The chicken crop is a marvelous grinding machine when properly outfitted with small pebbles. There is very little that is recognizable after it comes out of the other end of the chicken, maybe small bits of grass that manage to stay together.
While chickens can process grains as large as whole corn kernels, you have to be careful feeding them beans and peas. Raw dried beans and peas can contain
phytohemagglutinins which can be poisonous. Any chicken feed made with soy is made with
cooked soy meal for this reason.