Spent grain (as the name implies) has little to no food value for them. They get the fiber, but not the starch/sugar, which can actually make them sick. Unusable old grain is okay, but too much grain can make them fat. Cracked is better than whole, but it doesn’t have to be as fine as mash grain. Mix the grain with other things like sunflower and raisins and sprinkle that in the yard. They’ll be busy scratching for the mix and leave each other alone. Bored chickens can be very aggressive with their coop mates. If you wash the shells after using the eggs and get all the membrane out, then dry them in the oven to remove any raw egg taste, crush that and mix with the homemade scratch. The shells are easier to absorb and reuse than processing fresh limestone for layers. Layers do best with a high protein low fat diet but meat birds can have more fat. Too much fat can make them stop laying, which really isn’t a problem with meat birds.
Chickens will eat ANYTHING. They’re just domesticated scavengers. Use the grains to make treats for them and they will practically stampede every time they see you coming.