Genetic selection key to remarkable growth of chickens

Broilers are four times bigger than in the 1950s, ALES researchers find

Helen Metella - 27 January 2015

It's not your imagination-chickens are bigger and meatier today than when your mother or grandmother was the main cook in your family.

They're double the size they were in the late 1970s and a whopping four times bigger than they were in the late 1950s, according to a recent study by ALES researcher Martin Zuidhof, a poultry scientist from the Department of Agricultural, Food and Nutrition Science.

Not only that, chickens are eating half as much to achieve those sizes, and the size of their meat-dense breasts has increased by 80 per cent. And no, none of this is due to added hormones or steroids. The explanation, says Zuidhof, is simple genetics.

Zuidhof and his research team compared three breeds of mixed-sex chickens, which the scientists bred using identical diets and conditions.

The first group was from a strain developed in 1957, the second in 1978. Neither strain has been altered by selective breeding for several decades, so they've become a benchmark for commercial chickens of a specific time-period. The third strain, the Ross 308 broiler, was developed in 2005 and is the most modern commercial strain.

After 56 days, the 1957 breed weighed an average of 905 grams and the 1978 breed an average of 1,808 grams. But the average for the 2005 breed was 4,202 grams.

The results are easy to see in chickens because they are both fast-growing, have a short generation time, and have plenty of progeny (120 annually for a female, closer to 1,000 for a male). By choosing the best chickens from that many, scientists are able to select and multiply their desirable genetic qualities quickly.
In contrast, a cow may have only one calf per year.

"That's why we're not seeing the same gains in cattle," said Zuidhof.

The study was published in the journal Poultry Science last October and funded by the Poultry Research Centre. It was launched in order to counter widespread misinformation about chickens-that they're being pumped with steroids and antibiotics, or force-fed to improve size. This study characterized the degree of change in growth rates and meat yield that was due to genetic selection, and opened a lively conversation in the media.

Zuidof hopes that will lead to funding for another project, one in which these breeds are compared to a 1997 line. If it's done in 2017, it would complete a 60-year benchmark of commercial chicken lines in 20-year increments. That in turn would not only educate consumers, but "if we ever want to understand some of the unintended consequences of selection, we'd have an archive for 60 years," he said.