Modern supermarket broilers in the U.S. are roughly four times heavier at the same age than a 1957 bird, a University of Alberta study found. Experts attribute the change mainly to selective breeding, improved feed and tightly controlled rearing conditions — not hormones, which are banned in U.S. poultry. Faster maturation has cut time to market dramatically but raised concerns about antibiotic use, animal welfare and environmental impacts.
How America's Chickens Got Four Times Bigger — And Why Hormones Aren't to Blame
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Supermarket chickens in the United States look very different today than the birds families bought in earlier generations. A combination of selective breeding, improved feed and tightly managed production—not steroid hormones—explains much of the dramatic size increase.
Study Highlights Dramatic Growth
A University of Alberta study raised a broiler line preserved from 1957 side-by-side with a commercial broiler from 2005 under identical feeding and management conditions. At 56 days the 1957 line averaged 905 grams (2.0 pounds), while the modern line averaged 4,202 grams (9.3 pounds) — more than four times the weight.
"Although the chicken was domesticated during Neolithic times, the development of genetic programs designed for broiler performance was a 20th-century event," said Paul B. Siegel, a Virginia Tech professor who has studied broiler genetics for decades.
Why Size Increased
Scientists and industry experts say selective breeding for faster growth, greater breast yield and production efficiency accounts for most of the change. Hormones and steroid injections are not part of U.S. poultry production — they are banned — so genetics, optimized nutrition and controlled growth environments are the main drivers.
The industry shift began in the mid-20th century with breeding initiatives such as the "Chicken of Tomorrow" contests, which incentivized birds that matured faster and produced more meat at lower cost.
Faster Production, Bigger Impacts
The National Chicken Council reports that average time to market dropped from about 70 days to roughly 47 days; some broilers now reach slaughter age in as little as 35 days. At the same time, U.S. per-capita chicken consumption has risen — ZME Science reported chicken accounted for more than 40% of U.S. meat consumption and per-person consumption reached about 104.5 pounds in 2025.
- Antibiotic Use: Tightly managed growth systems often rely on antibiotics, raising concerns about antibiotic resistance and public health.
- Animal Welfare: Rapid growth and heavier birds are associated with leg problems, heart conditions and respiratory stress in flocks.
- Environmental Costs: Feed inputs, land use and the wider meat supply chain contribute significantly to the sector's climate footprint.
"There is a lot of misinformation about this topic," said Martin Zuidhof of the University of Alberta. "We did the study to provide scientific data on the reasons for higher growth rates in modern chickens. It is NOT due to steroid hormones (feeding or injection) but rather because of effective programs that selected chickens with higher growth rates, yield, and efficiency."
Producers are beginning to respond: primary breeders are now placing relatively greater emphasis on efficiency, livability and reproductive traits rather than simply maximizing size. Meanwhile, researchers and companies are exploring feed innovations — for example, grape pomace additives to improve flock health and reduce waste — and regulators and consumers are paying more attention to packaging chemicals and climate costs tied to cheap meat.
The modern broiler is the product of deliberate breeding and industrial-scale production choices. Understanding feed, welfare, antibiotic use and environmental impacts helps explain how chicken remained inexpensive even as the bird itself changed.
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