? Because of the lack of protein from animal origin in the diet, some amino acids like Lycine & Methionine need to be added to the diet. Other additives include a wide range of vitamins and trace elements. And you thought only humans need extra vitamins!