Here’s an eye-opening look at how breeding, hormones and who knows what else has dramatically accelerated the rate at which chickens grow (allowing them to be slaughtered much sooner):
In 1920, a chicken raised for meat was slaughtered at the age of about 112 days (less than 4 months) when he weighed about 2.2 pounds.
Since then, factory farming of chickens has continued a gradual but inexorable rise. With each new decade, chickens were fattened up faster and slaughtered earlier with little regard to the suffering of the chickens.
By 1950, a chicken was fattened up to weigh an average of over 3 pounds in just 70 days, the average age at which he would be slaughtered for his meat.
By 2000, the average chicken raised for his meat grew to weigh over 5 pounds by the time he was slaughtered at the age of 47 days.
Today, in 2013, we fatten them up even faster to weigh 5.89 pounds in just 47 days. Then, we slaughter them.
Even in 1920, chickens used for their meat were likely fattened up as fast as allowed by the know-how at the time. The natural weight of a chicken at 112 days of age, therefore, is no more than 2.2 pounds.
According to the Handbook of Poultry and Egg Statistics, published in 1937, the growth of a chicken during those times was approximately linear. This means that a 47-day old chicken in 1920 weighed approximately 2.2 × 47 ÷ 112 ≈ 0.923 pounds.
According to the USDA Poultry Slaughter reports, the average weight at slaughter in the first seven months of 2013 (January to July) was 5.89 pounds. Slaughtered at 47 days of age, these modern chickens weigh 5.89 ÷ 0.923 ≈ 6.38 times their 1920 counterparts.
Click on the graphic below to see an animated version: