美国国家公共电台 NPR Surf And Turf: To Reduce Gas Emissions From Cows, Scientists Look To The Ocean（在线收听）

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Livestock is a major source of greenhouse gases around the world. Scientists have been trying to find ways to reduce emissions from cows, and they think they might have found a solution in the ocean. NPR's Merrit Kennedy explains.

MERRIT KENNEDY, BYLINE: There's a pen at University of California, Davis where scientists are closely observing 12 research cows. Each is known by a four-digit number, except for the friendliest one.

BREANNA ROQUE: We just call her Ginger. She's the only one with a name.

KENNEDY: When cows like Ginger burp, out from their gut comes a lot of methane. It's a potent greenhouse gas, and livestock is one of the largest methane emitters in the U.S. These scientists are trying to reduce the emissions by tweaking the cow's diet. On this morning, graduate student Breanna Roque is mixing up breakfast for the research cows, and she's adding a secret ingredient.

ROQUE: So I'll sprinkle it in. I'll kind of rub it together with the hay, mix it around, and then we actually come through and pitchfork the whole ration.

KENNEDY: We'll get back to what Roque is adding in a minute, but first, why is it that cows emit methane? Animal science professor Ermias Kebreab, who is leading the experiment, says that the chemical reaction happens in the first chamber of the cow's four-part stomach. It's actually microbes living in the stomach that produce the methane, kind of like how yeast produces carbon dioxide in beer.

ERMIAS KEBREAB: It's a natural process of fermentation.

KENNEDY: The cows burp that methane out into the atmosphere. He says belches are a bigger problem than farts.

KEBREAB: Over 95 percent actually is from the mouth, from the front end of the cow.

KENNEDY: Which brings us back to that secret ingredient. What is it? Seaweed. Kebreab says they think seaweed has a chemical in it that can keep down methane levels in a cow's gut. To test it, they're feeding cows different amounts.

UNIDENTIFIED PERSON: Coming out now.

KEBREAB: Yep. There they are.

KENNEDY: The cows lumber up, each heading to its own specially prepared meal. Some cows don't like the salty taste of seaweed, so Roque added a little bit of molasses, too, which they seem to appreciate. To test whether the seaweed is working, the cows take a methane breathalyzer test at least three times a day. When they stick their heads in a machine that measures the gases in their breath, they get a cow cookie.

ROQUE: Well, when it has to do with feed, they can be trained (laughter).

KENNEDY: Kebreab says they noticed a difference in emissions right away.

KEBREAB: And the results are almost immediate. Like, you give them this today. The next day, you see the reduction.

KENNEDY: And the drop in methane was even more than they expected. He remembers the first numbers coming in.

KEBREAB: When Breanna was sending me those results, I was like, are you sure?

KENNEDY: Their experiment found that the high dose of seaweed reduced methane production by more than half.

KEBREAB: There's no doubt. There's - it's quite a dramatic reduction in methane emissions.

KENNEDY: But a dramatic reduction in emissions wouldn't do much good if it changed the taste of the cow's milk. Most people aren't looking for a salty, umami twist in their milkshakes, so researchers conducted another experiment. They brought in 25 panelists for a blind taste test who said the milk tasted normal. The relieved research team celebrated and made ice cream. It was also apparently delicious, but there's a long way to go.

The scientists are analyzing whether the seaweed changes the nutritional content of milk. They're also planning an experiment with beef cattle instead of dairy. And if all goes well, scientists and the livestock industry will have a new puzzle. What's the best way to grow a whole lot of seaweed? Merrit Kennedy, NPR News, Davis, Calif.