I have a family member who runs a commercial chicken farm. They have several large barns. each week they get a truckload of chicks that get thrown into a cleaned out barn where they live out their short life (the are literally thrown in there). These barns are "cage free", so all the chickens are running around on the floor all together. There are pipes that go through along the floor where the chickens go to a dispenser to get their water. And there is feed that is delivered through a similar method. Every morning, the workers walk through the barn to remove the chickens that are disfigured, injured or dead. You would be floored to know how many of these chicks have some sort of congenital defect. Have seen it all.
Anyway. the feed is carefully controlled. They get a predetermined mix of feed and/or antibiotics, hormones and whatever else they put in that stuff. What they get is determined by how old the chickens are and it changes the closer it gets to time to harvest them. I don't recall how many weeks they grow. It used to be 9 weeks but I know that time has shortened significantly over the years. It maybe 6-8 weeks, I honestly don't remember what it is now. But, about a week before they are harvested, their feed is changed to a mix that allows them to say these are antibiotic and/or hormone free or whatever it is that they like to advertise to make the consumer think it's all good.
The light, temperature and humidity are also carefully controlled in those barns. Each barnful of chickens are all at the same stage within the barn, but they rotate the barns, so there is always one barn getting cleaned out, while another barn receives the new chicks, a third barn is being harvested and all the other barns are in some stage of growth.
There is a huge waste area where they put all the chicken poop when they clean the barns out. They sell this as fertilizer...so whatever the chickens have been feed is now being spread onto the fields as fertilizer. you can see that stuff does get consumed by humans regardless if you eat meat or not, kwim?
Whenever I visit, I have to take a supply of prednisone as there is something in the air that causes me to break out in hives. don't know if its the feed, the chickens themselves, the excrement...who knows. thankfully, i don't go visit very often.