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The genetics of bigger chickens



Chickens owe this abundance, of course, to their place on our table. About 90 million tons of chicken meat are consumed every year, plus 67 million tons of eggs, compared with 110 million tons of pork and 67 million tons of beef. Between 1970 and 2005, world production of poultry meat more than quadrupled.

The red jungle fowl was domesticated around 4,000 years ago in India, where it still calls cock-a-doodle-doo in tiger-infested forests. But the triumph of the chicken is relatively recent. Until a couple of decades ago, beef and pork outweighed poultry in the average American diet. Fifty years ago, chicken was a scarce delicacy in many European countries.

What accounts for the rise of cheap chicken? First, motorized transport led to the growth of the intensive broiler industry, in which food is brought to the birds rather than vice versa. Second, selective breeding led to birds that were more efficient at converting grain into meat. Remarkably, this genetic improvement even now shows no sign of tailing off.

Ten years ago, Dr. Gerry Havenstein at North Carolina State University did a careful study of weight gain in chickens, comparing (under identical conditions) a modern 21st-century breed with a 1957 breed that had been kept going. He found that, at six weeks of age, the modern chicken was six times as heavy and had 9% more breast meat. Of that improvement, he found, 85% came from genetics and only 15% from better feed.

By 2001, when the study was done, a chicken reached the weight at which it would be killed in one-third of the time and after eating one-third of the food compared with the 1957 breed. That represents a considerable reduction in waste and in the amount of land devoted to growing feed per chicken.

In the decade since, there has been a consistent and linear increase in both weight gain and food-conversion efficiency in the broiler industry. Outside the lab, on the farm, chickens have accelerated their daily rate of growth by about 0.89 grams per year.

Nothing illustrates the power of selection in breeding better than this extraordinary change. But geneticists had expected the improvements to plateau by now. Shouldn't all of the best genetic combinations have been discovered? No less confounding is the fact that this effect has not been confined to chickens: The milk yield of dairy cattle shows similar linear improvement.

Two possible explanations for the continuing trend are that there is a lot more hidden genetic variability in a typical genome than scientists thought, or that brand new genetic mutations are happening fast enough to supply fresh variability to the selectors.

Whichever explanation is true, there's every chance that chicken growth rates may accelerate more, thanks to "genomic selection." This means testing many different birds to hunt down what mutations are boosting chickens' performance, using a gene chip with 60,000 single-letter DNA variants on it. You can then use that information to predict, even before they've hatched, which chicks will grow into successful fathers.

Many people would prefer chickens to be scratching, free-range, in farmyards. Given that this is impractical if the 3.5 billion people who live in cities are to eat affordable chicken meat, the genetic improvement of chickens is, on balance, good news. It means that they are fed from ever smaller acreages of land, leaving more for nature, and produce less waste. Nor is it clear that their living one-third as long is any more cruel, since they are killed either way-and they can be bred to be less fretful.