To city dwellers, cows may conjure up thoughts of a peaceful scene in the countryside - or dinner in the works.
The images that come to mind are quite different for rural Americans who live near cows, especially modern dairy farms with thousands of cattle and tons of manure.
According to a new report issued by government regulators in California, dairies are the No. 1 source of smog-producing pollution in the San Joaquin Valley, producing more than even cars and light trucks.
The San Joaquin Valley Unified Air Pollution Control District has determined that a cow annually emits 19.3 pounds of volatile organic compounds (VOCs), the gases that contribute to smog. That is 50 percent more than currently thought, the report said.
The new emission factor will force up to 250 more dairies to apply for permits, and force them to comply with regulations that are to be announced next summer.
Jared Fernandes, a dairy farmer who milks 3,000 cows in Tulare, said he finds the district's report hard to believe.
"It's a joke," he said. "Common sense tells me that I doubt cows are producing more than cars. Would you rather sit in your garage with your car running, or sit in a garage with a cow all night?"
Air pollution regulators say cow emissions cannot be directly compared to car emissions because they contain different types of VOCs.
Environmentalists, meanwhile, contend that the new emission factor doesn't reflect all the pollution created by dairies because it doesn't account for VOCs released by manure used as fertilizer, feed storage and other dairy processes.
"The number is a low-ball number," said Brent Newell, a lawyer for the Center for Race, Poverty and the Environment, a group that has sued to force the industry to apply regulations. "I think in the future it will be revised and revised upwards."
Several scientists involved in the research used to devise the new emission factor have criticized the report. They take issue with the way district staff determined the amount of VOCs known as volatile fatty acids.
The district relied on research conducted in Great Britain and a feedlot study from Texas for its data on the acids.
"We've been cautioning them that a large component of their estimate is something for which there was very little California data," said Charles Krauter, a researcher at California State University-Fresno.
Michael Marsh, head of Western United Dairymen, said his group will ask the district's Governing Board to review the findings.
"If they don't, and our farmers are caught being forced to rely upon on an emission factor for regulation that's not based on science, we will, of course, review all our legal options," he said.