Chickens in agriculture

In the United States, chickens were once raised primarily on family farms. Prior to about 1910, chicken was served primarily on special occasions or on Saturday, as the birds were typically more valued for their eggs than meat. As cities developed and markets sprung up across the nation, live chickens from local farms could often be seen for sale in crates outside the market to be butchered and cleaned onsite by the butcher. Some people still keep "free range" chickens for personal use, and may even sell the eggs and meat, but very few are raised on a large scale commercial basis this way.

With the advent of vertical integration and selective breeding of efficient meat-type birds, poultry production changed dramatically. Large farms and packing plants emerged that could grow birds by the thousands. Chickens could be sent to slaughterhouses for butchering and processing into pre-packaged commercial products to be frozen or shipped fresh to markets or wholesalers. Meat-type chickens currently grow to market weight in six to seven weeks whereas only fifty years ago it took three times as long. This is due exclusively to genetic selection and nutritional modifications (and not the use of growth hormones, which are illegal for use in poultry in the US and many other countries). Once a meat consumed only occasionally, the common availability and lower cost has made chicken a common and significant meat product within developed nations. Growing concerns over the cholesterol content of red meat in the 1980s and 1990s further resulted in increased consumption of chicken.

Another breed of chicken, the Leghorn chicken, was further developed to be an efficient egg layer. Egg production and consumption changed with the development of automation and refrigeration. Large farms were devoted solely to egg production and packaging. Today, eggs are produced on large egg ranches on which environmental parameters are well controlled. Chickens are exposed to artificial light cycles to stimulate egg production year-round. In addition, it is a common practice to induce molt through careful manipulation of light and the amount of food they receive in order to further increase egg size and production.

On average, a chicken lays one egg a day; however, this varies with the breed and time of year. For example, a Barred Plymouth Rock may lay one egg a day during the spring, summer and fall. But the same chicken may not lay at all during the winter. However, a chicken bred specifically for egg-laying may occasionally lay two eggs a day, and if housed correctly may lay all through the winter. Usually, egg laying hens are butchered after their first egg laying period that usually lasts from 12 to 14 months. The period begins when the hen is about 20 weeks old (depending on breed and season). Since roosters are not needed in the egg industry, all the males (roughly fifty percent of all chickens) are killed after their birth when producing birds for the egg industry.