The simple answer is chickens should be fed chicken feed. Commercial chicken feeds are a blend of several different kinds ingredients. The feed gives the chickens of the energy, protein, vitamins and minerals they need to grow and be healthy. A good diet is important for hens to lay eggs. Like us, chickens are omnivores. That means they can eat plant and animal foods.
The energy part of chicken feed comes from grains like corn, wheat and barley. Corn is most common in the United States while other countries like Europe and Canada use more wheat and barley. Some places in Africa use sorghum grain.
The protein portion of the feed is supplied by both plant and animal sources. Plant sources include the material that is left after oil has been removed from soybeans, canola, or sunflower seeds (to produce cooking oils). Another common plant protein source used in other countries is field peas. Some common animal sources include fishmeal and by-products from the slaughter industry known as meat and bone meal.
Most commercial chicken feeds also have vitamins and minerals added - much like when you take a vitamin pill once a day.
There are different kinds of feed. You should give the right feed to the right bird. There are feeds for growing chickens which are higher in protein than feeds for egg laying hens. Feed for egg laying hens are also higher in calcium. The calcium is needed for the production of the eggshell.
Even when given all the chicken feed they could eat, chickens will still forage for more. That is, they will scratch the ground looking for food. They are looking for insects and worm. They will eat a variety of bugs and worms as a treat. The bugs and worms are also a good source of extra protein for the chicken. The hens will also eat any small grains they can find while they are scratching around. They also will munch on some of the plant leaves. The chickens do not get a lot of nutrients from the grass or alfalfa/clover (because chickens are not cows), but they like to eat them. Adds a little fiber to their diet.
Reading a feed tag (University of Kentucky)
Dr. Jacquie Jacob
University of Kentucky