It's about a tossup. Methane, which is 23 times more potent than carbon dioxide as a contributor to global warming, is produced daily by cows. In fact, agriculture generates about 14 percent of the greenhouse gases today -- including two-thirds of all ammonia -- from cows. Even though other grazing animals also expel greenhouse gases, statistics suggest that cows produce 26 to 53 gallons (and some say up to 132 gallons) of methane every day -- the same amount of pollution emitted by a car in a day. And it's only going to worsen: The U.S. Food and Agriculture Organization predicts a 60 percent increase in agricultural methane output by 2030 [source: Times Online].
Curious readers may be interested to know just why it is that cows are such methane factories. It all has to do with a cow's digestion process, which takes place in its four stomachs. Cows, goats and other "ruminants" (heavy-duty chewers) are peculiar in that they ingest their food, regurgitate it as cud and eat it again. The reason for the second helping of cud is that the microorganisms in the cow's stomach can't break the food entirely down in one pass. So the cow sends it back to the stomachs to let the microbes finish their work. The microorganisms give off gases while they eat the second helping, which makes the cow belch, expelling methane. The food then moves among the rest of the cow's four stomachs, until it is finally digested, causing flatulence in the process and -- you guessed it! -- more methane being launched into the air [source: California State University Sacramento].
The nature of a cow's diet also contributes to the creature's tendency to emit methane. To help their businesses grow faster, farmers switched from natural feed to perennial ryegrass, because it grows abundantly and quickly. Ryegrass, however, is the "fast food" of grasses, lacking essential nutrients. It's also difficult for cows to digest and ultimately generates gas.
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