The environmental impact of eggs.

The environmental impact of eggs.

Illuminating answers to environmental questions.
June 1 2010 7:11 AM

Green Eggs vs. Ham

What are the environmental impacts of eggs?


Lantern, you've covered the environmental impact of meat a number of times. Where do eggs fit in? And would it be better for the planet to buy free-range or cage-free eggs?

The Lantern has good news for those of you who, like her, are devotees of the 24-hour breakfast menu: According to the best data we have so far, eggs are on the green end of the animal-protein spectrum. That doesn't mean they're particularly good for the planet, however. An omelet may have less of an impact than a strip steak or a pulled-pork platter, but eggs aren't necessarily any greener than the chickens that lay them.


In a report published in Livestock Science earlier this spring, two Dutch researchers gathered data from 16 different life-cycle analyses in order to compare the environmental impacts from five conventionally raised animal products: pork, chicken, beef, milk, and eggs. (All but one of the studies were from Europe, where the lion's share of food-related life-cycle analyses is conducted.) On a per-kilogram-of-protein basis, nearly all the data showed that beef production requires the most land and energy and produces the most greenhouse gas—often by a significant amount. When it comes to ranking pork, chicken, milk, and eggs, the differences weren't quite as clear.

Take land use, for example. Producing one kilogram of chicken protein (roughly the amount in three dozen chicken breasts) requires between 42 and 52 square meters of land. To get a kilogram of egg protein (what you'd get from about 10 dozen eggs) you need roughly the same amount: between 35 and 48 sq. m. Pork came in a bit higher, at 47 to 64 sq. m., and milk a little lower, at 33 to 59 sq. m. But there's enough overlap among the reported ranges that you can't always make a clear call one way or the other.

The reported ranges also overlapped when it came to greenhouse gas emissions. With regards to energy use, pork had the highest numbers and milk the lowest, with chicken and eggs ranked somewhere in the middle. (The report also looked at eutrophication—a kind of water pollution caused by excess nutrients—and acidification, but found conflicting data in these categories.)

What about different types of eggs—could you make a greener omelet by choosing cage-free or organic eggs over the conventional type?

Since the majority of the impacts associated with chicken-rearing come from producing their feed, one way to look at the question is as a matter of efficiency: Which type of bird does the best job turning the food it eats into the eggs we eat? In that sense, there's a pretty clear hierarchy, according to Hongwei Xin, director of the Egg Industry Center at Iowa State University.

Data from Europe indicate that chickens raised in conventional cage systems—the much-maligned "battery" cages now being phased out in the European Union—are the most efficient layers: It takes them about 2 kilograms of feed to produce 1 kilogram of eggs. Chickens raised in cage-free or barn systems—in which the birds are free to roam around the building but don't necessarily have access to the outdoors—require about 14 percent more food. Free-range birds, with access to the outdoors, require about 18 percent more than conventional caged birds. Organic chickens, whose feed is grown without chemical fertilizers or pesticides, need roughly 20 percent more food than birds kept in cages. (For assistance decoding the dizzying variety of egg labels out there, see this helpful guide from the New York Times.)

The differences in feed efficiency arise in part because cage systems allow farmers to carefully calibrate the birds' temperature, so the animals expend less energy keeping themselves warm. That energy can then go toward making more eggs. Chickens in noncage systems also waddle around and flap their wings a lot more than their cooped-up cousins, which leaves even less energy for egg-laying. Plus, mortality rates are often higher when birds are allowed to roam. Finally, in noncage systems, it's harder to regulate ammonia emissions from chicken poop—a major issue of environmental concern on poultry farms.

Of course, many green-minded consumers prefer eggs from uncaged hens precisely because of the things that reduce their efficiency as layers—like the freedom to exercise and engage in natural behaviors, like nesting and dust-bathing. (Keep in mind that "free-range" doesn't always mean that the hen spent its days frolicking in sun-kissed fields; it could mean the bird had some access to a concrete lot.) In addition, organic eggs may come with benefits that aren't necessarily reflected when you judge by efficiency alone—like a reduction in pesticide and fertilizer applications, which is good for soil and water quality.

  Slate Plus
Frame Game
March 4 2015 10:53 AM Slate Voice: “Shag the Dog” Listen to Will Saletan read his 2001 essay about the merits of zoophilia.