6 min read

Why Have Chickens Quadrupled In Size Since The 1950s?

<p><a class="checked-link" href="http://www.flickr.com/photos/26279436@N02/7010917319/in/photolist-bFwMsP-bsRh3j-bpne5y-5Jm3f9-4RHLbS-2atf7h-apXFV3-75PCq-HHJfz-fbVUDN-a9Rx5e-9qq9rL-5S82LK-2uNVSU-7M4LYu-7xYUDg-7B7RSo-4wp7KA-8t2sx7-8F4HPt-jVG7w-edSMCH-8hZjVy-boJnTj-2tkEx5-9sNczg-7YUtun-iYhCXs-9T9FDe-dP8ngy-4DD5tu-2fNR36-eBL6Ed-JaWdS-5k6C5d-bxapCQ-8G8maT-4i9Bgd-83RTdJ-drFiho-e1SnRA-fpfYGd-eVeaEb-eba4AF-aFgLW6-7pzaWM-nTAzYD-csfvnq-cD5o65-p5XuS">Matt Davis/Flickr</a></p>

Poultry chickens keep getting bigger and bigger every year, according to a new study. But bigger doesn't always mean better - especially in this case.

The study compared the growth of three different types of chickens to understand how the process of selective breeding has changed the size of modern chickens. Published in the journal Poultry Science, the research found that the first breed had not been changed by selective breeding since 1957; the second hadn't been changed since 1978; and the third was a modern breed of chicken common in today's factory farms.

Over 56 days, researchers gave all the animals the same amount of food, but found that growth rates were very different in the three breeds. At the end, the 1957 breed weighed an average of 905 grams (2 pounds), the 1978 breed averaged 1,808 grams (4 pounds) and the modern breed weighed 4,202 grams (9.3 pounds).

(USDA/Flickr)

The dramatic increase in growth is mirrored by a much shorter time needed to bring chickens to market today. In 1955, it took 70 days to let a chicken mature to sell for meat. Now, it takes just 47 days.

The poultry industry argues that bigger chickens means that fewer chickens need to be killed to produce the same amount of meat. But what effect is this extra mass having on the chickens themselves?

Martin Zuidhof, one of the authors of the study acknowledges that with greater size comes health problems for chickens.

"Twenty-five years ago was probably the low point in the industry in terms of negative consequences of selection," he told CTV News. "There were metabolic disorders, heart attacks, just difficulties that were associated with rapid growth."

But according to Paul Shapiro, Vice President of Farm Animal Protection of The Humane Society of the United States, that problem is far from solved.

"Chickens in the meat industry are sentenced to a lifetime suffering because of genetic manipulation that forces them to grow so rapidly," he told The Dodo. "All you need to do is go look at a shed of broiler chickens more than a month old. Most have difficulty walking and often collapse under their own weight."

(Compassion Over Killing)

Rapid growth in chickens can result in leg disorders, including bone deformities, lameness and ruptured tendons, as well as metabolic diseases. Shapiro noted that many factory-farmed chickens are unable to hold up their own weight, and are even missing feathers on their chests because they spend an unusual amount of time sitting.

One study found that when given the choice, broiler chickens will even choose to self-medicate with feed that contains painkillers, even if that feed tastes worse than their regular feed. In her book "Wild Health," biologist Cindy Engels pointed to this as evidence that the animals were in pain or suffering.

While growth hormones and steroids are illegal in the U.S. poultry industry (unlike other meat industries), farmers often use antibiotics as growth agents, along with breed selectivity. There are a small number of heritage-style breeders in the U.S. that don't use these strains of birds. If you eat meat, see the Animal Welfare Approved seal to find farms that adhere to this standard.