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That excess manure gives off air pollutants, and it is the country's fastest-growing large source of methane, a greenhouse gas.
And it washes down with the rain, helping to cause the 230 oxygen-deprived "dead zones" that have proliferated along the U.S. coast. In the Chesapeake Bay, about one-fourth of the pollution that leads to dead zones can be traced to the back ends of cows, pigs, chickens and turkeys.
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