that depends on what brand you get
in my situation i only have a few chickens and i'm in a city so i buy from
PETCO online, it's called Brown's Layer Booster:
Corn, Soybean Meal, Wheat Middlings, Dehydrated Alfalfa Meal, Soybean Oil, Dicalcium Phosphate, Calcium Carbonate , Salt, DL-Methionine, L-lysine, L-Threonine, zinpro availa zn-organic zinc, Aspergillus Oryzae Fermentation Extract Dehydrated, Amprolium Lactobacillus Acidophilus, Lactobacillus Casei, Bifidobacterium Thermophilum, Enterococcus Faecium, Choline Chloride, Naicin Supplement, Roughage Products,Vitamin E Supplement, Vitamin A-Acetate, Vitamin D3 supplement, d-calcium pantothenate, Thiamine Mononitrate, Riboflavini Supplement, Folic Acid, Menadione Nicotinamide Bisulfate, d-Biotin, Vitamin B12 Supplement, Ferrous Sulfate, Manganous Oxide, Zinc Oxide, Sodium Selenite, Copper Sulfate, Mineral Oil, Pyridoxine Hydrochloride and Ethylenediamine Dihydroiodide.
all the strange sounding stuff seems to be vitamins and minerals, so it's fine for me. it doesn't have any animal products so i let my girls out in the yard to get bugs and worms and i also feed canned tuna and kitchen scraps.
some might suggest making your own feed, which i tried but it's way too expensive if you don't have a mill nearby.... but it's the best way to know what's going into your chickens/eggs though.
you can try to ask your supplier what's in your feed if it's not on the bag.